Relay Logic

What you should see

The LED lights, with about 0.7 V across the 1N4001. Turn the diode round and all is dark.

Parts

9 V battery, 1N4001 from the drawer, 330 Ω, red LED

How it works

A diode lets current through one way and blocks it the other, like a valve. Forwards it takes about 0.7 V for itself and passes the rest along, which is why the LED is a shade dimmer than it would be without it. Turn it round and nothing flows at all.

Wiring

1. 1 → 46
2. 47 → 5
3. 6 → 28
4. 29 → 2

Open it on the bench: ohmsville.com/bench#a=one-way-street

Learn more: Classroom, Diodes and LEDs, paragraph 1 — ohmsville.com/classroom#diodes-and-leds-1

What you should see

Hold the button and the buzzer sounds; let go and it stops.

Parts

9 V battery, pushbutton, buzzer from the drawer

How it works

The buzzer has its own oscillator inside, so all it needs is nine volts the right way round and a closed loop to get them. The button is that loop's gap. Anything that can close a gap, a finger, a transistor, a relay contact, can sound an alarm.

Wiring

1. 1 → 38
2. 39 → 46
3. 47 → 2

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

Learn more: Classroom, The loop, paragraph 2 — ohmsville.com/classroom#the-loop-2

What you should see

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.

Parts

9 V battery, slide switch, 100 kΩ, CdS cell, Q1, Q2, 2 × relay and 2 × 1N4001 from the drawer, buzzer from the drawer

Slide switch ON.

How it works

Two relays, neither of them clever. The first is the dark alarm and gives the second a single push when the shadow crosses; the second's own contact is in parallel with that push, so once it pulls in it feeds itself and stays in. The shadow can leave and it makes no difference, because the thing holding the buzzer on is now the relay itself.

Wiring

1. 1 → 15
2. 16 → 33
3. 33 → 44
4. 45 → 2
5. 32 → 1
6. 34 → 36
7. 1 → 46
8. 47 → 35
9. 37 → 2
10. 51 → 47
11. 52 → 1
12. 1 → 40

13. 41 → 48
14. 41 → 55
15. 49 → 53
16. 56 → 53
17. 54 → 2
18. 58 → 2
19. 59 → 53
20. 53 → 60
21. 61 → 2

Open it on the bench: ohmsville.com/bench#a=ghost-in-the-wires

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

What you should see

Hold the button: the propeller winds up over about a tenth of a second and settles at 2 900 turns a minute, drawing 169 mA. Put the bare-shaft motor in instead and the same battery gives 4 100 turns and 20 mA.

Parts

9 V battery, pushbutton, motor with a propeller from the drawer

How it works

A motor that is turning is also a generator, and the voltage it makes opposes the battery. With nothing on the shaft it winds up until the two nearly cancel and there is hardly any current left over — twenty milliamps. Bolt a propeller on and the air fights it, so it settles slower, the cancelling is less complete, and eight times as much current flows. The load is not something extra you add to a motor; the load is what the current was for.

Wiring

1. 1 → 38
2. 39 → 46
3. 47 → 2

Open it on the bench: ohmsville.com/bench#a=what-the-propeller-costs

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

What you should see

Slide it on and the whole motor shivers instead of driving anything. It takes 20 mA — the same as the bare shaft — because a lump of brass on the end of a shaft is almost no load at all.

Parts

9 V battery, slide switch, motor with a weight from the drawer

Slide switch ON.

How it works

A weight bolted off-centre spends the whole rotation trying to fly outwards, and that force goes as the square of the speed: three grams of brass six millimetres off the shaft pulls about three and a half newtons at four thousand turns a minute, the same as hanging a third of a kilogram on it. The pull sweeps round with the shaft, so the motor and whatever it is screwed to shake rather than move. Turning a small weight costs almost nothing — twenty milliamps, the same as a bare shaft — which is how a pager did it on a watch battery.

Wiring

1. 1 → 40
2. 41 → 46
3. 47 → 2

Open it on the bench: ohmsville.com/bench#a=pager-in-your-pocket

Learn more: Classroom, The loop, paragraph 2 — ohmsville.com/classroom#the-loop-2

What you should see

Press the button and the motor jumps hard before it settles: the first instant draws far more than the 20 mA it holds once the shaft is turning.

Parts

9 V battery, pushbutton, motor with a weight from the drawer

How it works

A motor that has not started turning yet makes no back-EMF, so at the instant the button closes it looks like bare winding resistance to the battery and current surges. As the shaft spins up, the motor starts generating its own opposing voltage and the current falls, settling once the weight is turning fast enough to want 20 mA and pull 3.4 N outward. The jolt in the seat was the shake settling in, not the surge; the surge only lasted as long as the shaft took to catch up.

Wiring

1. 1 → 38
2. 39 → 46
3. 47 → 2

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

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

What you should see

Touch the plates dry and Q2 settles at 62 mA, well under its 100 mA limit, and the motor winds up. Touch them damp and Q2's current doubles in two solver steps, then it's gone — burnt about 34 microseconds after you touched, at roughly 88 mA.

