Part One “Basic Electricity”, dealt mainly with switches. Now, in Part 2, we are going to discuss a special kind of switch – the relay.

Notice in the above diagram that a relay uses an electromagnet. This is a device consisting of a coil of wire wrapped around an iron core. When electricity is applied to the coil of wire it becomes magnetic, hence the term electromagnet. The A B and C terminals are an SPDT switch controlled by the electromagnet. When electricity is applied to V1 and V2, the electromagnet acts upon the SPDT switch so that the B and C terminals are connected. When the electricity is disconnected, then the A and C terminals are connected. It is important to note that the electromagnet is magnetically linked to the switch but the two are NOT linked electrically.

There is another type of relay called a solenoid that basically works on the same principle. The solenoid electromagnet consists of wire wrapped around a tube containing an iron cylinder called a “plunger”. When electricity is supplied to the wire coil, the “plunger” moves through the tube and activates a switch.

At this point you might be wondering about the purpose of all this. Why switch an electromagnet just so it can control another switch? Why not just use one regular switch? One important application is illustrated in the diagram below.

NOTE: the symbol indicates a ground connection. Since a great percentage of an automobile consists of metal, using the automobile itself as one “side” of a circuit saves a tremendous amount of wire.

When the ignition key is turned all the way to the “start” position, it allows electricity to flow to the starter solenoid (relay) which then connects the battery to the starter motor. So why do we need this solenoid “middle man” ? Couldn’t we just eliminate it and connect the ignition wires to the + battery terminal and the other wire to the starter motor? The important point here is that the electromagnet is using a small amount of current to control a large amount of current to the starter motor. (Remember that the electromagnet and the switch are NOT connected electrically). Have you noticed that all of the wires (except the ignition wires) are purposely drawn with thick lines? The reason being that some circuits (such as the starter) in a car require a tremendous amount of current. (If you look at an automobile’s battery cables, you will notice they are quite thick.) Connecting the ignition wires to the battery and then to the starter motor would cause these thin wires to conduct much more current than they were designed for. These wires would become very hot and the insulation would start to smoke. (The same would hold true for the ignition switch). After starting the car for just a few times, the wires and the switch would be in bad shape.

We do have a second choice. We could use thick battery cables for the ignition wires and use a heavy duty ignition switch. This isn’t very practical either. Do you think it would be easy to squeeze cables into the steering column and to squeeze in a heavy duty ignition switch too? Therefore, the use of a solenoid is the most practical solution.

Another practical use of relays is for switching one circuit ‘on’ when another circuit has been switched ‘off’ or broken. What possible application requires such an odd switching arrangement? How about a burglar alarm?

Referring to the above diagram, let’s trace the electrical flow. Since the alarm loop wire connects points ‘V2’ to ‘C’, it can easily be seen that electricity flows from the negative battery terminal, goes to V1 then V2, then (because the alarm loop is unbroken) it goes to C and finally to the positive battery terminal. In this circuit, current is flowing through the electromagnet, causing the SPDT switch to make contact with terminal B. Because of this, the siren does NOT sound because there is NO current going to point A.

Now let’s suppose that the alarm loop is broken. The wire does not necessarily have to be cut to trigger the alarm. Perhaps one or more magnetic switches could be wired in series in the alarm loop and when one magnet moves, it causes the switch contact to break and then the alarm will sound. The electricity is now flowing across points C and A to the siren and NOT the electromagnet.

Alarm Circuit 1 does suffer from one serious flaw. Can you see what it is? If the alarm loop is reconnected, the siren shuts off. This is NOT recommended for any serious alarm system. After all, if a door with a magnetic switch is forced open, all a burglar would have to do is close the door. The siren then goes off ! Is there a better way to wire an alarm? Sure.

Alarm Circuit 2 looks very similar to Circuit 1, the only difference being that one side of the alarm loop now goes to point B instead of point C. What happens when current is applied to this circuit? The siren sounds off immediately and stays on continuously. Hmmm, that sure seems like an annoying alarm circuit. (Did Tim Conway’s father wire it? If you don’t get this joke, see Part I).

Now for the ‘beauty’ of this clever circuit. By using a scrap of wire, temporarily connect point B to point C. The alarm shuts off immediately. If you break the alarm loop, the siren sounds once again. What happens if the alarm loop is reconnected? The siren still blasts away. Now that’s a much better alarm circuit! Let’s see how it works. Temporarily connecting points B and C causes current to flow through the electromagnet, which attracts the switch to point B. As long as the alarm loop remains unbroken the alarm remains silent. Break the alarm loop, the alarm sounds. Unlike Circuit 1, reconnecting the alarm loop no longer causes current to flow through the electromagnet. The only way to activate the electromagnet is by connecting points B and C. All right!

In the ‘real world’, the relay, the power supply (battery) and the siren should be inaccessible to everyone except those authorized to ‘arm’ and ‘disarm’ it. You can easily see that the alarm could be ‘sabotaged’ in a number of ways if unauthorized persons had access to it. Incidentally, this type of circuit is called a supervised alarm system. Why? If the alarm loop were to be broken, you would never be able to arm it. Therefore, in a real world application, if the alarm cannot be armed, you would know something was wrong (a door might be open, a wire might be broken, etc.). Also, in real world applications, an alarm would be much more complex than the one shown here. The arming process would probably be done with a key switch. The alarm might also have flashing lights, an automatic phone dialer to the police and so on. The circuits shown are for demonstration and educational purposes only and NOT meant to be used in place of professional alarm systems.

One more circuit should be shown because in real life, the same power supply probably would not operate the alarm loop as well as the warning circuit. The diagram for such an arrangement is shown below in Alarm Circuit 3.

As far as building a science project, we would recommend Alarm Circuit 2, which probably could be built for about $5. (Yes, you could add some magnetic switches to the alarm loop but remember this will increase the cost very quickly). Though inexpensive and consisting of only 3 parts, Circuit 2 demonstrates some important electrical concepts. By all means, do some further research on relays, alarms and so on.

Just a few more words about relays. As is the case with many mechanical devices being replaced by their electronic equivalents, relays are being “phased out” by Solid State Relays (SSR’s). Mechanical relays do have their disadvantages when compared to an SSR:

1. switching is much slower
2. the contacts wear out
3. they make noise when they switch
4. their magnetic fields can cause problems for nearby components

Presently, their one advantage is their ability to switch high voltage and high current circuits. (the automobile starter solenoid for example). No doubt with time, even this will be surpassed by the SSR. At least mechanical relays can easily demonstrate the principles of electrical/electronic switching.