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Transistor Touch Switch Simple Science Project

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How to make a simple one transistor touch switch

Touch switch components on breadboard

How to turn on an LED using nothing more that the tiny conductivity of you finger placed across two wires.

This project illustrates how you can use a single transistor as a solid state switch. No moving parts. Except for your finger of course.

If you were to wire up an LED to a battery and complete the circuit with your finger then you are going to be unlucky if you thing that the LED will start to glow.

It is true that modern LEDs require very low currents to flow in order to see light output but you do have to give them some current however and your finger isn't going to do that. Not today at any rate.

The circuit behind the scarecrow
Circuit diagram for the simple touch switch

If you add a transistor to the circuit you can use it's gain or β to amplify the current flowing through your finger to illuminate the LED.

I used an LED that starts to glow faintly when as little as 10µA is passed through it. My general purpose NPN transistor has a typical gain of 100 which means that we only have to get a tiny 100nA to flow into the base of the transistor to start the LED illuminating when connected to the collector of the transistor.

Of course the LED won't be very bright at it's minimum but it is possible to make much more current flow just by pressing a finger between two contacts.

Lets go get the components and try it out.

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Comments (10)

Topic: Transistor Touch Switch Simple Science Project
Full StarFull StarFull StarFull StarHalf Star 4.5/5 (7)
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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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