Parts

touch plates from the drawer, 1 MΩ resistor from the drawer, Q1, Q2, motor with a weight from the drawer

How it works

A 2N2222 is rated for 100 mA of collector current, and the weight motor's running draw is only 20 mA, which looks safe. But a motor pulls its biggest current the instant it starts, not once it is spinning, because a stalled coil has no back EMF fighting it yet. That switch-on surge pushes Q2 past 88 mA in two solver steps and it is dead on the third, about 34 microseconds after the finger lands. Size a transistor for the moment the motor cannot turn yet, not the moment it can.

Wiring

1. 1 → 46
2. 47 → 33
3. 33 → 48
4. 49 → 2
5. 32 → 1
6. 34 → 36
7. 37 → 2
8. 1 → 50
9. 51 → 35

Open it on the bench: ohmsville.com/bench#a=motor-darlington-burns

Learn more: Classroom, Sensors, paragraph 1 — ohmsville.com/classroom#sensors-1

What you should see

Touch the plates, damp or dry, and Q1 never sees more than about 8 mA — nowhere near its 100 mA limit. The motor does not turn. A 10 kΩ resistor makes the transistor safe by making it too weak to move anything.

Parts

touch plates from the drawer, 10 kΩ resistor from the drawer, Q1, motor with a weight from the drawer

How it works

The motor will not turn on less than about 20 mA — that is the current it takes to beat its own bearing drag from a dead stop. This build's base resistor caps Q1 at roughly 7.5 mA even with damp skin on the plates, so the transistor stays cool and the motor stays still: safe and useless are the same 10 kΩ resistor here. Swap it for the Darlington next door and the safety margin disappears with it.

Wiring

1. 1 → 46
2. 47 → 48
3. 49 → 33
4. 1 → 50
5. 51 → 32
6. 34 → 2

Open it on the bench: ohmsville.com/bench#a=motor-base-resistor

Learn more: Classroom, Sensors, paragraph 1 — ohmsville.com/classroom#sensors-1

What you should see

Arm it with the slide. Touch the plates once — dry hands are enough — and the buzzer starts and keeps going after you let go. Only the slide shuts it up.

Parts

touch plates from the drawer, 1 MΩ resistor from the drawer, Q1, Q2, relay and 1N4001 from the drawer, buzzer from the drawer, slide switch

Slide switch ON.

How it works

Dry skin across the plates is about a megohm, which still pushes a few microamps through the 1 MΩ into the first transistor's base. Two transistors in a row multiply that into enough current to pull the relay in, and the relay's own contact then feeds its own coil straight from the battery. It holds itself in after the finger lifts, which is why only the slide can silence it.

Wiring

1. 1 → 46
2. 47 → 33
3. 33 → 48
4. 49 → 2
5. 32 → 1
6. 34 → 36
7. 37 → 2
8. 1 → 40
9. 41 → 50
10. 51 → 35
11. 55 → 51
12. 56 → 41

13. 52 → 51
14. 53 → 2
15. 57 → 41
16. 58 → 51

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

Learn more: Classroom, Sensors, paragraph 1 — ohmsville.com/classroom#sensors-1

What you should see

Touch the plates and the buzzer sounds and the lamp lights, for exactly as long as your finger is on them. Let go and it stops. That is the whole game: the plates are the rim, your finger is the tweezers, and the buzzer is the loop you just closed.

Parts

touch plates from the drawer, 1 MΩ resistor from the drawer, Q1, Q2, relay and 1N4001 from the drawer, buzzer and lamp from the drawer

How it works

The relay is there for the lamp. A cold filament is 15 Ω, so 9 V wants 0.6 A the instant it lights — six times what a 2N2222 will carry — and the relay's contact carries it without complaint while the transistors only have to lift the coil's 18 mA. With the buzzer alone you could leave the relay out and let Q2 drive it; the lamp is what earns the relay its place.

Wiring

1. 1 → 46
2. 47 → 33
3. 33 → 48
4. 49 → 2
5. 32 → 1
6. 34 → 36
7. 37 → 2
8. 1 → 50
9. 51 → 35
10. 55 → 51
11. 56 → 50
12. 57 → 1

13. 58 → 52
14. 59 → 1
15. 60 → 52
16. 53 → 2

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

Learn more: Classroom, Sensors, paragraph 1 — ohmsville.com/classroom#sensors-1

What you should see

Guide the wand's loop along the bent wire from one post to the other without letting the loop's edge touch the wire. Touch it, even for an instant, and the buzzer sounds and the lamp lights, for exactly as long as contact lasts. Reach the far post clean and you win; touch the wire five times and you start over.

