This video is about the electromagnetic relay, which is this device here, and this is its circuit symbol. It comprises of a magnet, which is this symbol here, and in reality it's this copper coil. A physical connection, represented by the dotted line, which is this metal armature, this bright metal armature, you can see here, and the spring. And the switch, in this case is a double pole switch, and this is just here, down at the bottom corner. Now, when I turn the magnet on, watch what happens to the switch. It moves. So the magnet becomes the magnet, it attracts the metal armature, it moves the switch. Turn it off. Turn it back on again. Turn it off. Turn it back on again. Turn it off. I could do this all day. This is a mechanical switch, controlled by an electro magnet, which is controlled by a different circuit. Here is my relay circuit in action. So I've got my relay in the middle. I've got it wired up to a 12 volt battery, and a push button. I've actually switched just here. And I've got the switch of the relay, wired up to a 6 volt AC supply, through these blue wires, and this bulb just here. So the bulb is using an AC supply. At 6 volts, I mean a 12 volt DC supply to control the relay. Let's have a look what happens. Turn the switch on. The relay turns on. The bulb comes on. Notice that the 12 volts from the switch is not what's powering the bulb. The bulb is being powered from its own power supply. Turn the relay off. The coil goes off. The spring pulls the switch back. The switch opens. The bulb goes off. So a relay allows one circuit operating at one voltage to control a second circuit, operating at a different voltage. Turn the bulb on. Turn the bulb off. This is an example of a circuit. In this case a 555A stable being used to control a relay. In this case a PCB mounted relay. This green wire here takes the output of the 555 and controls the relay coil. The relay contacts, the switches are connected through these blue wires to my light bulb, which is running off my AC supply. So things to note. The A stable is controlling the relay. The relay controls the switch. The switch controls the light. This red line shows that they are electrically separate. This circuit on this side is completely separate from the control circuit on this side. Different voltage. This one's AC. This one's DC. Second thing to note is that I added a diode. And my diode of the added, which is just here, is reverse bias. It's connected backwards across the supply. So it looks like it should never conduct. And it's here to protect the 555 when the relay turns off. And we've talked about that in our transistor section elsewhere. Now in our final circuit we have the same 555A stable, which is here. But this time the A stable is controlling a little NPN transistor. I'm using a BC107 in this case. So the A stable is turning the transistor on off a transistor. It's controlling the currents to the relay. So here's the transistor. Here's the relay. You can see it's wired up very slightly differently. Now the relay is connected to positive. And then the transistor switches down to ground. And my protection diode, which is still essential, is now connected just here up to the positive rail. And this protection diode is now protecting the transistor from when the relay turns off. We have exactly the same circuit on the other side. We have our 6 volts AC, our light bulb being controlled by this switch here. And again, these two circuits remain completely electrically separate. So this is the advantage of a relay. It's that a relatively complex electronic circuit here, working at one voltage, can use a transistor and a relay to control loads, which can take a high current or a high power, or work at a different voltage, work at AC, work at DC, work at hundreds of volts. And this small circuit can control this load through a relay.