This project explains how to build a highly sensitive Touch Switch circuit that uses the low electrical current conducted by the human body to switch an LED ON. The circuit uses two BC547 transistors in configuration to amplify this tiny current enough to light an LED brightly.
Components Required
- BC547 transistors-2
- Resistors-100kΩ & 220Ω
- LED-1
- Battery-9v-1
- Zero PCB-1
- Connecting wires
Circuit
How it works
- When you touch the input pads or wires on the circuit, your body acts like a small resistor allowing a tiny current to flow. This small current goes into the base of the first BC547 transistor, turning it on.
- When the first transistor switches ON, it allows current to flow from collector to emitter, which in turn activates the second BC547 transistor connected downstream. This causes current to flow through the LED, lighting it up.
- The 100 kΩ resistor limits the base current to a safe level so that the transistor isn’t damaged. The 220 Ω resistor limits the current flowing through the LED to protect it and keep it bright.
- Because the transistor amplifies the small current from your touch, the LED lights up brightly even though the touch current is very tiny. When you remove your finger, the circuit turns off, and the LED goes dark.
- This simple touch switch is a great way to learn how transistors can be used as switches and amplifiers in electronics, making touch-sensitive controls without complicated parts.
Connections Overview
- The touching wires connect to the Collector and Base of the first transistor (Q1).
- The Emitter of Q1 connects directly to the Base of the second transistor (Q2).
- The load (LED) is connected to the Collector of Q2.
- The Emitter of Q2 connects to the negative terminal (ground) of the battery, completing the high-current path.
Watch the full video tutorial here: How to make a Touch Switch using the transistor BC547