PAGE 142 OF 150 · PROJECTS
Build a Touch-Activated LED
A transistor amplifies a tiny touch current.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
How it works
Skin conducts a tiny current. Two metal contacts bridged by a fingertip let enough current reach a transistor base to switch it on.
Circuit
- Wire two NPN transistors as a Darlington pair, such as two BC547s.
- Connect one touch contact to the supply and the other to the first base through a 100 kilohm resistor.
- Put the LED and a 330 ohm resistor in the collector of the second transistor.
Only microamps pass through your finger at battery voltage, which is safe. A dry finger is a weaker conductor, so a damp finger lights it more brightly. Microcontrollers offer capacitive touch sensing as an alternative.
REMEMBER
The transistor pair turns a tiny finger current into LED current.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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