First Light

What you should see

The LED glows dimmer through 1 kΩ than through 330 Ω. Swap in 10 kΩ and it barely shows.

Parts

9 V battery, 1 kΩ resistor, red LED

How it works

Current is volts divided by ohms. The same nine volts through 330 Ω gives about 21 mA, through 1 kΩ about 7 mA, and through 10 kΩ under 1 mA, and an LED's brightness follows its current. Three resistors, three glows, one line of arithmetic.

Wiring

1. 1 → 9
2. 10 → 28
3. 29 → 2

Open it on the bench: ohmsville.com/bench#a=ohms-law

Learn more: Classroom, Resistance and Ohm's law, paragraph 2 — ohmsville.com/classroom#resistance-and-ohms-law-2

What you should see

Put 100 Ω straight across 9 V and watch it burn in a blink.

Parts

9 V battery, 100 Ω resistor

How it works

A resistor turns the energy it takes out of the current into heat, and it can only shed so much: this one is rated for a quarter of a watt. Nine volts across 100 Ω is 90 mA, and 9 times 0.09 is 0.81 W, more than three times what the part can get rid of. So it cooks, which is what a power rating is a promise about.

Wiring

1. 1 → 3
2. 4 → 2

Open it on the bench: ohmsville.com/bench#a=smoke-test

Learn more: Classroom, Resistance and Ohm's law, paragraph 5 — ohmsville.com/classroom#resistance-and-ohms-law-5

What you should see

Under a tenth of a milliamp into the base lights the LED with about 8 mA.

Parts

9 V battery, 100 kΩ, Q1, 330 Ω, red LED

How it works

The 100 kΩ lets only about 83 microamps into the transistor's base. That tiny current opens the path from collector to emitter for about 8 mA, roughly a hundred times more, and that is what lights the LED. The ratio is the transistor's gain, called beta, and it is why a small signal can drive a big load.

Wiring

1. 1 → 15
2. 16 → 33
3. 1 → 5
4. 6 → 28
5. 29 → 32
6. 34 → 2

Open it on the bench: ohmsville.com/bench#a=tiny-current

Learn more: Classroom, Transistors as switches and amplifiers, paragraph 2 — ohmsville.com/classroom#transistors-2

What you should see

Hover spring 18 and turn the knob: the voltage slides from 9 V down to 0.

Parts

9 V battery, 10 kΩ pot

How it works

Two resistances in a row share the battery's volts in proportion to their size, and the wiper is the point between them. Turn the knob toward one end and most of the pot is below the wiper, so the wiper sits near nine volts; turn it the other way and it sits near zero. That sliding voltage is what every volume knob and sensor readout starts from.

Wiring

1. 1 → 17
2. 19 → 2

Open it on the bench: ohmsville.com/bench#a=divider

Learn more: Classroom, Sensors, paragraph 4 — ohmsville.com/classroom#sensors-4

What you should see

The LED lights only when the slide is ON and the button is held.

Parts

9 V battery, pushbutton, slide switch, 330 Ω, red LED

How it works

Both switches are in the same loop, one after the other, so the current has to get through both. Open either one and the loop is broken and the LED is dark. Series is AND: this one and that one.

Wiring

1. 1 → 38
2. 39 → 40
3. 41 → 5
4. 6 → 28
5. 29 → 2

Open it on the bench: ohmsville.com/bench#a=series-switches

Learn more: Classroom, The loop, paragraph 4 — ohmsville.com/classroom#the-loop-4

What you should see

Either the slide or the button lights the LED.

Parts

9 V battery, pushbutton, slide switch, 330 Ω, red LED

How it works

Each switch is its own path around the other, so closing either one completes a loop to the LED. It does not matter which, and closing both changes nothing. Parallel is OR: this one or that one.

Wiring

1. 1 → 38
2. 1 → 40
3. 39 → 5
4. 41 → 5
5. 6 → 28
6. 29 → 2

Open it on the bench: ohmsville.com/bench#a=parallel-switches

Learn more: Classroom, The loop, paragraph 4 — ohmsville.com/classroom#the-loop-4

What you should see

The LED glows with the light slider toward the sun and goes dark toward the moon; or turn on the camera and cover it.

Parts

9 V battery, CdS photocell, 330 Ω, red LED

How it works

The photocell is a resistor whose ohms are set by light: about 1 kΩ in bright light, hundreds of thousands in the dark. It sits in the LED's loop like any other resistor, so more light means fewer ohms, more current and a brighter glow. It is a dimmer that light turns instead of a hand.

