Security door alarms are simple electronic devices that trigger when an intruder or trespasser tries to gain access to any designated area. They are a necessary part of today's home security systems. There are a number of electronic configurations available to implement such home security alarms. Here, we will take a look at implementing using a reed switch.
The heart of this circuit is a magnetic reed switch. A reed switch comprises two ferromagnetic blades, separated by only a few microns. When another magnet comes close to these blades, the two blades pull towards one another. Once touching, the blades close the normally open (NO) contacts, allowing electricity to flow. Some reed switches also contain a non-ferromagnetic contact, which forms a normally closed (NC) output. Any approaching magnet will disconnect the contact and pull away from the switching contact.
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Door Security Alarm Circuit
This circuit can be attached to any entry point such as doors, gates, etc, or any storage unit such as safes or drawers, where you want no intruders to trespass. As the safe/door with one terminal of reed switch attached to the surface is opened & brought close to the second terminal of the reed switch placed in close proximity. The magnetic connection is made through mutual inductance & sends a control signal to the base of the first BC547 transistor.