What’s a relay? You’ve probably seen a relay race where one runner hands off a baton to another runner. Similarly, an electronic relay hands off control from one circuit to another. A relay is a very simple device consisting of an electromagnet, an armature (a switch that closes when attracted by the electromagnet), and a spring that is connected to the armature.

(Diagram inspired by: http://electronics.howstuffworks.com/relay1.htm)

In Figure 1 (above), there are two circuits. The first circuit is a battery (3 volts), a switch and an electromagnet. The second circuit is a battery (6 volts), a light bulb and the relay’s armature. While the switch to the electromagnet is off, no current can flow from the 3 volt battery through the electromagnet. So, the armature-switch is off and no current can flow from the 6 volt battery to power the lamp in the second circuit.

In Figure 2 (above) the switch to the electromagnet circuit is switched on. When current from the 3 volt battery flows through the electromagnet, the electromagnet creates a magnetic field that attracts the armature to close the circuit to the lamp. Now current can flow from the 6 volt battery to the lamp, and the lamp lights up.

If you look closely at Figure 2, you’ll notice that while the armature-switch is closed allowing the current in the lamp circuit to flow from the 6 volt battery to the lamp, it does not come into contact with the electromagnet so, the 6 volts from the lamp circuit cannot flow into the electromagnet circuit. Thus, the 3 volts in the electromagnet circuit and the 6 volts in the lamp circuit remain separate.

Above is the electronic schematic diagram of a relay. The curly line represents the electromagnetic coil and the vertical lines represent the metal core the coil is wrapped around. The switch at the top of the diagram represents the armature.

Electronic schematic of the circuits from Figures 1 and 2.

Build the Relay Circuit With Snap Circuits

Parts Needed:

3 Battery Holder (2-AA) # 6SC B1
1 Base Grid (11” x 7.7”) # 6SC BG
1 Slide Switch # 6SC S1
1 Snap Circuits Lamp SOCKET # 6SC L2 (with bulb)
1 Relay # 6SC S3
1 Conductor with 2-snaps # 6SC 02
2 Conductor with 3-snaps # 6SC 03
1 Conductor with 4-snaps # 6SC 04

Build the following circuit:

When you press the press switch (S1) you will hear the relay click as current passes from the 3 volt battery block (B1) through the electromagnet. The electromagnet creates a magnetic field that attracts the metal armature and closes the circuit to power the lamp circuit.

When you release the press switch you will hear the relay click again since the current has been cut from the electromagnet. This causes the electromagnetic field to collapse so the armature is no longer attracted by the electromagnet and the spring attached to the armature returns the armature to its rest position. This opens the lamp circuit which cuts the current from the batteries to the lamp and the light goes out.

Often when using electronic relays in circuits it is recommended that you insert a flyback diode to protect sensitive electronics. The magnetic coil in the relay stores electricity like a capacitor and when the voltage is cut from the coil the current can flow back into the circuit in the wrong direction. You can build the following circuit to demonstrate the phenomenon.

Parts Needed:

1 Battery Holder (2-AA) # 6SC B1
1 Base Grid (11” x 7.7”) # 6SC BG
1 Slide Switch # 6SC S1
1 Red LED # 6SC D1
1 Relay # 6SC S3
3 Conductor with 2-snaps # 6SC 02
1 Conductor with 3-snaps # 6SC 03
1 Conductor with 4-snaps # 6SC 04

The following video will demonstrate the relay's magnetic field collapse sending voltage back through the circuit the wrong way. You may have to wait a moment for me to dim the lights a bit to see the led flash, but when I push the button, this engages the electromagnet in the relay and you may be able to hear the relay click on. When I release the button this cuts off the voltage to the relay (you may be able to hear it click off) and the magnetic field collapses and the current flows back into the circuit causing the LED to flash.

Here’s another circuit you can build that includes the snap circuits motor:

This next video demonstrates how the circuit on the left side of the relay (B1 battery block and Slide Switch S1) is used to switch on the circuit on the right side of the relay (another B1 battery block and motor with fan attached).

Again, an electronic relay hands off control from one circuit to another. The voltage in the first circuit remains separate from the voltage in the second circuit. This can be useful if you need to control a higher voltage circuit such as driving a motor with a lower voltage circuit from, say, a microcontroller and where connecting a motor directly to the microcontroller might damage it.

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Here's a bit of fun trivia:

First Computer Bug

In 1947, Grace Murray Hopper was working on the Harvard University Mark II Aiken Relay Calculator (a primitive computer).

On the 9th of September, 1947, when the machine was experiencing problems, an investigation showed that there was a moth trapped between the points of Relay #70, in Panel F.

The operators removed the moth and affixed it to the log....The entry reads: "First actual case of bug being found."

...

The word went out that they had "debugged" the machine and the term "debugging a computer program" was born.

Although Grace Hopper was always careful to admit that she was not there when it actually happened, it was one of her favorite stories.

Source: http://www.jamesshuggins.com/h/tek1/first_computer_bug.htm