Wiring

1. 1 → 44
2. 45 → 5
3. 6 → 28
4. 29 → 2

Open it on the bench: ohmsville.com/bench#a=light-meter

Learn more: Classroom, Sensors, paragraph 2 — ohmsville.com/classroom#sensors-2

What you should see

Slide the light under the CdS cell toward the moon: the LED comes on in the dark and goes off in the light.

Parts

9 V battery, 100 kΩ, CdS photocell, Q1, 330 Ω, red LED

How it works

The 100 kΩ pulls the transistor's base up toward nine volts and the photocell pulls it down toward zero, and whichever has fewer ohms wins. In the light the cell is about 1 kΩ and wins easily, so the base sits near zero and the transistor stays off. In the dark the cell lets go, the base rises past 0.7 V, and the transistor switches the LED on.

Wiring

1. 1 → 15
2. 16 → 33
3. 33 → 44
4. 45 → 2
5. 1 → 5
6. 6 → 28
7. 29 → 32
8. 34 → 2

Open it on the bench: ohmsville.com/bench#a=night-light

Learn more: Classroom, Sensors, paragraph 5 — ohmsville.com/classroom#sensors-5

What you should see

Turn the speaker on, then tap the button: a click on press and another on release.

Parts

9 V battery, pushbutton, 330 Ω, speaker

How it works

A speaker only makes a sound when the current through it changes. A steady 27 mA is silent; the jump from nothing to 27 mA when you press is a click, and the jump back when you let go is another. Make that jump seven hundred times a second and the clicks blur into a tone.

Wiring

1. 1 → 38
2. 39 → 5
3. 6 → 42
4. 43 → 2

Open it on the bench: ohmsville.com/bench#a=click

Learn more: Classroom, Oscillators, paragraph 4 — ohmsville.com/classroom#oscillators-4

What you should see

Hold the button and the lamp lights — and watch the current: it starts near 520 mA and settles at 63 mA.

Parts

9 V battery, pushbutton, lamp from the drawer

How it works

A lamp's filament is a resistor, but not a fixed one: cold tungsten is about 15 Ω and hot tungsten about 142 Ω. The instant the loop closes the cold filament pulls about 520 mA, then it heats in a few hundredths of a second and settles near 63 mA. A resistor's value can depend on how hot it is, and this is one you can watch change.

Wiring

1. 1 → 38
2. 39 → 46
3. 47 → 2

Open it on the bench: ohmsville.com/bench#a=light-the-lamp

Learn more: Classroom, Resistance and Ohm's law, paragraph 1 — ohmsville.com/classroom#resistance-and-ohms-law-1

What you should see

The lamp burns steadily until you press the key, and goes out for as long as you hold it.

Parts

9 V battery, 330 Ω, 10 kΩ, red LED, Q1, pushbutton

How it works

The 10 kΩ feeds the transistor’s base all the time, so at rest it is switched hard on and the lamp is lit. The key is wired straight across base and emitter: press it and the base is shorted to the minus rail, the 10 kΩ’s current drains to ground instead, and the transistor shuts. Input up, output down, and one transistor is a NOT gate.

Wiring

1. 1 → 5
2. 6 → 28
3. 29 → 32
4. 34 → 2
5. 1 → 11
6. 12 → 33
7. 33 → 38
8. 39 → 2

Open it on the bench: ohmsville.com/bench#a=heritage-backwards-lamp

Learn more: Classroom, Transistors as switches and amplifiers, paragraph 4 — ohmsville.com/classroom#transistors-4

What you should see

With the loop closed the speaker says nothing. Open the loop and it howls until you close it again.

Parts

9 V battery, two 330 Ω, two 10 kΩ, two 0.1 µF, Q1, Q2, speaker, slide switch

Slide switch ON.

How it works

The first fourteen wires are the ordinary two-transistor tone circuit. The slide switch, standing in for a loop of wire around whatever you are guarding, pins one transistor’s base to the minus rail while it is closed, and a base that cannot rise past 0.7 V can never switch, so the whole oscillator is held still. Open the loop and the base is free; the circuit starts itself and the speaker howls.

Wiring

1. 1 → 5
2. 6 → 32
3. 1 → 7
4. 8 → 42
5. 43 → 35
6. 1 → 11
7. 12 → 33
8. 1 → 13
9. 14 → 36
10. 32 → 24
11. 25 → 36
12. 35 → 26

13. 27 → 33
14. 34 → 2
15. 37 → 2
16. 36 → 40
17. 41 → 2

Open it on the bench: ohmsville.com/bench#a=heritage-loop-alarm

Learn more: Classroom, Transistors as switches and amplifiers, paragraph 3 — ohmsville.com/classroom#transistors-3

What you should see

The lamp is on when you power up. Tap the key once and it goes out — and stays out, however long you wait.

