What it shows
Dry skin has a resistance of hundreds of kilohms, far too high to light an LED directly. Two transistors in a Darlington pair multiply the microampere current through a fingertip by about 40,000, so a light touch across two pads switches the LED on.
The science
One touch pad connects to 9 V and the other to the base of the first transistor. When a finger bridges the pads, a current of a few microamperes flows through the skin into the base. The first BC547 amplifies this about 200 times at its emitter, which feeds the second base, giving another 200 times at the second collector, where the LED and its 470 Ω resistor sit. The 100 kΩ resistor from the first base to ground drains stray charge so the LED is off when nothing is touched.
Materials
Cost: under Rs 100. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two BC547 transistors
- A 470 Ω resistor and a 100 kΩ resistor
- One red LED
- A 9 V battery with a snap connector
- Two small foil pads or two bare wire ends 5 mm apart on a card
- A breadboard and jumper wires
Safety
For this project: A 9 V battery through dry skin is far below any level you can feel; still, keep the circuit away from cuts and wet hands.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Place both BC547s in the breadboard with their legs in separate rows, flat faces towards you.
Connect the emitter of the first transistor to the base of the second to make the Darlington pair.
Connect both collectors together, then to the LED short leg; connect the LED long leg through the 470 Ω resistor to the 9 V rail.
Connect the emitter of the second transistor to the ground rail and the battery negative to the same rail.
Connect the 100 kΩ resistor from the first base to ground.
Connect one touch pad to the 9 V rail and the other pad to the first base.
Snap on the battery: the LED is off. Press a fingertip across both pads: the LED lights.
Try touching with a wet fingertip, then through a sheet of paper, and note which works.
What you should see
The LED lights whenever a fingertip bridges the two pads and goes off when the finger is removed.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why will a single transistor not usually work here?
A gain of 200 turns a few microamperes into only a milliampere, too little for a bright LED.
2What is the total current gain of the pair?
Roughly the product of the two gains, about 40,000.
3Why is a wet finger more effective?
Wet skin has a much lower resistance, so more base current flows.
4Why does paper stop it working?
Dry paper is an insulator, so no base current can flow.
Take it further
Swap the LED for a 5 V relay module and let the touch switch a small motor on.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each