Alarm Bells

Alarm bells have a dome size of between six and nine inches, they work on 12vDC and draw about 180mA. There are two types of alarm bells in use:

• Electromagnetic Bells
• Motorised Bells

ELECTROMAGNETIC BELLS

If you refer to the next page you will see a diagram of 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 magnetisim. A metal bar is placed inside the cylinder and this bar acts as a hammer to strike the bell dome. When a voltage is applied to the coil it becomes magnetised, this forces the bar out from inside the cylinder.

OPERATION

In the above diagram we see how an electromagnetic bell operates. We know that 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 where upon the coil will become demagnitised. The bar is then returned to its original position and the circuit is made once again thus repeating the process.

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 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.

MOTORISED BELLS

This is the second type of alarm bell in common use. However 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. This is as much as we will say about these bells as they are very simple in their operation.