# Ohmsville

> The early-80s spring-terminal electronics lab, alive in your browser. Real parts, real physics — LEDs glow, the speaker sings, and things smoke when you get it wrong. A shop teacher sits in the manual. Live at https://ohmsville.com.

## Opening a circuit by URL
The bench reads its whole state from the URL hash, so a circuit is a link:

`/bench#a=<activity>&b=<board>&w=<wire pairs>&c=<campaign>`

- `a=` — the manual page to open on load: an experiment (`exp-<n>`) or a recipe id (see Recipes below).
- `b=` — the board layout, present only when it differs from the standard kit. Parts are listed in spring order, comma-separated: a standard part is `id@col.row`, a part pulled from the drawer is `+kind:value@col.row` (e.g. `+resistor:2200@3.1`).
- `w=` — wires, comma-separated `spring-spring` pairs, e.g. `1-5,6-28`.
- `c=` — a campaign id (see Explore below), applied once when the page opens.

Example — experiment 1's reference wiring: `/bench#a=exp-1&w=1-5,6-28,29-38,39-2`

## Levels and experiments
### Level 1 · First Light (30 in ONE)
Batteries, resistors, LEDs and the loop that joins them.
  - exp-1 — Light the LED: Press the button and the red LED lights. Let go and it goes dark.
  - exp-2 — Dim it with the pot: Turn the knob and the LED goes from bright to almost out.
  - exp-3 — Two-transistor flasher: The red and green LEDs take turns blinking, about once a second.
  - exp-4 — Make a tone: Turn the speaker on and hear a steady tone around 700 Hz.
  - exp-5 — Light-controlled tone: Drag the light slider under the CdS cell (or turn on the camera and cover it) and the pitch changes.

### Level 2 · Timing & Control (55 in ONE)
Series and parallel, a transistor as a switch, and capacitors that set the pace.
  - exp-6 — Series LEDs: Both LEDs glow, and each is dimmer than the red one was alone through the same 1 kΩ.
  - exp-7 — Parallel LEDs: Both LEDs at full brightness at once, each on its own 330 Ω.
  - exp-8 — A knob that carries no load: Turn the pot and the LED dims, but the pot itself only ever handles microamps.
  - exp-9 — Slow on, slow off: Hold the button: the LED fades up over a couple of seconds. Let go: it stays lit and fades out slowly.
  - exp-10 — Siren knob: Turn the speaker on and turn the new pot: the pitch slides from a low buzz up to the tone of experiment 4.

### Level 3 · Sound & Sense (100 in ONE)
Stack batteries, store charge, and turn light and knobs into sound.
  - exp-11 — Twice the push: With two batteries in series the LED through 1 kΩ is about twice as bright as with one.
  - exp-12 — Capacitor battery: Hold the button to charge the 470 µF, let go, and the LED keeps glowing for about half a second as the capacitor empties.
  - exp-13 — Metronome: Turn the speaker on and hear a steady click about three times a second.
  - exp-14 — Two-tone: Turn the speaker on: the slide switch flips the tone between two pitches.
  - exp-15 — Light theremin: Turn the speaker on. The light slider plays the pitch, and the new pot tunes it: each sets one half of every cycle.

### Level 4 · Relay Logic (200 in ONE)
An electromagnet that throws a switch: NOT, memory, an alarm, and the bell.
  - exp-16 — One coil, two circuits: Green is lit at rest. Hold the button: a click, green goes out and red comes on. Let go: click, back to green.
  - exp-17 — Kickback: Hold the button: click, LED on. Let go: click, LED off, and Q1 is still alive. Now pull the diode's wires and try again: Q1 dies the moment you let go.
  - exp-18 — Dark alarm: Slide the light toward the moon: click, and the buzzer sounds. Back toward the sun: click, silence.
  - exp-19 — The relay that remembers: Slide ON. Tap the button: click, LED on, and it stays on after you let go. Slide OFF: click, LED out. Slide ON again: still off until you tap.
  - exp-20 — The electric bell: Turn the speaker on and slide ON: a low buzz, around 90 Hz, and the relay blade shivers.

### Level 5 · Logic Lane (250 in ONE)
A chip that keeps time, gates that decide, and a digit the room can read.
  - exp-21 — The pad beacon: The red lamp blinks on its own, about two thirds of a second on and two thirds off, and nothing you do to the board changes it.
  - exp-22 — Range safety: Turn the speaker on: a tone. Turn the knob and the pitch climbs from a low hum near 690 Hz to a shriek past 3 kHz.
  - exp-23 — Go / No-Go: Slide ON and hold the button: the lamp lights. Either one alone: nothing. That is AND.
  - exp-24 — Set the digit: Nothing pressed: 0. Button: 1. Slide: 2. Both: 3.
  - exp-25 — Zero: Hold the button and the clock sits at 9. Let go: 9, 8, 7, all the way to 0, about one step a second, with the lamp ticking. Then it rolls round to 9 and starts again.

## Recipes
Short circuits that show what one part does, opened the same way as an experiment (`a=<recipe-id>`):
- ohms-law — Ohm's law, by eye: The LED glows dimmer through 1 kΩ than through 330 Ω. Swap in 10 kΩ and it barely shows.
- smoke-test — Smoke test: Put 100 Ω straight across 9 V and watch it burn in a blink.
- exp-1 — Light the LED: Press the button and the red LED lights. Let go and it goes dark.
- exp-2 — Dim it with the pot: Turn the knob and the LED goes from bright to almost out.
- exp-3 — Two-transistor flasher: The red and green LEDs take turns blinking, about once a second.
- exp-9 — Slow on, slow off: Hold the button: the LED fades up over a couple of seconds. Let go: it stays lit and fades out slowly.
- exp-4 — Make a tone: Turn the speaker on and hear a steady tone around 700 Hz.
- tiny-current — Tiny current, big current: Under a tenth of a milliamp into the base lights the LED with about 8 mA.
- exp-8 — A knob that carries no load: Turn the pot and the LED dims, but the pot itself only ever handles microamps.
- divider — Voltage divider: Hover spring 18 and turn the knob: the voltage slides from 9 V down to 0.
- series-switches — Two switches in series: The LED lights only when the slide is ON and the button is held.
- parallel-switches — Two switches in parallel: Either the slide or the button lights the LED.
- light-meter — Light meter: The LED glows with the light slider toward the sun and goes dark toward the moon; or turn on the camera and cover it.
- night-light — Night light: Slide the light under the CdS cell toward the moon: the LED comes on in the dark and goes off in the light.
- exp-5 — Light-controlled tone: Drag the light slider under the CdS cell (or turn on the camera and cover it) and the pitch changes.
- click — Make it click: Turn the speaker on, then tap the button: a click on press and another on release.
- one-way-street — One-way street: The LED lights, with about 0.7 V across the 1N4001. Turn the diode round and all is dark.
- exp-17 — Kickback: Hold the button: click, LED on. Let go: click, LED off, and Q1 is still alive. Now pull the diode's wires and try again: Q1 dies the moment you let go.
- alarm-button — Alarm button: Hold the button and the buzzer sounds; let go and it stops.
- exp-18 — Dark alarm: Slide the light toward the moon: click, and the buzzer sounds. Back toward the sun: click, silence.
- exp-16 — One coil, two circuits: Green is lit at rest. Hold the button: a click, green goes out and red comes on. Let go: click, back to green.
- exp-19 — The relay that remembers: Slide ON. Tap the button: click, LED on, and it stays on after you let go. Slide OFF: click, LED out. Slide ON again: still off until you tap.
- one-shot — One shot: Tap the button and the lamp comes on and stays on for about a second, then goes out by itself. Tapping again during the second changes nothing.
- exp-21 — The pad beacon: The red lamp blinks on its own, about two thirds of a second on and two thirds off, and nothing you do to the board changes it.
- exp-22 — Range safety: Turn the speaker on: a tone. Turn the knob and the pitch climbs from a low hum near 690 Hz to a shriek past 3 kHz.
- inverter — NOT, from a NAND: The lamp is lit at rest and goes dark while you hold the button. That is NOT.
- exp-23 — Go / No-Go: Slide ON and hold the button: the lamp lights. Either one alone: nothing. That is AND.
- count-by-hand — Count by hand: Tap the button and the digit goes up by one: 0, 1, 2 … 9, and then round to 0 again.
- exp-24 — Set the digit: Nothing pressed: 0. Button: 1. Slide: 2. Both: 3.
- exp-25 — Zero: Hold the button and the clock sits at 9. Let go: 9, 8, 7, all the way to 0, about one step a second, with the lamp ticking. Then it rolls round to 9 and starts again.
- flickering-candle — Flickering candle: A yellow LED that never sits still: two clocks running at once, and the brightness is their sum.
- eyes-in-the-bushes — Eyes in the bushes: Two red LEDs blinking the opposite way round: when one is lit the other is out, about three times every two seconds.
- lightning — Lightning: A rattling burst of flashes for about three quarters of a second, then nothing for about the same, over and over.
- creaky-door — Creaky door: Hold the button to wind it up. Let go and the tone groans downward for ten seconds, slower and slower, until the door is shut.
- tripwire-scream — Tripwire scream: Keep a light on the cell and the shop is quiet. Break the beam — slide the light toward the moon, or cover the camera — and it screams.
- ghost-in-the-wires — Ghost in the wires: Slide ON to arm it. Something crosses the shadow once and the buzzer starts — and keeps going after whatever it was has gone. Only the slide shuts it up.

