Design & Technology
Switches, relays and transducers
Switches, relays and transducers are mechanical or electrical devices which are used to connect and disconnect circuits (switches and relays) or to produce an output from a circuit by changing one form of energy into another (transducers).
Switches
Switches are devices for connecting and disconnecting circuits. Different types of switches have different numbers of poles [poles: output terminals or contacts in a switch ] and throws [throws: circuits or conducting positions controlled by each pole of a switch ].
There are four main types of mechanical switches used in circuits. The description given to these depends on the function of the switch.
| Switch type | Diagram |
|---|---|
| Single pole single throw (SPST) | |
| Single pole double throw (SPDT) | |
| Double pole single throw (DPST) | |
| Double pole double throw (DPDT) |
The single throw switches are either open or closed. Double pole single throw switches are often used to switch between two outputs. The double throw switches are used to change from one part of a circuit to another. The double pole double throw can be used to reverse the direction of a motor. The dotted lines on the symbols indicate that both sides move together.
Switches are available in a wide range of forms that include rotary, key, membrane, mercury, tremble and toggle.
Push switches
Push switches are a type of SPST switch. They can be either Push to make switches, which are normally open (NO) or Push to break switches, which are normally closed (NC).
Push switches
Relays
Relays are a special type of switch which is turned on and off by an electromagnet. Below is a diagram of a simple relay. When a current flows through the coil an electro-magnetic field is set up. The field attracts an iron armature, whose other end pushes the contacts together, completing the circuit. When the current is switched off, the contacts open again, switching the circuit off.
Relay switch
A useful property of relays is that the circuit powering the coil is completely separate from the circuit switched on by the relay. For this reason relays are often used where a safe low-voltage circuit is being used to control a high-voltage circuit. The symbol for a relay makes the separation of the two circuits clear by separating the coil symbol from the switch symbol:
Relay symbol
The relays used in schools generally a have voltage between 6V and 15V. When using a diode with a relay, be sure that it is connected across the poles (terminals) of the relay to prevent damage to the transistor from back EMF generated when the relay coil switches off.
Output Transducers
An output transducer is an output device which works by changing one form of energy (eg electrical energy) into another form of energy (eg sound or light energy).
Loudspeakers
Loudspeakers (and headphones) are output transducers that change electrical energy into sound. Most loudspeakers are moving coil types. The core of the loudspeaker vibrates in and out very quickly. This produces sound waves that can be picked up by the human ear.
Most loud speakers have a resistance value of 8 Ohms. You need to take this value into account when matching the loudspeaker up with the electronic circuit that will operate it.
Digital displays
Digital displays are transducers that change electrical energy into light energy. Digital displays can be found in watches, microwave ovens, mobile phones and car dashboard displays.
Two common types of digital display are seven-segment displays and liquid-crystal displays. Resistors should be used in series with the displays to prevent the diode segments from being damaged.
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