Relay: Basics and DIY

Hello Friends,

This instructable deals with electromagnetic relays. They are electromagnetic switches working based on the principle electromagnetic induction.

E-M Relays are used where any circuit with a different voltage level has to be controlled using a low voltage level circuit. Just like controlling a 230 VAC bulb with a 5 V DC output of a microcontroller.Relays play a very crucial role in many categories of electronic circuits.

For more information about relays, go through this.

For doing this, we need:

• 5 V relay.
• BC 547 transistor.
• 1N 4007 diode.
• Perf board/Universal PCB.
• LED.
• 4.7 Kohms resistor.
• 470 ohms resistor.
• Terminal connector.

Electromagnetic relays work based on the principle of electromagnetic induction.

When the voltage is applied to the coil, it magnetizes the core and pulls the switch closed.

NC: a normally closed connection which already closed even before applying the voltage.

NO: a normally open connection which is closed when the voltage is applied.

The circuit consists of LED with a series current limiting resistor. And transistor BC 547 for increasing the microcontroller output current to a high value for the relay coil. The diode 1N 4007/4001 is connected across the relay coil to protect the transistor from back emf of the coil.

To do this circuit on the board, we need to:

• Cut a piece of perf board.
• Mark on the board as shown in the pictures
• Solder the 470 ohms resistor as shown and the LED.
• Solder the 547 transistor.
• Solder the 4.7 kohms resistor with one led to the input end and the other to the middle pin of the transistor.
• Solder the relay. Connect one end of the coil to the collector pin of the 547 transistor and leave the other end free.
• Solder the diode across the relay as shown.
• Solder a 2 pin terminal connector near the relay.
• Now joint the resistor ends for the input signal.
• Joint the negative pin of the LED and emitter pin of the transistor.
• Connect the common pin and the NO pins of the relay to the terminal connector.

Now we have to take 3 wires out.

1. Solder a red wire to the one end of the relay coil (the one we left). This is the Vcc.
2. Solder a black wire to the point where emitter pin of the transistor and negative of the led meets. This is the Ground.
3. Solder one more wire to the point where both the resistors meets.

Strictly follow the steps and the pictures attached.

That is how we did this DIY.

The wires colored red, black and green are for the Vcc supply, the ground, and the microcontroller signal respectively.

I wish this could help for your DIY projects, Thanks, and Happy making :)