## Lessons
The classroom's course, one lesson per idea, each with a demo on the bench:
### Charge and the loop
- what-electricity-is — What electricity is: Voltage is the push (energy per charge, measured in volts); current is the flow of charge (measured in amps). Charge does not leave a cut wire the way water leaves a cut pipe; the wire is already full of charge and the whole column moves together.
- the-loop — The loop: Current needs a closed loop from + through the parts back to −; a switch is a gap in the loop; switches in series both need to close (AND) while switches in parallel need only one (OR).

### Ohm's law
- resistance-and-ohms-law — Resistance and Ohm's law: Ohm's law: I = V / R (volts divided by ohms gives amps). A resistor's colour bands give its value; its power rating in watts is the heat it can shed, and P = V × I above that rating burns it.
- series-and-parallel — Series and parallel: In series the same current flows through each part and voltages add (resistances add). In parallel each part sees the same voltage and currents add (combined resistance drops). Batteries in series add volts; capacitors in parallel add farads.

### Parts that do something
- capacitors-and-time — Capacitors and time: A capacitor stores charge (farads = charge per volt). Charging through a resistor takes time set by R × C, about 63% per time constant. Capacitors pass changes and block steady current; electrolytics have polarity.
- diodes-and-leds — Diodes and LEDs: A diode conducts one way only and takes a fixed forward voltage (about 1.9 V for a red LED) rather than a proportional share; an LED needs a series resistor to set its current, and reversed it stays dark but undamaged.
- transistors — Transistors as switches and amplifiers: An NPN transistor lets a small base current control a collector current about beta (around 100) times larger; with the base fully on or off it is a switch, and in between it is an amplifier where the output follows the input.

### Circuits that move
- oscillators — Oscillators: The two-transistor astable multivibrator flips because each transistor turns the other off through a capacitor that recharges through a resistor; period ≈ 1.4 × R × C, so smaller C or R means a faster flip (a flasher at 100 µF, a 700 Hz tone at 0.1 µF).
- sensors — Sensors: A sensor turns a physical quantity into resistance (photocell: about 1 kΩ bright, hundreds of kΩ dark). In a voltage divider the sensor's share of the volts changes with it, and a transistor at the midpoint turns that into a switch or a control.

### The shop
- reading-schematics — Reading schematics: A schematic shows connections, not positions: the same schematic wires many ways. Symbols carry meaning (curved capacitor plate is the electrolytic's −, LED triangle points the way current flows, NPN arrow points out of the emitter), and reading starts at the battery + and follows the loop.
- measuring — Measuring: A voltmeter goes across a part (parallel, very high resistance); an ammeter goes in the loop (series, very low resistance) and must never be clipped across a part. Voltages around a loop add to the source voltage; currents into a junction equal currents out.
- safety-and-real-parts — Safety and real parts: Danger is set by current through the body, not volts alone: 9 V through skin is under a tenth of a milliamp and safe, mains at 120–240 V can push tens of milliamps across the chest and is not. Heat, polarity (electrolytics, LEDs, battery), and part tolerance are the real-world details the simulator only hints at.

## Parts catalogue
Every part in the drawer, unlimited supply, grouped by kind:
- **resistor**: 10 Ω, 22 Ω, 47 Ω, 100 Ω, 220 Ω, 330 Ω, 470 Ω, 1 kΩ, 2.2 kΩ, 4.7 kΩ, 10 kΩ, 22 kΩ, 47 kΩ, 100 kΩ, 220 kΩ, 470 kΩ, 1 MΩ
- **capacitor**: 1 nF, 10 nF, 100 nF, 1 µF, 10 µF, 100 µF, 470 µF
- **led**: LED red, LED green, LED yellow
- **npn**: NPN 2N2222
- **pot**: 10 kΩ pot, 100 kΩ pot
- **switch**: Pushbutton, Slide switch
- **speaker**: Speaker 8 Ω
- **photocell**: CdS cell
- **diode**: 1N4001
- **buzzer**: Buzzer
- **relay**: Relay 9 V
- **timer555**: NE555
- **gate**: NAND gate, NOR gate
- **sevenseg**: 7-segment
- **counter**: Counter up, Counter down
- **battery**: 9 V, 1.5 V

## Explore
- [Home](/)
- [The Ladder](/ladder) — the levels, in order, with what each unlocks.
- [Classroom](/classroom) — the course above, read as lessons.
- [Mission Control](/campaign/mission-control) — Five circuits stand between you and a launch.
- [Spooky Shack](/campaign/spooky-shack) — The shop is shut, the lights are off, and something in the 200-in-1 is still running.
- [Halloween](/halloween) — the Spooky Shack pack: seven circuits, the October mission, and the write-up.
- [Blog](/blog) — dev log, how the kit works, and the history behind the campaigns.
- [For agents & developers](/agents) — this file, read as a page, plus the URL grammar worked out, the API and the MCP connector setup.