Parts

wand loop and bent wire from the drawer, 1 MΩ resistor from the drawer, Q1, Q2, relay and 1N4001 from the drawer, buzzer and lamp from the drawer

How it works

The 1 MΩ leg keeps this game gentle: 9 V through 1 MΩ is only about 9 µA, far below what skin can feel, yet enough to turn on Q1's base and wake the two-transistor amplifier. A relay does not let go the instant contact clears — drop-out takes tens of milliseconds while the coil's field collapses — so a touch shorter than that will not always register, but a wire held against the loop for a heartbeat will. That same collapse keeps the buzzer from sticking on: once the field falls, the contact opens and the loop goes quiet, same as letting go of the touch plates.

Wiring

1. 1 → 46
2. 47 → 33
3. 33 → 48
4. 49 → 2
5. 32 → 1
6. 34 → 36
7. 37 → 2
8. 1 → 50
9. 51 → 35
10. 55 → 51
11. 56 → 50
12. 57 → 1

13. 58 → 52
14. 59 → 1
15. 60 → 52
16. 53 → 2

Open it on the bench: ohmsville.com/bench#a=steady-hand

Learn more: Classroom, Sensors, paragraph 1 — ohmsville.com/classroom#sensors-1

What you should see

Slide it on: the motor takes the frame disc to 4 061 turns a minute and the lamp lights it steadily. Twelve drawings cross the window 812 times a second, and all you see is a grey ring.

Parts

9 V battery, slide switch, motor from the drawer, lamp from the drawer

Slide switch ON.

How it works

The motor free-runs at 4 061 turns a minute — 67.7 turns a second — because a bare shaft is almost no load at all. Twelve drawings on the rim means 812 of them cross the window every second, and the eye holds each one for about a twentieth of a second, so it sums forty of them into a grey ring. A steady lamp cannot show you one frame. Something has to pick.

Wiring

1. 1 → 40
2. 41 → 46
3. 47 → 2
4. 41 → 48
5. 49 → 2

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

Learn more: Classroom, The loop, paragraph 2 — ohmsville.com/classroom#the-loop-2

What you should see

Hold the button: click, and the LED on the relay’s contact comes on. Let go: click, and it goes out. The bite lives in the half millisecond between the two clicks.

Parts

9 V battery, pushbutton, relay and 1N4001 from the drawer, 330 Ω, red LED

How it works

A coil hates having its current cut. Hold the button and 17.96 mA is circling in it; open the button and that current has nowhere to go, so the coil makes 396 V trying to keep it moving. All the energy behind that number is 40 microjoules, which is why it is a snap and not an injury. The diode across the coil is a one-way detour that opens exactly when the coil needs it, and the same release then peaks at 0.73 V: the difference between a gag and a circuit you can build things on.

Wiring

1. 1 → 38
2. 39 → 46
3. 47 → 2
4. 51 → 47
5. 52 → 46
6. 1 → 48
7. 49 → 5
8. 6 → 28
9. 29 → 2

Open it on the bench: ohmsville.com/bench#a=kickback-pen

Learn more: Classroom, Diodes and LEDs, paragraph 1 — ohmsville.com/classroom#diodes-and-leds-1

What you should see

Turn the speaker on: a steady low buzz starts by itself and never stops on its own.

Parts

9 V battery, speaker, relay from the drawer, 1N4001 from the drawer

How it works

The coil only stays pulled in while current is flowing through the very contact its own pull just opened, so the relay can never settle — it is always mid-swing between shutting its circuit and breaking it. Each swing takes a few milliseconds, fast enough that the clicks blur into a tone instead of a ticking. The diode across the coil has nothing to do with the buzz itself; it just gives the collapsing field somewhere to go each time the contact lets go, so the spark doesn't eat the contact after a few thousand cycles.

Wiring

1. 1 → 42
2. 43 → 50
3. 48 → 46
4. 47 → 2
5. 51 → 2
6. 52 → 46

Open it on the bench: ohmsville.com/bench#a=heritage-self-tripped-relay

Learn more: Classroom, The loop, paragraph 1 — ohmsville.com/classroom#the-loop-1

What you should see

Light the LED only when the key and the switch are both on together — either one alone leaves it dark.

Parts

9 V battery, 330 Ω resistor, red LED, 330 Ω pull-up resistor from the drawer, two 1N4001 diodes from the drawer, pushbutton from the drawer (key), slide switch from the drawer (select), two 22 Ω pulldown resistors from the drawer

How it works

Each diode can only pull the output down to match its own input, never up, so a single low input is enough to hold the output low regardless of the other one. The output only rises when neither diode has a reason to conduct, which takes both inputs high at once — the definition of AND, built from parts a lot older than a logic-gate chip.

Wiring

1. 1 → 46
2. 47 → 48
3. 47 → 50
4. 47 → 5
5. 6 → 28
6. 29 → 2
7. 49 → 52
8. 53 → 1
9. 49 → 56
10. 57 → 2
11. 51 → 54
12. 55 → 1

13. 51 → 58
14. 59 → 2

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

Learn more: Classroom, Diodes and LEDs, paragraph 1 — ohmsville.com/classroom#diodes-and-leds-1