7.5 Reed Switches and Potential Dividers

A reed switch is a special switch; one that is usually not controlled manually. A reed switch looks something like this:

(It is called a reed switch because the two strips of metal resemble the reeds in a wind instrument) A reed switch is a small glass tube with two separate wires in it. When a magnet is brought near a reed switch, each pole of the magnet induces one of the two wires. Since they are opposite, they are attracted towards one another, and therefore move towards each other, and close the switch. What is even better about the switch, is that we can make a magnet - an electromagnet - whenever desired.

To start a car, it is necessary to use the battery. But we know that it could be dangerous to touch a metal key that is closely linked with the large E.M.F of a car battery. Here is where the reed switch comes in handy.

The switch in the circuit diagram represents the key of the car. When it is closed, current runs through the solenoid to create an electromagnet. This magnet then closes the reed switch beside it, which then lets the current in the second circuit flow, and allows electricity to reach the motor.

This way of using a reed switch to link two circuits is called a reed relay. The switch in the first circuit, in fact, need not be a switch at all. An L.D.R. (Light-Dependent Resistor) could well be used instead (though perhaps not in a car!). When the light on it becomes strong enough its resistance decreases, and so does its proportional voltage. This then allows enough to reach the electromagnet, closing the switch.

A potential divider is a simple a method of dividing the potential difference in a circuit (or part of it). As you will later see, it is a very good way to control a transistor. The potential divider is basically two resistors connected in series, or else one resistor with a movable separation (that controls the size of each one by its position along the resistor). Two equations can be combined to find the unknown

resistance, or unknown voltage in any such circuit: , and : using the example circuit

, or, more usefully,

These formulae may be used to find any of the five unknowns above. (In fact, there are really only four unknowns, as V1 and V2 can be substituted for one another in the equation). However, using both equations mentioned above often proves to be just as easy.