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 and 
throws. 
 
There are four main types of mechanical switches used in circuits. The description given to these depends on the function of 
the switch. 

 

 
The single throw switches are either open or closed. Double pole single throw switches are often used in mains circuits to 
disconnect or isolate a connection. The double throw switches are used to change from one part of a circuit to another. 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. 

 

 
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: 

 

 
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. 
 
 
Transducers 
A 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.