Figure 1 open circuit Figure 2 closed circuit


The purpose of this project is to study basic Relay operation, and demonstrate how the old fashioned clapper type code sounder worked.

This Relay is very small and covered with a sound deadening dust cover, but you can get the idea. Using just two wires you can apply a voltage to make the Relay close with a click, and then when the voltage is removed the Relay opens with a click.

This "click", "click" sound can be arranged into a telegraph code which could be used to send messages. Can you imagine trying to learn this "language"?

The code used to send these messages of clicks and clacks was called the American Morse Code. Men got so proficient that this was like a new language to them. This method has mostly been replaced by the telephone and teletype systems today.

The Relay used here has a resistance of about 500 ohms and requires a minimum of about 6mA (milliamps) to initially pull the armature (movable part) in. This means that the voltage required (minimum) is 0.5 X 6 = 3 volts. A fresh set of 3V batteries (two 1.5V cells) should just barely be able to pull the Relay in (energize it).

Try it. It is usual for the Relay to not quite pull in at this voltage. If yours does, you are lucky to have one of the more sensitive units.

ow series the 3V and 9V Batteries to obtain 6V. (Did you figure that one out? Connecting the Batteries in series opposing results in 9-3 = 6 volts.) This should pull the Relay in easily. If it doesn't, you may need to replace the Batteries.

Try a 100 ohm Resistor in series with the Relay. It (does or does not?) pull in. Usually it does.

Now try the 9V Battery, alone. A series 470 ohm Resistor should still allow Relay operation, but a 1K will not. Insert a 1K Resistor in series with the Relay and then momentarily short circuit across the 1K with a piece of wire.

The Relay should operate and stay energized with the 1K in the circuit. The reason for this is that it takes more current to energize the Relay (typically 6 to 9mA), than to hold the contacts in place (typically 1 to 3mA). The is because the magnetic air gap is much smaller when the Relay is energized.







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