## API
The same content as JSON, plus a simulator that runs the real solver. No key, no
sign-up, CORS open to any origin. Base: https://ohmsville.com/api

- `GET /api/experiments`, `GET /api/experiments/{id}` — the manual, with each experiment's goal, wiring, circuit document, bench link and schematic.
- `GET /api/recipes`, `GET /api/recipes/{id}` — the short per-part circuits.
- `GET /api/lessons`, `GET /api/lessons/{id}` — the classroom's course.
- `GET /api/parts` — the standard board, the drawer, and the pin names of every part kind.
- `POST /api/links` — a circuit document in, a bench link out (plus the canonical document that link carries).
- `GET /api/circuit?url=` — a bench link in, a circuit document out.
- `POST /api/simulate` — settle a circuit, apply knob changes step by step, and read volts, milliamps, LED brightness, transistor region, relay state and speaker pitch. Caps: 40 wired parts, 120 wires, 3 000 ms of circuit time, a 6 s wall clock; over the wall clock you get the steps that finished and `"complete": false`.
- `POST /api/ask` — ask the shop teacher about a circuit. Needs an `X-Api-Key`; `POST /api/keys` with your email mails you one, good for 50 questions a UTC day.
- `GET /api/openapi.json`, `GET /api/docs` — the whole thing, machine- and human-readable.

A circuit is a JSON document, not a URL hash: its schema is at `/api/schema/circuit.json`. The 21 standard
parts are always on the board, so `parts` lists only what you took from the drawer, and every wire endpoint is
`"<part>.<pin>"` (or a bare spring number). Experiment 1, in full:

```json
{
  "format": "circuit-city.circuit",
  "version": 1,
  "activity": "exp-1",
  "wires": [
    [
      "battery.+",
      "r330a.a"
    ],
    [
      "r330a.b",
      "led-red.a"
    ],
    [
      "led-red.k",
      "pushbutton.a"
    ],
    [
      "battery.-",
      "pushbutton.b"
    ]
  ]
}
```

### MCP

`https://ohmsville.com/mcp` is a remote MCP server (Streamable HTTP, stateless). Add it as a connector in
Claude.ai or ChatGPT, or in Claude Code with
`claude mcp add --transport http ohmsville https://ohmsville.com/mcp`.

Tools: `list_experiments`, `get_experiment`, `list_recipes`, `list_lessons`, `list_parts`,
`simulate_circuit`, `make_link`, `parse_link`, `ask_teacher`. Resources:
`ohmsville://experiments/{id}`, `ohmsville://lessons/{id}`, `ohmsville://schema/circuit`.
Prompt: `design_circuit`. Everything but `ask_teacher` is keyless; `ask_teacher` takes the same
key `POST /api/ask` does, as an `Authorization: Bearer` header or an `apiKey` argument.

## Every experiment, in full
### exp-1 — Light the LED (Level 1)
Parts: 9 V battery, 330 Ω resistor, red LED, pushbutton
Goal: Press the button and the red LED lights. Let go and it goes dark.
Concept: A circuit is a loop. Current leaves the battery +, goes through the resistor and LED, through the button, and back to −. The resistor limits the current so the LED gets about 20 mA instead of burning. Try wiring the LED backwards, or leaving the resistor out.
Wiring: w=1-5,6-28,29-38,39-2

### exp-2 — Dim it with the pot (Level 1)
Parts: 9 V battery, 10 kΩ potentiometer, 330 Ω resistor, red LED
Goal: Turn the knob and the LED goes from bright to almost out.
Concept: The potentiometer is a resistor with a moving contact (the wiper). Between end 1 and the wiper you get anywhere from 0 to 10 kΩ. More ohms, less current, dimmer LED. That is Ohm's law: I = V / R.
Wiring: w=1-17,18-5,6-28,29-2

### exp-3 — Two-transistor flasher (Level 1)
Parts: 9 V battery, 2 × 330 Ω, 2 × 10 kΩ, 2 × 100 µF, red + green LED, Q1, Q2
Goal: The red and green LEDs take turns blinking, about once a second.
Concept: This is an astable multivibrator. Each transistor turns the other off through a capacitor. The capacitor charges through the 10 kΩ until the next transistor switches on, then the whole thing flips. Flash rate is set by R × C: about 1 / (1.4 × 10 kΩ × 100 µF) ≈ 0.7 Hz.
Wiring: w=1-5,6-28,29-32,1-7,8-30,31-35,1-11,12-33,1-13,14-36,32-20,21-36,35-22,23-33,34-2,37-2

### exp-4 — Make a tone (Level 1)
Parts: 9 V battery, 2 × 330 Ω, 2 × 10 kΩ, 2 × 0.1 µF, speaker, Q1, Q2
Goal: Turn the speaker on and hear a steady tone around 700 Hz.
Concept: Same flasher, but with capacitors 1000 times smaller it flips 1000 times faster: too fast to see, so we listen instead. The speaker sits in Q2's collector so it gets a square wave. Swap a 0.1 µF for a 100 µF and it is a flasher again.
Wiring: w=1-5,6-32,1-7,8-42,43-35,1-11,12-33,1-13,14-36,32-24,25-36,35-26,27-33,34-2,37-2

### exp-5 — Light-controlled tone (Level 1)
Parts: everything from 4, CdS photocell instead of one 10 kΩ
Goal: Drag the light slider under the CdS cell (or turn on the camera and cover it) and the pitch changes.
Concept: A CdS photocell is a resistor that changes with light: about 1 kΩ in bright light, hundreds of kΩ in the dark. Put it where a timing resistor was and the light level sets the pitch. In the dark the cell climbs past 100 kΩ, so little base current reaches Q2 that it can never turn on hard enough to flip the pair, and the tone stops. Slide the light up and it sings again. That is a sensor: something physical turned into ohms.
Wiring: w=1-5,6-32,1-7,8-42,43-35,1-11,12-33,1-44,45-36,32-24,25-36,35-26,27-33,34-2,37-2

### exp-6 — Series LEDs (Level 2)
Parts: 9 V battery, 1 kΩ resistor, red LED, green LED
Goal: Both LEDs glow, and each is dimmer than the red one was alone through the same 1 kΩ.
Concept: Parts in series share one current, and their voltage drops add up. The red LED takes about 1.9 V and the green 2.1 V, so the resistor only has 5 V left to push with instead of 7. Less push through the same ohms means less current, and the same current runs through both LEDs. Hover the resistor before and after adding the second LED.
Wiring: w=1-9,10-28,29-30,31-2

### exp-7 — Parallel LEDs (Level 2)
Parts: 9 V battery, 2 × 330 Ω, red LED, green LED
Goal: Both LEDs at full brightness at once, each on its own 330 Ω.
Concept: In parallel, each branch gets the whole 9 V, so each LED runs exactly as it would alone. The battery just supplies the sum: about 21 mA plus 21 mA. Hover the battery and compare with experiment 6. This is how a string of lights stays bright when you add more.
Wiring: w=1-5,6-28,29-2,1-7,8-30,31-2

### exp-8 — A knob that carries no load (Level 2)
Parts: 9 V battery, 10 kΩ pot, 100 kΩ, Q1, 330 Ω, red LED
Goal: Turn the pot and the LED dims, but the pot itself only ever handles microamps.
Concept: The pot makes a voltage between 0 and 9 V at its wiper. That voltage pushes a tiny current through 100 kΩ into Q1's base, and Q1 lets about 100 times as much flow from collector to emitter for the LED. The control never touches the load current. This is how a small knob moves a big motor.
Wiring: w=1-17,19-2,18-15,16-33,1-5,6-28,29-32,34-2

