Generally speaking, alarm bells used for intruder alarm systems have a dome size of between six and nine inches, the bigger the louder. As will most other devices in alarm systems they work on 12vDC and draw about 180mA. Again the bigger the bell the more current it will draw. Check when installing to ensure you have a big enough back up battery. (see section on power supplies and batteries).

There are two types of alarm bells in use:

1. Electromagnetic Bells 

2. Motorised Bells 

This is the second type of alarm bell it is not as loud and therefore not as popular as the electromagnetic type. It operates by using a motor to spin a hammer which strikes the bell dome. They are very simple in their operation and rarely used in alarms these days.

The main bell type is the electromagnetic type so this is the one we'll go through in bit more detail...

ELECTROMAGNETIC BELLS

If you refer to the diagram you will see how an electromagnetic bell works.

An electromagnet is simply a hollow cylinder with a number of turns of wire wrapped around it. When a voltage is applied to this coil it becomes a magnet, when the voltage is removed the coil looses its magnetism. A metal bar is placed inside the cylinder and this bar acts as a hammer to strike the bell dome.

OPERATION

In the diagram we see how an electromagnetic bell operates. When a voltage is applied to the coil this will become magnetised and push the bar out from the center of the coil striking the bell dome. The circuit is made through the bar itself and as it moves it breaks this circuit whereupon the coil will become demagnitised. The bar is then returned to its original position whit the hel of a spring and the circuit is made once again thus repeating the process. All this is housed inside the bell dome so as to prevent tampering.

NOTE...

This type of bell has one drawback and that is it produces a lot of E.M.I. (electro-magnetic interference). This can disrupt the operation of microprocessor based control panels and so a suppressor is fitted to these bells to counteract this problem. As a result most alarm bells will have their terminals marked with a positive (+) and negative (-) symbol.

INSTALLATION

Most bells will be installed inside a bell box and come with a detachable back plate. This is screwed to the surface first and the wiring connections made. The bell itself is then fixed to this backplate by means of a retaining screw.