Touch switch circuit using a transistor

Introduction

In this circuit project, we will create a simple touch switch using only one transistor. When someone touches the ends of the circuit, the LED Turn ON, and the LED stops when they remove their finger. This circuit is very useful and can be used in many applications. For example, you can use this circuit to make a doorbell or a touch switch.

Making Video

This simple circuit allows you to turn on an LED with just the touch of your finger! Today I am going to make a simple touch sensor using the transistor BC547. There is nothing complicated about this circuit and it is very interesting.

How to make this prject

The circuit is very simple to build. First, we need to connect the collector of the transistor to the 470 Ohm resistor. We also need to connect the base of the transistor to the Copper wire (Touch Pad 1) also connect another touch pad to the positive terminal of the 9v battery. To connect the LED, you should attach the other end of the resistor to its negative terminal. Additionally, you need to establish a connection between the emitter of the transistor and the negative terminal of the 9v power supply. Finally, connect the positive terminal of the LED to the positive terminal of the 9v battery. When you touch the base of the transistor with your finger, it will complete the circuit, causing the LED to light up.

This circuit is also very easy to make and requires few components. So, why don’t you give it a try and make your own touch sensor?

Components Required

BC 547 Transistor

A transistor, made of semiconductor material, performs the function of amplifying or switching electronic signals and electrical power. It features at least three terminals that connect it to an external circuit. When you apply a voltage or current to one set of terminals, the transistor actively controls the current flowing through another set of terminals.

There are two types of transistors, PNP and NPN

The transistor type indicates the semiconductor material employed in the transistor. The two primary types are NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). NPN transistors, being the most prevalent type, actively find application in numerous digital and analog circuits. On the other hand, PNP transistors, though less common, possess distinct advantages over NPN transistors and actively serve specific purposes. For this project, we utilize an NPN Transistor BC 547.

You will use an NPN transistor for this project. NPN stands for negative-positive-negative transistor. Transistors are semiconductor devices that amplify or switch electrical signals.

Transistors have three pins:

• Emitter (E)
• Base (B)
• Collector (C)

A transistor can be compared to a valve that controls the flow of water between two pipes, and the hand wheel is used to control the flow of water. Transistors do not have a hand wheel to control current flow, but there is a way to do it.

Comparing a Transistor to a Valve

Let’s learn how transistors work through an example. The image above shows a pipe carrying water and a valve controlling its flow. Here, water enters through the collector section. The flow of water is controlled by a value called Base, which comes from a section marked emitter. Learn more about transistor

Circuit Diagram

Let’s Build the circuit on the breadboard

Step 1

Connect the battery (9V) to the power (+Ve / Red wire) and ground (-Ve / Black wire) rails of the breadboard.

Step 2

Identify the Buzzer anode (long leg / +ve) and cathode (short leg / -ve) and connect it to the breadboard.

Step 3

Insert the resistor into the board with one end (First leg) in the same vertical column as the Buzzer cathode (short). You can insert the resistor in either direction because it isn’t polarized.

Step 4

Plug the transistor into the breadboard. And you need to connect the collector pin of the transistor to the resistor 470 Ohm.

Step 5

You will need copper wire to make a touch pad. Connect one end to a transistor base pin and the other end to any area of the breadboard like the above figure.

Step 6

Wire up the emitter pin (-ve) leads using black jumper wires. One end of the jumper wire should be connected to the transistor emitter pin and the other end to the power rail cathode (-ve).

Step 7

You will need another copper wire to make a touchpad. Connect one end power rail anode (+ve) pin and the other end to any area of the breadboard like the above figure.

When someone touches the 9V positive wire and the Transistor base wire, it actively establishes a connection between the base of the transistor and the positive terminal of the battery. In other words, this action effectively closes the circuit. Consequently, a small amount of current is readily permitted to flow from the base to the emitter, allowing for proper operation of the transistor.

NOTE: The human skin contact resistance is a maximum of 10,000 ohms and 1,000 ohms when wet. This depends on the contact area, moisture, and condition of the skin.

Not Working?

      If your circuit isn’t working, make sure you haven’t made any of these mistakes: