The circuit was constructed to provide an alarm circuit that is reliable, simple, small size, and low in power consumption.A cut off time for the alarm is not set on the circuit since it is purposely built to be handy and can be easily manipulated.
There are two identical transistors used in this circuit which could be any of the NPN general purpose audio switches or amplifiers like the family of BC107/108/109 or similar family of 2N3904 that are particularly suitable for use in AF input and driver stages. Every alarm in the circuit is combined to the buzzer with a sound intensity of 82 dB. There are sensors present in the circuit that can be made of wire loops or close reed switches which operates due to the employed magnetic field. To prevent a signal from each sensor from triggering both LEDs, two 1N148 diodes were utilized.
Construction is not critical and there aren’t any traps for the novice. The two 100n capacitors aren’t really necessary, I just included them to make sure that there is no noise interference coming from the long wire loops. For transistors, you can use any NPN general-purpose audio amplifiers/switches (BC 107/108/109, BC 237/238, 2N2222, 2N3904…). Assemble the circuit on perf board. Together with the buzzer and a 9V battery, it should easily fit in a pocket-sized plastic box smaller than a pack of cigarettes. A fresh battery should suffice for weeks of continuous operation
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