Project 13 — You Are a Wire (The Touch Switch)

Text written by Claude Fable 5 (claude-fable-5).

Project 13 — You Are a Wire (The Touch Switch)

The one idea: electricity can flow through you — a trickle far too small to feel — and two transistors can amplify that whisper into a lit LED.

Zero new parts: the two NPN transistors from the sensing arc are enough. In Project 5 the child was the switch, closing a metal loop with a metal handle. Today the loop closes through their own body.

What you need

• 4×AA battery holder (6 V) — 3×AA works too
• Two NPN transistors (BC547 or 2N2222)
• One LED + ~330 Ω resistor (or an active buzzer for the loud version)
• Two “touch pads”: just two jumper wires with bare ends, or tape each to a coin for something satisfying to press

Build it — the Darlington pair

One transistor’s amplification isn’t enough for skin; stacked, the first one’s output feeds the second one’s input, and their powers multiply:

  +6V ──┬────────────[330 Ω]──[LED]──┐
        │                            │ (LED + resistor into Q2 collector)
     touch pad A                     │
                              Q1 C ──┤        Q1, Q2 = NPN
     touch pad B ── Q1 base          ├── Q2 C
                    Q1 emitter ── Q2 base
                                  Q2 emitter ──┐
  GND ─────────────────────────────────────────┘

1. Q2: emitter → GND. Collector → LED cathode; LED anode → 330 Ω → +.
2. Q1: emitter → Q2’s base. Collector → Q2’s collector (tie them).
3. Touch pad A → +. Touch pad B → Q1’s base.
4. Flat side facing you on a BC547: legs are C–B–E left to right; 2N2222 (2N2222A TO-92): E–B–C. When in doubt, the wiring explainer habits apply — look it up, don’t guess twice.

Nothing happens until the child presses one pad with each hand: the LED lights. Let go — dark. Their body is the missing wire.

Now play:

• Grip harder / lick a fingertip → brighter. (Brightness is a moisture meter.)
• One fingertip on both pads → dimmer but working — a centimetre of skin.
• The human chain: child holds pad A, you hold pad B, join free hands — it lights through both of you. Add everyone in the house. It still works, a little dimmer per person. Nobody feels a thing.

What to say to the child

“There’s a tiny bit of electricity going through you right now — from this hand, across your arms, out the other hand. It’s so small you can’t feel it, way smaller than what a battery pushes through an LED. But these two little transistors are like two hearing aids in a row: they take that tiny whisper of electricity and make it ten thousand times stronger — strong enough to light the lamp. Want to see if it works through Grandma too?”

For you — the physics

• Skin is a ~1 MΩ resistor, give or take a lot. Dry fingertip-to-fingertip is ~1–5 MΩ; damp skin ~100 kΩ. So the base current is $I_B \approx 6\,\text{V}/1\,\text{M}\Omega \approx 5\ \mu\text{A}$ — three orders of magnitude below the ~1 mA perception threshold and five below dangerous. That’s why it’s imperceptible, and why this demo is honest rather than lucky. (The buzz-wire game never shocked anyone for the same reason: at these voltages your body barely counts as a conductor.)
• The Darlington multiplies gains: Q1’s emitter current is $\beta_1 I_B$, and all of it is Q2’s base current, so $I_C \approx \beta_1 \beta_2 I_B \approx 10^4 \times 5\ \mu\text{A} = 50$ mA — far more than the LED’s 330 Ω path allows, so Q2 simply saturates and the LED runs at full brightness. Dimming with dry skin or long chains is the circuit falling out of saturation: brightness becomes a live readout of $\beta_1\beta_2\,V/R_{\text{skin}}$.
• Brightness-as-moisture is a real instrument: galvanic skin response — sweat lowers skin resistance — is the core of the classic polygraph. The child’s “grip harder, it glows more” is GSR, unironically.
• Two transistors sold as one: Darlington pairs come packaged (TIP120, BC517) and drive motors and relays everywhere — worth knowing the trick has a name.

Try next

• Loud version: replace LED + resistor with the active buzzer — a touch alarm. Doorknob wire optional, sibling reactions guaranteed.
• The everything-tester, revisited: with this much gain, “non-conductors” give up their secrets — a pencil line on paper, a damp tissue, a lemon wedge, tap water vs. salty water. (Project 1’s LED-only tester called all of these “no”.)
• The thirsty plant: stick the two pads into a flowerpot’s soil — wet soil lights it, dry soil doesn’t. Water the plant and watch it come back. A genuinely useful machine, made of two transistors.

→ Next bridge project: Project 14 — the song that rides on a light beam.

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