Sound & Sense

What you should see

Eighteen volts on a nine-volt lamp. It is beautiful for four milliseconds.

Parts

two 9 V batteries, lamp from the drawer

How it works

Two batteries in series push eighteen volts through a filament made for nine. Twice the volts through the same filament is twice the current and four times the heat, far more than the wire can shed. It gets brighter than it ever should, hotter than it can stand, and opens in a few milliseconds.

Wiring

1. 1 → 49
2. 48 → 46
3. 47 → 2

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

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

What you should see

Flip the slide switch and the lamp lights at nine volts — flip it back and the beam goes dark, nothing else in the loop.

Parts

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

Slide switch ON.

How it works

The switch is the whole interface a flashlight has: closed, the loop runs 9 V through 15 Ω cold and the lamp pulls near 520 mA before settling at 63 mA hot; open, the loop breaks and every one of those milliamps stops at once. Nine volts is what this lamp is rated for — the second battery that pushed it to eighteen and burned it out stays on the shelf.

Wiring

1. 46 → 40
2. 41 → 48
3. 49 → 47

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

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

What you should see

Slide the light to sun and the LED lights. Slide back to a room and it dies — and it is not the LED's fault.

Parts

solar cell from the drawer, red LED

How it works

A battery uses a chemical reaction to pile charge onto one terminal; a solar cell uses light to do the same job. Four cells in a row make about two volts and tens of milliamps in full sun, and almost nothing in an ordinary room. It needs no resistor because the panel can only supply so much current, so it limits itself.

Wiring

1. 46 → 28
2. 29 → 47

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

Learn more: Classroom, What electricity is, paragraph 3 — ohmsville.com/classroom#what-electricity-is-3

What you should see

While there is light the panel holds Q1 on and the LED stays dark. Let the room go dim — past dusk, about 60 lux — and the LED comes on.

Parts

solar cell from the drawer, 10 kΩ, 100 kΩ, 330 Ω, Q1, Q2, red LED

How it works

Here the panel is a sensor, not a supply. Even a little light gives it a few tenths of a volt across the 100 kΩ, enough to hold the first transistor on and the second off, so the LED is dark. Let the room go dim and the panel's voltage falls to nothing, the first transistor lets go, the second takes over, and the LED lights: a divider and a transistor again, with a source doing the sensing.

Wiring

1. 46 → 33
2. 47 → 2
3. 33 → 15
4. 16 → 2
5. 1 → 11
6. 12 → 32
7. 34 → 2
8. 32 → 36
9. 37 → 2
10. 1 → 5
11. 6 → 28
12. 29 → 35

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

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

What you should see

Hover spring 11 and drag the heat slider: 2.06 V at freezing, exactly half the battery at 25 °C, 6.62 V on a hot day.

Parts

thermistor from the drawer, 10 kΩ resistor

How it works

The thermistor is a resistor whose ohms fall as it warms, and it sits above an ordinary 10 kΩ as the top half of a divider. At 25 °C the bead is 10 kΩ too, so the middle sits at exactly half the battery. Colder, the bead's ohms climb and the middle drops; warmer, they fall and the middle rises. A sensor turns heat into ohms, and a divider turns ohms into volts.

Wiring

1. 1 → 46
2. 47 → 11
3. 12 → 2

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

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

What you should see

Take the battery right out of the loop and squeeze the igniter: the red LED flashes. Half a millisecond at 1.2 milliamps, and nothing on the board is powering it but your hand.

Parts

piezo igniter from the drawer, red LED, no battery at all

How it works

Squeeze a piezo and it makes a fixed amount of charge, not a fixed voltage. Where that charge goes decides everything: an LED takes it at under two volts and flashes, a dry finger takes the same charge at twelve hundred. That is why the volts on a spark are not the thing to be frightened of, and why nothing here needs a battery.

Wiring

1. 46 → 28
2. 29 → 47

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

Learn more: Classroom, What electricity is, paragraph 2 — ohmsville.com/classroom#what-electricity-is-2

What you should see

Two plates for somebody to hold, an igniter hidden under your thumb. Squeeze it and they get 1 199 volts, which is a snap and nothing more — the whole thing carries 0.72 millijoules, about what a car door gives you.

Parts

piezo igniter from the drawer, touch plates from the drawer, no battery at all

How it works

Volts are not what bites — energy is. The igniter makes a fixed lump of charge, and a dry finger turns that into more than a thousand volts because a megohm needs a lot of volts to pass a milliamp. The whole snap is under a millijoule, and it is over before you can lift your hand. Nothing in this build stores anything, which is the only reason it is a gag.

Wiring

1. 46 → 48
2. 49 → 47

Open it on the bench: ohmsville.com/bench#a=finger-zap

Learn more: Classroom, Safety and real parts, paragraph 3 — ohmsville.com/classroom#safety-and-real-parts-3