Parts

9 V battery, 330 Ω, two 10 kΩ, 100 kΩ, red LED, Q1, Q2, pushbutton

How it works

Two transistors feed each other’s bases through resistors, with no capacitor to time anything, so the pair simply stays in whichever state it is in. At switch-on the first transistor wins and holds the second off, which is what keeps the first on. The key pulls the first base to ground; the second transistor takes over and holds the first down by itself, so releasing the key changes nothing.

Wiring

1. 1 → 5
2. 6 → 28
3. 29 → 32
4. 34 → 2
5. 1 → 11
6. 12 → 35
7. 37 → 2
8. 32 → 15
9. 16 → 36
10. 35 → 13
11. 14 → 33
12. 33 → 38

13. 39 → 2

Open it on the bench: ohmsville.com/bench#a=heritage-sulks

Learn more: Classroom, Transistors as switches and amplifiers, paragraph 4 — ohmsville.com/classroom#transistors-4

What you should see

The lamp comes on when you light the cell and goes out when you shade it — the opposite way round from the night light.

Parts

9 V battery, CdS cell, 10 kΩ, 330 Ω, red LED, Q1

How it works

It is the night light with the two halves of the divider swapped: the photocell pulls the base up from the plus rail and a 10 kΩ pulls it down. In bright light the cell is about a kilohm and beats the 10 kΩ, so the base rises past 0.7 V and the lamp comes on; in the dark the cell is hundreds of kilohms, the 10 kΩ wins, and the lamp is out. Which side of the divider the sensor sits on decides which way round the whole circuit works.

Wiring

1. 1 → 44
2. 45 → 33
3. 33 → 11
4. 12 → 2
5. 1 → 5
6. 6 → 28
7. 29 → 32
8. 34 → 2

Open it on the bench: ohmsville.com/bench#a=heritage-daylight-switch

Learn more: Classroom, Sensors, paragraph 4 — ohmsville.com/classroom#sensors-4

What you should see

The lamp winks about one and a half times a second, and the speaker pops on the same beat.

Parts

9 V battery, two 330 Ω, 10 kΩ, 1 kΩ, two 100 µF, red LED, speaker, Q1, Q2

How it works

A two-transistor flip-flop with the big 100 µF capacitors, so it flips about one and a half times a second. The LED hangs on one transistor’s collector and the speaker on the other’s, and since the two are always in opposite states, every flip does both jobs at once: the lamp dies as the speaker current jumps. A jump in speaker current is a pop, so you hear every flip you see.

Wiring

1. 1 → 5
2. 6 → 28
3. 29 → 32
4. 1 → 7
5. 8 → 42
6. 43 → 35
7. 1 → 11
8. 12 → 33
9. 1 → 9
10. 10 → 36
11. 32 → 20
12. 21 → 36

13. 35 → 22
14. 23 → 33
15. 34 → 2
16. 37 → 2

Open it on the bench: ohmsville.com/bench#a=heritage-popping-lamp

Learn more: Classroom, Oscillators, paragraph 4 — ohmsville.com/classroom#oscillators-4

What you should see

Slide the switch and the tone jumps to a second note about an octave up, and stays there.

Parts

9 V battery, two 330 Ω, 1 kΩ, two 10 kΩ, two 0.1 µF, Q1, Q2, speaker, slide switch

How it works

One transistor’s base is fed through a 1 kΩ and a 10 kΩ in a row, and the slide switch is wired straight across the 10 kΩ. Close it and the current takes the wire instead of the resistor, so that half of the cycle refills through 1 kΩ alone and takes a fraction of the time. The other half never changes, which is why the note jumps about an octave rather than elevenfold.

Wiring

1. 1 → 5
2. 6 → 32
3. 1 → 7
4. 8 → 42
5. 43 → 35
6. 1 → 9
7. 10 → 11
8. 12 → 33
9. 11 → 40
10. 41 → 12
11. 1 → 13
12. 14 → 36

13. 32 → 24
14. 25 → 36
15. 35 → 26
16. 27 → 33
17. 34 → 2
18. 37 → 2

Open it on the bench: ohmsville.com/bench#a=heritage-two-notes

Learn more: Classroom, Oscillators, paragraph 5 — ohmsville.com/classroom#oscillators-5