Electronics School · Build It Yourself · Project 5

Touch Switch

Light an LED with the touch of a fingertip, using two transistors to amplify the tiny current through your skin.

What you’ll make

Touch two bare wire ends with one fingertip, and an LED lights up. Two transistors, connected so that one feeds the other, amplify the tiny current that flows through your skin until it’s enough to light the LED.

The touch switch. Q1 and Q2 are both 2N3904 transistors. Q1 amplifies the current through your finger; Q2 amplifies it again.

Before you begin: ask your parent, guardian or a responsible grown-up for permission. Some activities need a grown-up to stay and help. Not sure? Ask first!

What you need

A battery holder for 3 AA batteries, with a switch, and 3 AA batteries (4.5 V)
A breadboard and jumper wires
2 NPN transistors, 2N3904
1 resistor, 1 kΩ (bands: brown, black, red; five-band: brown, black, black, brown, brown)
1 resistor, 220 Ω (bands: red, red, brown; five-band: red, red, black, black, brown)
1 LED, 5 mm, any colour that lights on 4.5 V with 220 Ω (red or yellow is best)
2 jumper wires to use as touch wires, with bare metal ends

Cost: low.

Safety numbers for this build

Supply	4.5 V (3 AA batteries) only
Current through your finger	a few microamps (millionths of an amp): far too small to feel
LED current	about 7 to 8 mA when on
Protection	the 1 kΩ resistor limits Q1’s base current, and because both collectors connect through the LED and its 220 Ω resistor, the total current stays around 8 mA even if the touch wires touch each other

This is only safe because the battery voltage is so low. Never try a touch circuit with anything that plugs into the wall, or with a higher-voltage battery. Touch the wires with one hand only, and don’t put them in your mouth.

Know your transistors

Hold each 2N3904 with its flat side facing you, legs down: left to right, the legs are E (emitter), B (base), C (collector). Most 2N3904s use this order, but check the seller’s datasheet.

Build it, step by step

Rails: battery pack off. Red lead to the red (+) rail, black lead to the blue (−) rail.
Place Q1 and Q2 in the breadboard, a few rows apart, each leg in its own row.
Q2’s emitter to the blue (−) rail with a jumper wire.
Q1’s emitter to Q2’s base with a jumper wire.
Q1’s collector to Q2’s collector with a jumper wire.
LED and resistor: 220 Ω from the red (+) rail to an empty row; LED long leg into that row, short leg into Q2’s collector row.
The 1 kΩ resistor from Q1’s base row to an empty row.
Touch wires: push one into the 1 kΩ resistor’s free row, and the other into the red (+) rail. Bend the free ends so they point up, about a finger-width apart.
Switch on.

Try it

Press one fingertip across both bare wire ends. The LED lights. Lift your finger and it goes out.

Predict, then test: does the LED get brighter if you press harder, or if your finger is slightly damp? Try two fingers of the same hand, close together, then farther apart.

Not working? Check each transistor is the right way round, that Q1’s emitter really goes to Q2’s base, and the LED polarity. Very dry skin conducts poorly: try a slightly damp fingertip.

LED glows faintly with nothing touching? A dangling wire can pick up tiny stray signals. That’s normal for such a sensitive circuit.

Why it works

BeginnerAmplifying a trickle

Your skin conducts electricity, just very poorly. When your finger bridges the wires, a tiny current flows from + through your skin and the 1 kΩ resistor into Q1’s base. Q1 multiplies it, perhaps a hundred times, and feeds the result into Q2’s base. Q2 multiplies it again, enough to light the LED.

IntermediateThe Darlington pair

Two transistors connected this way are called a Darlington pair, after Sidney Darlington, who patented it in 1953. The total gain is roughly the two gains multiplied together: with two transistors of gain 100, about 10,000. A fingertip might pass only a few microamps; times 10,000, that’s tens of milliamps available, plenty for a 10 mA LED.

The pair needs about 1.4 V at Q1’s base to turn on (two base-emitter junctions of about 0.7 V each), and when fully on it leaves somewhat more voltage across itself than a single transistor does.

For teachers and parents

Open the teacher guide

Time: 30 to 45 minutes.

Learners will: show that skin conducts weakly; explain amplification; build a two-transistor circuit from a schematic.

Risk and supervision: low at 4.5 V. Make the safety message explicit: this works only because the voltage is very low, and must never be tried with wall power or larger batteries.

Discussion questions: Why doesn’t the LED light from your finger alone, without the transistors? Where might a touch-sensitive switch be useful? Why is it important that the voltage is low?

Common misconceptions: “Electricity is flowing through my whole body” (only a tiny current, mostly through the skin of one fingertip). “The transistor makes current” (it controls current from the battery).

Worksheet with answers:

If each transistor has a gain of 100, what is the combined gain? (About 10,000.)
If 2 µA flows through your finger, about how much current could the pair control? (About 20 mA.)
What keeps the circuit safe if the touch wires touch each other directly? (The 1 kΩ resistor limits Q1’s base current, and the 220 Ω resistor and LED limit the total current through both transistors to about 8 mA.)

Learn more

← Project 4: The Fading Light Stage 3: Series and parallel →

Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.

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