A simple capacitive touch-sensor circuit walkthrough — parts list, pinout, breadboard steps, and an interactive touch pad you can try in the browser.
Beginner build · 3 components
A resistor, a transistor and an LED — that's the whole circuit. Bridge two exposed wires with your finger and your own skin resistance does the switching.
Hold your mouse (or finger) on the TOUCH HERE pads to light the LED
Parts bin
Everything here is common stock at any electronics shop — nothing exotic, nothing that needs soldering to test.
Operating principle
Nothing mystical — it's ordinary transistor switching, with your finger standing in for one resistor.
Skin isn't a perfect insulator. Measure across two fingertips with a multimeter and you'll see real resistance — usually somewhere in the hundreds of kilohms, dropping a lot once your skin is slightly damp.
Bridge the two touch wires with a fingertip and that resistance becomes part of the circuit. A small trickle of current can now flow from the battery's positive rail, through your finger, and into the transistor's base.
That tiny base current is the trigger. It switches the transistor on, which opens a much larger path from collector to emitter — and that's the path the LED sits in. Enough current flows through R1 and the LED to make it glow.
Base current — microamps, routed through your finger. This is the signal that says "turn on."
Collector current — milliamps, drawn straight from the battery through R1 and the LED. This is the power that actually makes light.
The transistor is the go-between: a whisper of current at the base unlocks a much stronger current at the collector.
Assembly
Five short steps. Test it on the breadboard first — solder it later once you know it works.
Place the BC547 across the centre gap of the breadboard. Collector, base and emitter each get their own row so nothing else shares that connection yet.
Run the 470 Ω resistor from the top supply row down to the row just above the transistor. It should land one row short of the collector — it isn't connected to the transistor directly, the LED will bridge the gap.
Trim two legs from a spare LED down to about 1 cm and bend them into a small pad — that pad is what your finger will touch. Insert the real LED so its anode meets the resistor and its cathode meets the collector.
Run one jumper from the supply-side pad to the base region, and one from the other pad toward the transistor's base leg. These two wires are the only path between them — your skin will complete it.
Clip in the 9V battery. Nothing should happen yet. Press a fingertip across both exposed pads at once and the LED should glow. A slightly damp fingertip lowers your skin resistance and makes it easier to trigger.
If nothing happens
In order of how often they actually turn out to be the problem.