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Transistor Touch Switch Simple Science Project construction

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.

Typical transistor packages LED terminal identification

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.

LED terminal identification

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.

Inserting the transistor

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.

Inserting the resistors

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.

Inserting the LED

Add the LED

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

Grounding the emitter

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.

Add the touch sensitive wires

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.

Connect the battery

Battery power

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

Touch the sensor wires to illuminate the LED

Try out the touch switch

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

When you place your finger on the two wires that make up the touch sensor you should see the LED glow. This is due to the tiny current passing through your finger being amplified by the transistor to deliver a larger current into the LED.

See what happens when you press harder on the touch wires or when you dampen your finger before touching. If you see the LED glow brighter then it's because you are conducting more current into the base of the transistor.

You have just built a very simple touch switch that uses a single transistor. We could make the circuit more sensitive by adding another transistor to increase the current gain or amplification. That is for another day however.



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Comments (10)

Topic: Transistor Touch Switch Simple Science Project
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Chad (US) says...
Studying capacitors as well as transistors right now. Would adding one cause the light to blink?
19th February 2017 6:17pm
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divya (India) says...
what is the principal for touch switch?
17th December 2016 7:53am
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sravanthy (India) says...
can i place the same circuit on pcb
8th April 2016 5:25pm
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Jhajha Bokarowasi (India) says...
Done! Perfect. If Anyone needs help connect with me through Quora. Thanks Steve. Love you ♥
26th November 2015 10:50am
megha (India) says...
will dis be perfect???? or if possible.. can u suggest me some better topics....
23rd October 2015 7:44pm
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megha (India) says...
hey, my question is.. where can i get a complete information on this in detail... since i need to submit it for my 12th boards... pls help...
23rd October 2015 7:42pm
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sumit (Bangladesh) says...
Cryingwhich NPN transistor did i use???
5th May 2014 8:13am
Steve (UK) says...
Hi Sumit,

Any NPN from the pack listed in the parts list on page 2 of this article should do just fine.
15th May 2014 8:33am
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nisha (India) says...
its gud bt i wonder whether its useful in dae to dae life!
6th April 2014 3:12pm
Steve (UK) says...
Hi Nisha, This circuit is meant to demonstrate the use of a transistor as a switch. It can be used anywhere that a small current signal is available but it must switch a larger current. This requirement crops up every day when interfacing microprocessors with external equipment for instance. Even turning on an indicator LED from an output pin of a microcontroller is often done using a circuit like this with the output pin driving the base of the transistor. I hope that this helps you to think ... Read More
6th April 2014 3:40pm

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