Transistor Touch Switch Simple Science Project construction

Solderless Breadboard

Get a solderless breadboard. Almost any size breadboard will work for this project because there are so few components to place. If you are going to buy a new one then I suggest that you go for an 830 point breadboard. I find this size to be a good compromise and will come in handy for more complex projects later.

Which type of transistor do you have?

Examine your transistor and make sure that you know which pin is which. You will need to know this so that you can plug it into your breadboard in a moment.

Identify the LED terminals

Examine your LED. This component must also be placed into your board in the correct way. All diodes including LEDs have an anode (+ve) and a kathode (-ve).

There are three common ways of identifying the leads of an LED and they all apply to the kathode. The kathode is usually the terminal that has:

1. The shortest lead. Not much use if you have cut the leads to length.
2. A flat spot on the body. Not always easy to see.
3. The larger electrode inside the LED. This is not guaranteed but it's how I generally identify my LEDs and it has worked for me every time so far.

Remember that the anode should be connected to the positive side of the circuit.

Insert the transistor

Insert the three leads of the transistor into the breadboard with the emitter on the left, the base in the middle and the collector to the right.

Put the two resistors in

I used 470Ω resistors but you can use anything from 100Ω to 1kΩ.

Connect a resistor between the anode of the LED and the positive power line running across the top of the breadboard.

Connect the other resistor from the base of the transistor to a free connection. This will eventually connect to your touch device.

Add the LED

Insert the LED with the Kathode lead connected to the collector of the transistor.

Ground the emitter of the transistor to the -ve power rail

Connect a wire between the emitter of the transistor and the negative power rail running along the bottom of the breadboard.

Connect the touch sensitive wires

Cut two longish bare wires to use as the touch sensitive device. You can strip the insulation from solid hook up wire if you don't have any uncovered wire.

Place them horizontally and parallel on your breadboard. Connect one of them to the base resistor of the transistor.

Connect the other long wire to the positive power line using another piece of wire.

Battery power

Connect the battery into the circuit. Connect the negative to the lower power rail and the positive to the top one.

Try out the touch switch

Place your finger on the touch wires and see what happens.