### exp-9 — Slow on, slow off (Level 2)
Parts: 9 V battery, pushbutton, 10 kΩ, 100 µF, 100 kΩ, Q1, 330 Ω, red LED
Goal: Hold the button: the LED fades up over a couple of seconds. Let go: it stays lit and fades out slowly.
Concept: The button charges the 100 µF through 10 kΩ, about one second per 63%. The capacitor voltage feeds Q1 through 100 kΩ, so the LED follows the charge. Released, the only way out is back through that 100 kΩ, ten times slower. A capacitor is a bucket for charge; the resistor is the width of the hose.
Wiring: w=1-38,39-11,12-20,21-2,20-15,16-33,1-5,6-28,29-32,34-2

### exp-10 — Siren knob (Level 2)
Parts: everything from 4, 100 kΩ pot from the drawer
Goal: Turn the speaker on and turn the new pot: the pitch slides from a low buzz up to the tone of experiment 4.
Concept: The tone circuit flips when a capacitor charges through its base resistor. Put the 100 kΩ pot in series with Q2's 10 kΩ and that half of the cycle takes anywhere from 10 kΩ to 110 kΩ worth of time. More ohms, slower charge, lower pitch. One knob, and a fixed circuit becomes an instrument.
Wiring: w=1-5,6-32,1-7,8-42,43-35,1-11,12-33,1-13,14-$pot2.0,$pot2.1-36,32-24,25-36,35-26,27-33,34-2,37-2 (needs from the drawer: pot2=pot:100000)

### exp-11 — Twice the push (Level 3)
Parts: 2 × 9 V battery, 1 kΩ resistor, red LED
Goal: With two batteries in series the LED through 1 kΩ is about twice as bright as with one.
Concept: Batteries in series add their voltages: the second battery's + sits on top of the first's, so the loop sees 18 V. Ohm's law does the rest: twice the volts through the same ohms is twice the current. Never try this with the 330 Ω: 18 V would push about 50 mA and burn the LED.
Wiring: w=1-$bat2.1,$bat2.0-9,10-28,29-2 (needs from the drawer: bat2=battery:9)

### exp-12 — Capacitor battery (Level 3)
Parts: 9 V battery, pushbutton, 330 Ω, 470 µF from the drawer, 1 kΩ, red LED
Goal: Hold the button to charge the 470 µF, let go, and the LED keeps glowing for about half a second as the capacitor empties.
Concept: A capacitor stores charge and gives it back. The button fills it through 330 Ω in a fraction of a second. Released, the only path for that charge is out through 1 kΩ and the LED, and the glow fades as the voltage falls: 1 kΩ × 470 µF is about half a second per 63%. That is a battery that lasts half a second. Swap in the 470 Ω and it empties twice as fast.
Wiring: w=1-38,39-5,6-$c470.0,$c470.1-2,$c470.0-9,10-28,29-2 (needs from the drawer: c470=capacitor:0.00047)

### exp-13 — Metronome (Level 3)
Parts: 9 V battery, 2 × 330 Ω, 2 × 22 kΩ from the drawer, 2 × 10 µF from the drawer, speaker, Q1, Q2
Goal: Turn the speaker on and hear a steady click about three times a second.
Concept: The same two-transistor flip-flop as the flasher and the tone, tuned for the ear instead of the eye: 22 kΩ × 10 µF sits between the flasher's slow 10 kΩ × 100 µF and the tone's fast 10 kΩ × 0.1 µF. Each flip is a jump in speaker current, and a jump is a click. Swap a 10 µF for a 100 µF and one side of the beat stretches ten times.
Wiring: w=1-5,6-32,1-7,8-42,43-35,1-$r22a.0,$r22a.1-33,1-$r22b.0,$r22b.1-36,32-$c10a.0,$c10a.1-36,35-$c10b.0,$c10b.1-33,34-2,37-2 (needs from the drawer: r22a=resistor:22000, r22b=resistor:22000, c10a=capacitor:0.00001, c10b=capacitor:0.00001)

### exp-14 — Two-tone (Level 3)
Parts: everything from 4, slide switch, 0.1 µF from the drawer
Goal: Turn the speaker on: the slide switch flips the tone between two pitches.
Concept: Capacitors in parallel add up. The slide puts a second 0.1 µF beside one of the tone circuit's timing capacitors, so that half of the cycle has 0.2 µF to charge and takes twice as long. The whole period grows by half, and the pitch drops by a third. A doorbell's ding-dong is two tones like this.
Wiring: w=1-5,6-32,1-7,8-42,43-35,1-11,12-33,1-13,14-36,32-24,25-36,35-26,27-33,34-2,37-2,24-40,41-$c100n.0,$c100n.1-25 (needs from the drawer: c100n=capacitor:1e-7)

### exp-15 — Light theremin (Level 3)
Parts: everything from 5, 100 kΩ pot from the drawer
Goal: Turn the speaker on. The light slider plays the pitch, and the new pot tunes it: each sets one half of every cycle.
Concept: Each half of the tone's cycle has its own timing resistor. The photocell owns Q2's half, as in experiment 5; the pot, in series with Q1's 10 kΩ, owns the other. Light shortens one half, the knob stretches the other, and the pitch is one over the sum. With the pot turned all the way up this is experiment 5 again; turned all the way down under bright light it stalls, because one side then switches a hundred times harder than the other and the flip-flop cannot flip. A theremin is played without touching it; here the hand shades the sensor.
Wiring: w=1-5,6-32,1-7,8-42,43-35,1-11,12-$pot2.0,$pot2.1-33,1-44,45-36,32-24,25-36,35-26,27-33,34-2,37-2 (needs from the drawer: pot2=pot:100000)

### exp-16 — One coil, two circuits (Level 4)
Parts: 9 V battery, pushbutton, relay from the drawer, 2 × 330 Ω, red + green LED
Goal: Green is lit at rest. Hold the button: a click, green goes out and red comes on. Let go: click, back to green.
Concept: An electromagnet pulls a switch blade. About 18 mA in the coil loop throws a contact in a second loop that never touches the first. At rest the blade sits on NC, so the green LED is lit; pull in and it swings to NO, so the red one is. NC is the opposite of the coil: held means red, not held means green. That opposite is what computers call NOT.
Wiring: w=1-38,39-$relay.0,$relay.1-2,1-$relay.2,$relay.3-5,6-28,29-2,$relay.4-7,8-30,31-2 (needs from the drawer: relay=relay)

### exp-17 — Kickback (Level 4)
Parts: 9 V battery, pushbutton, 10 kΩ, Q1, relay from the drawer, 1N4001 from the drawer, 330 Ω, red LED
Goal: Hold the button: click, LED on. Let go: click, LED off, and Q1 is still alive. Now pull the diode's wires and try again: Q1 dies the moment you let go.
Concept: A coil hates a change in its current. Cut its path and it makes whatever voltage it takes, hundreds of volts, to keep the current going for an instant. Q1 is rated 40 V. The diode is a one-way detour that is closed while the coil is powered and opens exactly when the coil needs it, so the current circles round and dies out in half a millisecond instead of punching through the transistor. Every coil driven by a transistor gets one.
Wiring: w=1-$relay.0,$relay.1-32,34-2,1-38,39-11,12-33,$d1.0-$relay.1,$d1.1-1,1-$relay.2,$relay.3-5,6-28,29-2 (needs from the drawer: relay=relay, d1=diode)

