A relay is a switch that is electrically operated with the switch being isolated from the power that causes the switch to operate. The relay I am going to show is operated by low DC power. The switch is capable of handling high voltage AC power. In other words you can turn on and off a 110 volt appliance with a 5 volt power source. But in the interest of safety I am only going to explain how to use the relay to turn on and off low power LEDs.
Step 1: Parts Used
Here is a list of the components I will use in this demonstration.
A Solderless Breadboard
Mini Relay SPDT 5 Pins 5VDC 10A 120V Contact
2 - Resistor 30 uF
2 - AA Batteries
5 volt power supply
1 Red LED
1 Green LED
Jumpers - Clips -
Step 2: The Relay
The relay has 5 pins. It is a SPDT relay. SPDT means single pole double throw. Pins A and B are the pins where you apply the 5 volts. The other three pins make up the two switches. 1 and 2 make up one switch and 1 and 3 make up another switch. Even when no power is applied to pins A and B, pins 1 and 2 are connected. They are Normally Closed or NC. When you apply the 5 volt power source to pins A and B the connection between 1 and 2 opens and 1 and 3 connect. You can set the polarity of the 5 volts to A B either way, when the power is applied the change will occur. The connection between pins 1 and 3 are Normally Open (NO) meaning no connection exists between those two when the relay is unpowered.
Step 3: Power!
In the photo where the green LED is glowing the circuit has 5 volts of power being applied to pins A and B from my 5 volt regulated Altoid power supply. The power from the two AA batteries is flowing to pin 1 and 3, then through the LED and back to the batteries.
In the photo where the red LED is glowing no change has taken place in the circuit with the AA batteries other than the connection change in the relay. There is no longer any voltage being applied to pins A and B. This closes the connection between pins 1 and 2 and opens the connection between pins 1 and 3. The connection between pins 1 and 2, or 1 and 3 are capable of handling enough voltage to turn on a 110 volt alternating current (AC) light or appliance. But the voltage that operates this relay should not exceed 5 volts. So you can turn on and off a household appliance with a 5 volt power source. But please leave dangerous voltage alone if you are not absolutely sure you know what you are doing because that is easily enough voltage to kill you.
Thanks for looking at my Instructable and if I can answer any questions please feel free to ask.