Fun With Snap Circuits 7: History is a relay of revolutions
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 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.]
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. You can see in the diagrams above how the relay works.
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 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.
Above is the electronic schematic of the circuits from figures 1 and 2.
The above diagram shows how to build the circuit from Snap Circuits.
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. This allows current to flow from the two 3 volt battery blocks (also labeled B1)connected in series (for a total of 6 volts) through the lamp and it lights up.
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.
Here’s another circuit you can build that includes the snap circuits motor:
Here’s a video of the circuit in action: