Home

School Projects for GCSE We often receive letters from students looking for information for various alarm projects for their GCSE's. The sort of equipment an alarm company uses would be too sophisticated and expensive so we have provided some simple alarm circuits which may be of some use to them. Basic Alarm Circuit Basic Electronic Alarm Circuit May we suggest that the another way forward would be to purchase an inexpensive book such as 'Practical Alarm Projects' which is available from Maplin Electronics who would in all probability, also be able to supply the components required. Maplin have a number of shops including one in City Road, Cardiff. Maplins Web Site is:  www.maplin.co.uk   Basic Alarm Circuit   This very basic alarm system is suitable for battery operation as no power is drawn until the alarm is triggered. The key operated (or hidden) switch is used to set and switch off the alarm. When the alarm is switched on, if one of the normally open alarm switches is operated power is passed to the relay which will energise and close the contact switches. The # 1 contact supplies a feed to the relay coil and causes the relay to latch and remain energised until the alarm is switched off. The # 2 contact provides a feed to the siren which will also sound until the alarm is switched off or the battery runs out. A low power consumption siren shoud be chosen and the operating voltage of both the siren and relay should match the battery voltage. Basic Electronic Alarm Circuit     This circuit allows for the use of both normally open and normally closed detecion devices but some power is consumed as soon as the system is switched on. When the alarm is set and one of the detection devices is operated, the relay will energise and latch until the alarm is switched off. If this circuit is used with a mains derived power supply, ensure that the alarm could not create a noise nuisance as the addition of a cut off timer would then be manditory.