Touch Sensor Circuit

How to Build a Touch Sensor Circuit

In this project, we will show you how you can build a touch sensor circuit.

Touch sensors have tremendous application in real life. You've probably once had or used a device that by touching a certain part of it turns it on.

In this project, the touch sensor circuit we'll build will turn on and light a LED.

Components Needed

220Ω Resistor
150Ω Resistor
2N2222 BJT Transistor
1 LED
2 Wires
9-volt Battery or Power Supply

In this project we're going to use use two wires, in this project, we use one red and the other blue, which serve as the touch sensor wires. When a person touches both wires, the circuit is then closed and current runs through the circuit and lights the LED. For the power source, you can either use a 9-volt battery or you can use a power supply to supply the 9 volts.

Below is the schematic of the circuit we will be building:

This is the more real life representation of it:

How It Works

When the wires aren't touched, the circuit is not closed and, thus, no current can pass through the circuit. When a user touches the wires, he closes the circuit, so now current can pass through and light the LED. The person's body acts as the closer of the circuit. Because a person offers so much resistance, the current is very small and doesn't cause any shock. But the small current is amplified by the transistor and thus it's sufficient current to light an LED.

Troubleshooting

Problems can arise while doing this circuit, so this is why it's recommended that you use the exact parts or as close as possible. First of all, if you place too much voltage into the circuit, the LED will light up without you even touching the wires. This defeats the whole purpose of having touch sensor. Under too much bias voltage provided into the circuit, the collector of the transistor breaks down and conducts across into the emitter, even without any base current at all. This is why the levels need to set up exactly, or else this circuit may not function.

In the same way, too little voltage will not make the circuit function as well. If too little voltage is used, even when you touch the wires, the LED will not light, because the current isn't sufficient to light the LED.

Note: This circuit has been run, tested, and proven, and should work for anyone trying it.