### exp-18 — Dark alarm (Level 4)
Parts: 9 V battery, 100 kΩ, CdS cell, Q1, Q2, relay from the drawer, 1N4001 from the drawer, buzzer from the drawer
Goal: Slide the light toward the moon: click, and the buzzer sounds. Back toward the sun: click, silence.
Concept: One transistor is not enough here: in the dark the divider gives Q1 about 80 µA of base current, which is 8 mA of coil, under the 10 mA the relay needs. So Q1 feeds Q2, and the pair has a gain of about 10 000. The base then needs two junctions' worth, 1.4 V, and the divider crosses that when the cell is near 18 kΩ: room light (10 kΩ) is off, dark (300 kΩ) is on. The relay's own hysteresis keeps it from chattering at dusk.
Wiring: w=1-15,16-33,33-44,45-2,32-1,34-36,1-$relay.0,$relay.1-35,37-2,$d1.0-$relay.1,$d1.1-1,1-$relay.2,$relay.3-$bz.0,$bz.1-2 (needs from the drawer: relay=relay, d1=diode, bz=buzzer)

### exp-19 — The relay that remembers (Level 4)
Parts: 9 V battery, slide switch, pushbutton, relay from the drawer, 1N4001 from the drawer, 330 Ω, red LED
Goal: Slide ON. Tap the button: click, LED on, and it stays on after you let go. Slide OFF: click, LED out. Slide ON again: still off until you tap.
Concept: The relay's own contact feeds its coil, so once it is in, it holds itself in. The button is SET, the slide is RESET, and one relay is one bit of memory: a flip-flop, the way the first computers stored numbers.
Wiring: w=1-40,41-38,41-$relay.2,39-$relay.0,$relay.3-$relay.0,$relay.1-2,$d1.0-2,$d1.1-$relay.0,$relay.0-5,6-28,29-2 (needs from the drawer: relay=relay, d1=diode)

### exp-20 — The electric bell (Level 4)
Parts: 9 V battery, slide switch, speaker, relay from the drawer, 1N4001 from the drawer
Goal: Turn the speaker on and slide ON: a low buzz, around 90 Hz, and the relay blade shivers.
Concept: The coil is powered through its own NC contact. Pull in, and the contact that fed it opens; let go, and it closes again. The relay switches itself for as long as there is power: an oscillator with a hinge, and every doorbell and buzzer before the transistor worked this way. Compare with the flasher: there the pace is set by R × C, here by how far the blade has to travel.
Wiring: w=1-40,41-42,43-$relay.4,$relay.2-$relay.0,$relay.1-2,$d1.0-2,$d1.1-$relay.0 (needs from the drawer: relay=relay, d1=diode)

### exp-21 — The pad beacon (Level 5)
Parts: 9 V battery, NE555 from the drawer, 10 kΩ, 100 kΩ, 10 µF from the drawer, 330 Ω, red LED
Goal: The red lamp blinks on its own, about two thirds of a second on and two thirds off, and nothing you do to the board changes it.
Concept: The capacitor charges through both resistors until THR passes two thirds of the supply; the chip then pulls OUT low and drains the cap through DIS and the 100 kΩ until TRIG falls below one third, and starts again. R and C set the pace: 110 kΩ on the way up, 100 kΩ on the way down, 10 µF either way, so each half takes about 0.7 seconds. The chip keeps its own time whether or not anyone is watching.
Wiring: w=1-$t1.0,$t1.3-2,1-11,12-$t1.4,$t1.4-15,16-$t1.2,$t1.1-$t1.2,$t1.2-$c10.0,$c10.1-2,$t1.5-5,6-28,29-2 (needs from the drawer: t1=timer555, c10=capacitor:0.00001)

### exp-22 — Range safety (Level 5)
Parts: 9 V battery, NE555 from the drawer, 1 kΩ, 10 kΩ pot, 0.1 µF, 330 Ω, speaker
Goal: Turn the speaker on: a tone. Turn the knob and the pitch climbs from a low hum near 690 Hz to a shriek past 3 kHz.
Concept: The same circuit as the beacon with a capacitor a hundred times smaller, so the same charging runs a thousand times faster. The pot sits between DIS and THR as a rheostat, and that resistance sets both halves of the period, so turning the knob is turning the pitch. A siren, a metronome and a doorbell chime are all this circuit at different speeds.
Wiring: w=1-$t1.0,$t1.3-2,1-9,10-$t1.4,$t1.4-17,18-$t1.2,$t1.1-$t1.2,$t1.2-24,25-2,$t1.5-7,8-42,43-2 (needs from the drawer: t1=timer555)

### exp-23 — Go / No-Go (Level 5)
Parts: 9 V battery, 2 × NAND gate from the drawer, pushbutton, slide switch, 2 × 10 kΩ, 330 Ω, red LED
Goal: Slide ON and hold the button: the lamp lights. Either one alone: nothing. That is AND.
Concept: A NAND says no only when everything says yes, so on its own it is the opposite of what the console needs. Feed its output into a second NAND with both inputs tied together, which makes an inverter, and you have AND: both stations GO, or no launch. Every input needs a resistor to ground, or an open switch leaves the pin floating and the gate cannot tell you anything.
Wiring: w=1-$g1.3,$g1.4-2,1-$g2.3,$g2.4-2,1-38,39-$g1.0,$g1.0-11,12-2,1-40,41-$g1.1,$g1.1-13,14-2,$g1.2-$g2.0,$g2.0-$g2.1,$g2.2-5,6-28,29-2 (needs from the drawer: g1=gate:nand, g2=gate:nand)

### exp-24 — Set the digit (Level 5)
Parts: 9 V battery, 7-segment display from the drawer, pushbutton, slide switch, 2 × 10 kΩ
Goal: Nothing pressed: 0. Button: 1. Slide: 2. Both: 3.
Concept: The four input pins are a number written in binary — D0 is 1, D1 is 2, D2 is 4, D3 is 8 — and the decoder inside turns it into the seven lamps that make a digit. Two switches reach 3; tie D2 and D3 to switches instead and the same display counts to 9. A pin left unwired reads low, which is why D2 and D3 go straight to ground here rather than being left alone.
Wiring: w=1-$disp.4,$disp.5-2,1-38,39-$disp.0,$disp.0-11,12-2,1-40,41-$disp.1,$disp.1-13,14-2,$disp.2-2,$disp.3-2 (needs from the drawer: disp=sevenseg)

### exp-25 — Zero (Level 5)
Parts: 9 V battery, NE555, decade counter (down), 7-segment display, 100 kΩ, 10 kΩ, 10 µF, pushbutton, 330 Ω, red LED
Goal: Hold the button and the clock sits at 9. Let go: 9, 8, 7, all the way to 0, about one step a second, with the lamp ticking. Then it rolls round to 9 and starts again.
Concept: Three chips, each doing one thing. The 555 makes edges, the counter remembers a number and changes it on every edge, the display turns that number into a digit. The four wires between counter and display are the number itself in binary, moving from one chip to the next — a bus, which is what every computer is made of. It rolls past zero because a decade counter has no idea what a launch is; deciding to stop is the gates’ job.
Wiring: w=1-$t1.0,$t1.3-2,1-$ctr.2,$ctr.3-2,1-$disp.4,$disp.5-2,1-15,16-$t1.4,$t1.4-11,12-$t1.2,$t1.1-$t1.2,$t1.2-$c10.0,$c10.1-2,$t1.5-$ctr.0,1-38,39-$ctr.1,$ctr.1-13,14-2,$ctr.4-$disp.0,$ctr.5-$disp.1,$ctr.6-$disp.2,$ctr.7-$disp.3,$t1.5-5,6-28,29-2 (needs from the drawer: t1=timer555, ctr=counter:down, disp=sevenseg, c10=capacitor:0.00001)

## Every lesson, in full
### what-electricity-is — What electricity is (Charge and the loop)
Everything on this bench is made of atoms, and every atom carries charge: a positive core and negative electrons around it. In a copper wire the outermost electrons are barely held. They wander from atom to atom all the time, in every direction, going nowhere in particular. That is a wire with nothing happening in it.

