# Simple Touch Switch 3904 NPN

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## Introduction: Simple Touch Switch 3904 NPN

Hello :)

This project will show you how to make a simple touch switch using a 2N3904 (NPN) transistor.

A newbie special 10 minutes project!

### Teacher Notes

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## Step 1: Parts List:

1. 1 LED
2. 1 2N3904 Transistor
3. Jumper Wires
4. 2 680 Ohm Resistors *or 1K or less
6. 5V Power Supply

## Step 2: Connecting the Transistor

• Place the 2N3904 transistor on the breadboard with the flat side facing you.
• Connect a resistor from the right >>>>> most pin of the 2N3904 to the (+) short leg of the LED.
• Connect the (-) long leg of the LED to ground.
• Place a jumper wire in a spare slot where you placed your resistor.
• Connect the 2nd resistor to the left <<<<< most 2N3904 pin, power (5V) the other end of the resistor.
• Place a jumper wire in a spare slot where the left most <<<< pin of the 2N3904 is connected.

## Step 3: Test the Circuit

Now that all the parts are connected test the circuit by simply tapping the wires together.

Did it work? YAY!!! Now try touching a wire with your left hand and touching the other wire with your right hand.

You can even have a friend join in, just hold hands and complete the circuit. The current travels through your body completing the circuit and lites the LED. Cool huh?

## Step 4: Watch Me Build the Project!

Enjoyed this project? Subscribe to my YouTube Channel

Bits4Bots

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## 12 Discussions

the schemetic you have is incorrect, for NPN transistor collector is connected to the load then to power, not ground!!!!!!!!

Isn't it better to use a Darlington transistor?

Much of the comments above I recognize as the circuit is not sensitive enough to detect the human body's resistance...

Darlingtons may solve that. An alternative is to use the transistor twice where the first emitter is directly connected to the second base and both collectors are connected to the +. (a.k.a. poor man's Darlington)

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Bits4Bots got his LED legs backwards. Short pin is always ground.

The led doesn't light up when I am touching the green wires but light up when I touch the green wires to each other. What is a possible issue? I am using all the same parts except I am using a 9v battery instead of a 5v. Thank you.

what is that app you are using in the video?

Virtual Bread Board and NI Multisim

Doesn't work. touching the two green wires together and it works. But Why not just use a switch instead.

There are many ways to build this circuit. The circuit in this project has been tested by myself and other student engineers during class. It will solely depend on the type of transistor.