Relay LED Blink

2019-09-25 - Authored by: Iain Campbell

This project will demonstrate how a relay can be used in a circuit to make a LED blink.

About relays

A relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. The heart of a relay is an electromagnet (a coil of wire that becomes a temporary magnet when electricity flows through it).

Note: Tinkercad doesn’t simulate LED relays operation very well so it’s best this circuit is built using real components. We’ve found it doesn’t work properly in Tinkercad!

Read further information about relays here

Components required for this experiment include:

Circuit diagram

Breadboard layout

The breadboard diagram is shown below:

How it works


  1. You can use any 9V relay for this example, but note that the pin arrangement on a relay varies between relay types. On many relays, the pin arrangement is printed on top of the relay, so that you know which pin is which.
  2. You may need to lookup the pinout of the relay in the manufacturer’s data-sheet for the relay you are using.
  3. Note: Not all relays are fitted for easy use on a breadboard, so you might need to bend the legs a little bit to get it to fit into the breadboard.
  4. Mentors may provide some relays which has wires soldered to it as an alternative.
  5. Start by connecting the battery connector to the supply rail on the side. The red wire is the positive terminal, the black wire is the negative one.
  6. Next, place the relay on the breadboard. Then connect the resistor (R2) from pin 4 to pin 2 of the relay. Connect a wire from the positive supply rail to the relay input (pin 3) and from the negative supply rail to the relay coil (pin 1).
  7. Add the capacitor over the relay coil. The relay coil doesn’t have a positive or negative side, but this type of capacitor, polarized capacitor, does. So place the capacitor with the negative pin at the lower end (pin 1) since we have connected the negative battery terminal here.
  8. The negative pin of the capacitor is usually marked with a “-” (minus sign) or a zero
  9. Now, if you power up the circuit you should hear a clicking sound. If not, check your connections before proceeding. A smart thing to check is the voltage on the relay coil. If there is no voltage there, something is wrong.
  10. When you have gotten the clicking sound, it’s time to connect the resistor (R1) and the Light Emitting Diode (LED). Connect them from the second output of the relay (pin 5) and back to the negative battery terminal (as shown in the diagram above).


Further experiments