Current is what you get when those electrons drift one way on purpose. We measure it in amperes, and one amp is a lot of electrons: about six billion billion passing a point every second. The kit works in thousandths of an amp, milliamps, written mA. A red LED wants about 20 mA. The pushbutton passes that easily; your finger does not.

Voltage is the push. A battery uses a chemical reaction to pile up extra electrons on one terminal and leave the other short of them, and the difference between the two is the voltage, measured in volts. Nine volts means each bit of charge that makes the trip from one terminal to the other gives up nine joules per coulomb of energy along the way, in the resistor as heat and in the LED as light.

The water analogy is the one every shop teacher reaches for, and it is good: voltage is pressure, current is the flow in litres per second, and a thin pipe is resistance. A pump is the battery. It gets you through the first half of this course.

Here is where it breaks. Cut a pipe and water pours out. Cut a wire and nothing pours out, because charge cannot leave the metal; the electrons just stop drifting. And a battery does not squirt electrons down an empty hose. The wire is already full of them, end to end, so when you close the switch the whole column starts moving at once. That is why the LED lights the instant you press the button, even though any one electron crawls along at less than a millimetre a second.

One more habit to pick up now. We draw current flowing from + to −, the way it was guessed before anyone knew about electrons. The electrons actually go the other way. Both descriptions give the same answers, so the shop uses the old arrow and does not lose sleep over it.

Concept: Voltage is the push (energy per charge, measured in volts); current is the flow of charge (measured in amps). Charge does not leave a cut wire the way water leaves a cut pipe; the wire is already full of charge and the whole column moves together.

### the-loop — The loop (Charge and the loop)
Current only flows around a complete loop. From the battery +, through every part in turn, and back to the battery −. Break the loop anywhere and the whole thing stops, not just the part after the break. The battery in experiment 1 is connected to the LED the whole time, and the LED stays dark until the button closes the gap.

That is what a switch is: a gap you can open and close. The pushbutton closes while you hold it. The slide switch stays where you put it. Neither one does anything clever; each is a bit of metal that either touches or does not.

A short circuit is a loop with nothing in it to slow the current down. Wire the battery + straight to the battery − and the full nine volts pushes through almost zero ohms. In a real kit the wire gets hot and the battery goes flat in minutes. Every resistor and lamp in a loop is there partly to keep that from happening.

Put two switches in the same loop, one after the other, and both have to be closed before anything flows. That is series. Give each switch its own path around the other and either one will do. That is parallel. Computers are built from millions of exactly this: series is AND, parallel is OR.

Get in the habit of tracing the loop with a finger before you wire it. Start at +, name each spring you pass through, and make sure you arrive at −. If you cannot trace it, the electrons cannot either.

Concept: Current needs a closed loop from + through the parts back to −; a switch is a gap in the loop; switches in series both need to close (AND) while switches in parallel need only one (OR).

### resistance-and-ohms-law — Resistance and Ohm's law (Ohm's law)
Resistance is how hard a part makes the current work to get through. We measure it in ohms, written Ω. Copper wire is a fraction of an ohm. A resistor is a part built to have a known resistance and nothing else: a little carbon or metal film sized so that 330 Ω is 330 Ω, give or take five percent.

Ohm's law ties the three together: volts equal amps times ohms, V = I × R. Turn it around and current is volts divided by ohms, I = V / R. Nine volts across 1 kΩ pushes 9 / 1000 = 0.009 A, which is 9 mA. Across 330 Ω it is about 27 mA. Across 10 kΩ under 1 mA. Same battery, three resistors, three brightnesses. That is the whole of the Ohm's law recipe.

When an LED sits in the loop it takes about 1.9 V for itself before the resistor sees the rest, so experiment 1 runs at (9 − 1.9) / 330, about 21 mA. You will do that subtraction a lot. It is the only wrinkle in an otherwise one-line law.

The stripes tell you the value. The first two bands are digits, the third is how many zeros to add, and the fourth is the tolerance. Black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Orange orange brown is 3, 3, one zero: 330 Ω. Brown black red is 1, 0, two zeros: 1 kΩ. Brown black orange is 10 kΩ, brown black yellow is 100 kΩ. A gold fourth band means five percent, which is why the kit's second 330 Ω actually measures 336.

A resistor turns the energy it takes out of the current into heat, and the power is volts times amps, P = V × I. The kit's resistors are rated for a quarter of a watt. Put the 100 Ω straight across 9 V: 90 mA, and 9 × 0.09 is 0.81 W. That is more than three times what the part can shed, so it smokes. The rating is a promise about heat, and the smoke test breaks it on purpose so you know what it smells like.

Concept: Ohm's law: I = V / R (volts divided by ohms gives amps). A resistor's colour bands give its value; its power rating in watts is the heat it can shed, and P = V × I above that rating burns it.

### series-and-parallel — Series and parallel (Ohm's law)
Two parts in series sit one after the other in the same loop, so the same current passes through both. There is nowhere else for it to go. The battery's volts get shared out between them, and the resistances simply add: 330 Ω after 1 kΩ is 1330 Ω.

That is why two LEDs in series through one 1 kΩ glow dimmer than one did. Each LED takes about 1.9 V, so the resistor is left with 9 − 3.8 = 5.2 V and passes about 5 mA instead of 7. Batteries in series work the same way in reverse: the second battery's + sits on top of the first's − and the loop sees 18 V. Twice the volts through the same ohms is twice the current, which is why experiment 11 uses the 1 kΩ and not the 330 Ω.

Two parts in parallel sit side by side between the same two points, so each sees the full voltage and each draws its own current. The currents add up at the battery. Two LEDs each on their own 330 Ω both run at full brightness, and the battery supplies about 42 mA instead of 21.

Resistors in parallel let more current through together than either would alone, so the combined resistance is lower than the smallest one: two 330 Ω side by side act like 165 Ω. Capacitors are the odd ones out. Side by side they add their farads, because you have simply made a bigger bucket. Experiment 14 uses that to drop a tone by a third.

Do not share one resistor between two parallel LEDs. It looks thrifty, but the two LEDs then split one current between them, and since no two LEDs are quite alike the one with the slightly lower forward voltage hogs most of it. One resistor per LED, every time.

Concept: In series the same current flows through each part and voltages add (resistances add). In parallel each part sees the same voltage and currents add (combined resistance drops). Batteries in series add volts; capacitors in parallel add farads.

### capacitors-and-time — Capacitors and time (Parts that do something)
A capacitor is two metal plates very close together with an insulator between. Push charge onto one plate and it pulls the opposite charge onto the other, but nothing crosses the gap. It is a bucket for charge. Capacitance, in farads, is the size of the bucket: how much charge it holds per volt. A farad is enormous, so the kit's parts are in microfarads, µF. The 100 µF is a million times smaller than a farad and still the biggest bucket on the board.

Filling a bucket takes time, and the resistor in front of it is the width of the hose. Multiply the ohms by the farads and you get seconds: that is the time constant, R × C. In one time constant the capacitor charges to about 63 percent of the way, and it is essentially full after four or five. 10 kΩ × 100 µF is one second. 1 kΩ × 470 µF is about half a second. 10 kΩ × 0.1 µF is one thousandth of a second.

