Simple Capacitive Touch Sensor

Overview

Your body is a capacitor! When you touch a conductive surface, you add capacitance to a circuit, changing how it behaves. This project uses a Darlington pair (two transistors) to detect the tiny currents that flow when you touch a wire. It's the same principle behind smartphone touchscreens!

Components Needed

• 2x NPN Transistor (2N3904 or 2N2222)
• 1x Resistor (1MΩ - one megohm)
• 1x Resistor (1kΩ)
• 1x Resistor (100kΩ)
• 1x LED (any color)
• 1x 9V Battery
• 1x 9V Battery clip
• 1x Breadboard
• 1x Small metal plate or coin
• severalx Jumper wires

Instructions

1. Build the Darlington Pair

   Place two transistors on the breadboard. Connect the emitter of the first transistor to the base of the second. Connect the emitter of the second transistor to ground. This creates a super-high-gain amplifier.

2. Add the Touch Input

   Connect the 1MΩ resistor from power (9V) to the base of the first transistor. This "pulls up" the base weakly. Connect a wire from the base—this is your touch sensor wire. Optionally attach a coin or metal plate to the end.

3. Add Sensitivity Resistor

   Connect the 100kΩ resistor from the base of the first transistor to ground. This sets the sensitivity threshold—adjust if needed.

4. Add the LED

   Connect the LED positive (long leg) to 9V through the 1kΩ resistor. Connect the LED negative to the collector of the second transistor.

5. Test Your Touch Sensor

   Power up the circuit. Touch the sensor wire or metal plate—the LED should light up! Release and it turns off. You're detecting the AC noise your body picks up from nearby wiring.

Challenges

• Make a touch-activated night light that stays on for a few seconds (add a capacitor!)
• Create multiple touch pads and control different LEDs
• Measure your body's capacitance using an RC time constant circuit
• Build a "proximity" sensor that activates before you touch by increasing sensitivity
• Add a 555 timer to create a toggle (touch on, touch off)