Magnetic Reed Contacts

These are probably the most extensively used detection devices in alarm systems due to their simplicity reliability and resistance to tampering. The main use for contacts is in detecting the opening of doors, windows, skylights, roller shutters, etc.

OPERATION

If you refer to the above diagram you will see that the contact is made up of two parts. One is a strong magnet and the other is a reed switch. This reed switch is composed of two metal strips enclosed in a sealed glass tube. These strips are placed so that they are overlapping but not touching. When the magnet is brought to within 15mm (approx) of the reed, the strips become magnetised and attract each other thus closing the circuit. When the magnet is moved away the strips become demagnetised and the circuit opens again.

WIRING ARRANGEMENTS FOR CONTACTS

Dry terminals: on many contacts extra screw terminals are provided to secure wires which have been joined together to complete a circuit. These terminals are not connected electrically to the detection device, they are simply provided for the installation engineers convenience