That one line explains half the kit. Experiment 9 fades an LED up over a couple of seconds because it charges 100 µF through 10 kΩ. Experiment 12 keeps an LED glowing for half a second after you let go, because the 470 µF empties out through 1 kΩ and the LED. Swap in 470 Ω and it empties twice as fast. Swap a 100 µF for a 0.1 µF and something that blinked now sings.

Once a capacitor is full, no more current flows into it. So a capacitor passes changes and blocks anything steady. That is why it can pass a click or a tone from one transistor to the next while keeping their steady voltages apart, and it is the trick behind every oscillator in Unit 4.

The big ones, 10 µF and up, are electrolytic and have a + and a − leg. They only work one way round. Wire one backwards on a real bench and it heats, swells, and can pop. The little 0.1 µF ceramics do not care which way they go.

Concept: A capacitor stores charge (farads = charge per volt). Charging through a resistor takes time set by R × C, about 63% per time constant. Capacitors pass changes and block steady current; electrolytics have polarity.

### diodes-and-leds — Diodes and LEDs (Parts that do something)
A diode is a one-way valve for current. Push the right way, from anode to cathode, and it conducts. Push the wrong way and it is a wall. Nothing moves, so nothing lights. Inside is a junction between two flavours of silicon, and the junction only lets charge across in one direction.

A diode is not a resistor. It does not take a share of the volts in proportion to the current. It takes a fixed bite, its forward voltage, and then lets almost anything through. For an ordinary silicon diode that bite is about 0.6 or 0.7 V. For the kit's red LED it is about 1.9 V, and for the green about 2 V. Above that the current climbs very steeply.

That steepness is why an LED must never go straight across a battery. Nine volts onto a part that only knows how to hold 1.9 leaves seven volts with nowhere to go, the current runs away, and the LED burns in a blink. The resistor in series soaks up the difference and sets the current: (9 − 1.9) / 330 Ω is about 21 mA, a comfortable glow. Brightness follows current, not volts, so the resistor is the dimmer.

The LED's light comes from the junction itself: electrons crossing it drop to a lower energy and the difference leaves as a photon. The colour is set by the semiconductor, not by the plastic, and a higher energy colour needs a bigger forward voltage. That is why the green takes a little more than the red.

On a real LED the longer leg is the anode, and the flat edge on the rim marks the cathode. On the board the springs are labelled A and K. Wire it backwards and the LED simply stays dark; it is not damaged, it is just holding the door shut.

Concept: A diode conducts one way only and takes a fixed forward voltage (about 1.9 V for a red LED) rather than a proportional share; an LED needs a series resistor to set its current, and reversed it stays dark but undamaged.

### transistors — Transistors as switches and amplifiers (Parts that do something)
The transistor is the part that made everything else possible. The kit's Q1 and Q2 are NPN transistors with three legs: base, collector, and emitter. A small current into the base lets a much larger current flow from collector to emitter. Take the base current away and the collector current stops. A little current controls a big one.

How much bigger is a number called beta, or hFE, and for the kit's transistors it is around 100. In the Tiny current recipe the 100 kΩ lets about 83 µA into the base, and the LED lights with about 8 mA: a hundred times more. Hover both and check the ratio yourself.

The base to emitter path behaves like a diode, and it needs about 0.6 or 0.7 V before it opens. Below that the transistor is off. That is why there is always a resistor in front of the base: without it the base would be an LED across a battery all over again.

Used as a switch, you give the base either nothing or plenty. Nothing, and the transistor is an open gap. Plenty, and it is closed as far as it can go, saturated, with the collector current set by the load and not by the base at all. The night light in Unit 4 works this way: the photocell decides whether the base gets enough, and the LED is either on or off.

Used as an amplifier, you keep the base in between, where the collector current follows the base current up and down. Experiment 8 does this: the pot sets a voltage, a tiny current trickles through 100 kΩ into the base, and the LED dims smoothly as the collector current follows. The knob never carries the LED's current. It only steers it. Every volume control, motor controller, and radio in the world is some version of this.

Concept: An NPN transistor lets a small base current control a collector current about beta (around 100) times larger; with the base fully on or off it is a switch, and in between it is an amplifier where the output follows the input.

### oscillators — Oscillators (Circuits that move)
Everything so far sits still once you wire it. An oscillator does not. It flips back and forth on its own, and the kit's flasher, tone, siren, metronome, and theremin are all the same oscillator with different parts in it.

It is called an astable multivibrator, and it is two transistors that take turns. When Q1 switches on, its collector drops, and a capacitor carries that drop to Q2's base and turns Q2 off. Q2's capacitor then slowly charges back up through a 10 kΩ until Q2's base reaches 0.7 V and Q2 switches on. That drops its collector, the other capacitor kicks Q1 off, and the whole thing flips the other way. Neither side can stay put, so it never stops.

Each half of the cycle lasts about 0.7 × R × C, so the whole period is about 1.4 × R × C. With 10 kΩ and 100 µF that is about 1.4 seconds per cycle, a blink of 0.7 Hz: the flasher. With 10 kΩ and 0.1 µF the capacitors are a thousand times smaller, so it flips a thousand times faster: about 700 Hz, too fast to see, so we put a speaker on it and listen. The flasher and the tone are the same circuit.

A speaker only makes sound when the current through it changes. A steady 27 mA is silent; the jump from zero to 27 mA is a click. Seven hundred jumps a second is a tone, and this circuit gives a square wave: fully on, fully off. That is why it sounds a little buzzy.

Change the R or the C in either half and the pitch follows. The siren knob puts a 100 kΩ pot in series with one 10 kΩ: more ohms, slower charge, lower note. Experiment 14 adds a second 0.1 µF beside one timing capacitor for a lower second tone. Experiment 5 swaps a timing resistor for the photocell, and light plays the tune.

Concept: The two-transistor astable multivibrator flips because each transistor turns the other off through a capacitor that recharges through a resistor; period ≈ 1.4 × R × C, so smaller C or R means a faster flip (a flasher at 100 µF, a 700 Hz tone at 0.1 µF).

### sensors — Sensors (Circuits that move)
A sensor is something physical turned into ohms, or volts, so a circuit can act on it. The kit has two. The pot turns a position into ohms: from one end to the wiper you get anywhere from 0 to 10 kΩ depending where the knob sits. The CdS photocell turns light into ohms: about 1 kΩ in bright light and hundreds of kΩ in the dark.

The simplest use is to put the sensor straight in the loop as a variable resistor. The Light meter recipe is the photocell in series with an LED: more light, fewer ohms, more current, brighter glow. It is a dimmer that light turns. Experiment 2 is the same idea with the pot.

The second use is to put the sensor where a timing resistor was. In experiment 5 the photocell replaces one of the tone circuit's 10 kΩ, so the light level sets how fast one half of the cycle charges, and the pitch follows your hand.

The most useful trick is the voltage divider. Two resistances in a row split the battery voltage in proportion: a big one on top and a small one below puts most of the volts across the big one, and the point between them sits low. Swap their sizes and it sits high. The Voltage divider recipe shows the pot doing exactly this, sliding its wiper from 9 V to 0.

The night light is a divider with a transistor listening at the middle. The 100 kΩ pulls Q1's base up; the photocell pulls it down. In light the photocell is about 1 kΩ, it wins easily, the base sits near 0 V and Q1 stays off. In the dark the photocell lets go, the base rises past 0.7 V, and Q1 switches the LED on. A sensor, a divider, and a transistor is a complete automatic device, and most of the ones in your house are this with more decoration.

Concept: A sensor turns a physical quantity into resistance (photocell: about 1 kΩ bright, hundreds of kΩ dark). In a voltage divider the sensor's share of the volts changes with it, and a transistor at the midpoint turns that into a switch or a control.

### reading-schematics — Reading schematics (The shop)
A schematic is a map of the loops, not a picture of the board. It shows which terminal joins which, and nothing about where the parts sit or how long the wires are. The same schematic can be wired a dozen ways on the springs and every one of them is the same circuit. That is why the teacher grades the outcome and not your wiring.

Learn the symbols and you can read any manual from 1978 or today. Battery: a stack of long and short lines, long is +. Resistor: a zigzag. Capacitor: two parallel plates; if one plate is curved the part is electrolytic and the curved plate is −. LED: a triangle pointing the way current flows, a bar across the point for the cathode, two little arrows for the light. Switch: a gap with a hinged bar. Speaker: a box with a cone. Photocell: a resistor in a circle with light arrows landing on it. Pot: a resistor with an arrow for the wiper.

The transistor is a circle with three legs. The flat bar inside is the base. The leg with the arrow is the emitter, and the arrow points out for NPN, the way current leaves. The remaining leg is the collector.

Wires that meet are joined; in these drawings a corner or a junction is a connection. Two lines that merely cross without joining are not. Labels beside each symbol match the labels printed on the board, so 330 Ω on paper is the spring pair marked 330 Ω in front of you.

Read a schematic the way you trace a loop: find the battery, put your finger on +, and follow the wire through each part to −. Then look for the side loops. In the flasher you will find two of them, one per transistor, mirror images of each other. Once you see that, the whole circuit is two copies of one idea.

Concept: A schematic shows connections, not positions: the same schematic wires many ways. Symbols carry meaning (curved capacitor plate is the electrolytic's −, LED triangle points the way current flows, NPN arrow points out of the emitter), and reading starts at the battery + and follows the loop.

### measuring — Measuring (The shop)
On this bench you hover a part and read its volts and milliamps off the card. A real meter makes you choose, and the choice teaches you something.

Voltage is a difference between two points, so a voltmeter goes across a part: one probe on each end, in parallel with it, while the circuit runs. A voltmeter has a very high resistance, millions of ohms, so it takes almost no current and does not disturb the loop. Clip it across the LED in experiment 1 and hold the button: about 1.9 V.

Current is a flow through a point, so an ammeter goes in the loop: you break the wire and let the current pass through the meter on its way. An ammeter has almost no resistance, so it does not slow the current it is counting. That is exactly why you must never clip it across a part like a voltmeter: a near-zero-ohm meter across a battery is a short circuit, and the meter's fuse, if it has one, is the only thing that goes.

An ohmmeter puts a little current of its own through a part and works out the resistance. Use it on a part that is out of the circuit and powered down. Measure a resistor while the battery is still driving current through it and you get nonsense, or a dead meter.

Two rules let you check any reading. Go once around a loop and the volts across the parts add up to the battery's volts: in the Ohm's law recipe about 7.1 V across the 1 kΩ plus 1.9 V across the LED is 9 V. And current going into a junction equals current coming out: in experiment 7 the battery's 42 mA is two 21 mA branches rejoining. If your readings do not obey those two, one of the readings is wrong.

Concept: A voltmeter goes across a part (parallel, very high resistance); an ammeter goes in the loop (series, very low resistance) and must never be clipped across a part. Voltages around a loop add to the source voltage; currents into a junction equal currents out.

### safety-and-real-parts — Safety and real parts (The shop)
You can hold a 9 V battery's terminals in your fingers all day. Dry skin is around 100 kΩ or more, so 9 V pushes well under a tenth of a milliamp through you, far below anything you can feel. Touch it to your tongue and you will feel it, because a wet tongue is a few kΩ, but that is a tingle, not a danger.

Mains is different in kind, not just degree. A wall outlet is 120 or 240 V, and a few tens of milliamps across the chest is enough to stop a heart. Ohm's law says 240 V through even 10 kΩ of damp skin is 24 mA. Never open, probe, or wire anything that plugs into the wall while it is plugged in, and until you have been taught how, do not work on the mains side at all. The low-voltage side of a wall adapter is fine. The adapter's insides are not.

Heat is the everyday hazard. A quarter-watt resistor asked for a watt gets hot enough to burn a fingertip in seconds before it smokes. A shorted 9 V battery gets hot and goes flat; a shorted lithium cell can catch fire. When something in a real circuit is hotter than it should be, take the battery out first and think second.

Polarity matters on three parts: the battery, every LED, and every electrolytic capacitor. The LED backwards is harmless. The electrolytic backwards heats, swells, and can burst, so check the stripe before you connect it. The battery backwards into a circuit with transistors and electrolytics can wreck several parts at once.

Real parts are not the numbers printed on them. Five percent tolerance means the kit's second 330 Ω is really 336, the second 10 kΩ is 9.7, and a 100 µF can be anywhere from 80 to 120. Circuits that work are designed so that does not matter, which is one reason the flasher blinks at about, not exactly, 0.7 Hz. When you build for real: solder in a ventilated room, keep water off the bench, and wear eye protection when you clip leads. Springs and 9 V forgive nearly everything. The real world forgives a little less.

Concept: Danger is set by current through the body, not volts alone: 9 V through skin is under a tenth of a milliamp and safe, mains at 120–240 V can push tens of milliamps across the chest and is not. Heat, polarity (electrolytics, LEDs, battery), and part tolerance are the real-world details the simulator only hints at.

## The schematic drawing DSL
A schematic in `kit/schematics.ts` is symbols placed on a canvas plus wires drawn as polylines between them, not a picture of the board. Each symbol names a part kind, an id, a position, and an optional rotation (0/90/180/270°) and flip; terminal positions (a battery's +, a transistor's base/collector/emitter, a relay's coil/NC/NO/COM) are computed from the kind and that rotation, so a wire is authored between named terminals, never magic coordinates. Parts an activity pulls from the drawer (not yet on the board) are drawn from the same `x-<ref>` id used in `wiringRefs.ts` and carry an explicit label, since they have no board-assigned one yet.

## Pin names
Every wire endpoint in a circuit document is `"<part name>.<pin>"`. The pin names are the tags printed
beside the springs, lower-cased, with a 1-based ordinal where a kind repeats a tag and `a`/`b` for an
untagged two-terminal part. A bare spring number works too.

| kind | pins |
| --- | --- |
| `battery` | `+` `-` |
| `resistor` | `a` `b` |
| `pot` | `1` `w` `2` |
| `capacitor` | `+` `-` |
| `led` | `a` `k` |
| `npn` | `c` `b` `e` |
| `switch` | `a` `b` |
| `speaker` | `a` `b` |
| `photocell` | `a` `b` |
| `diode` | `a` `k` |
| `buzzer` | `+` `-` |
| `relay` | `coil1` `coil2` `com` `no` `nc` |
| `timer555` | `v+` `trig` `thr` `g` `dis` `out` |
| `gate` | `a` `b` `y` `v+` `g` |
| `sevenseg` | `d0` `d1` `d2` `d3` `v+` `g` |
| `counter` | `clk` `rst` `v+` `g` `q0` `q1` `q2` `q3` |