Simple Alarm Generator. Design Trick 8

You can make a simple alarm generator based on an oscillator to use in burglar alarm circuits. It uses only two transistors and a few other components. Oscillator is a circuit that generates AC pulses from DC. The AC pulses thus generated can be converted into audio frequency using a speaker. Let us see how it works.

C2 and VR1 are the RC network in the oscillator. C2 is a disc capacitor and VR1 is a variable resistor (Preset). By adjusting the wiper of VR1, its resistance can be varied to select different tones. When power is switched on, a potential difference appears across C2. The point A on one side of C2 becomes positive (current flows through VR1) and point B becomes negative (Grounded through speaker). Since T1 gets a positive voltage in its base, it conducts. This pulls the base of T2 and it also conducts. The point B then gets a higher positive potential and lower negative potential. This is due to the resistance of the speaker coil. As a result C2 discharges and point A becomes negative which turns off T1. This switches off T2 and oscillation stops. C2 again charges via R1 and VR1 to repeat the process. Since the value of C2 is too low, the charging/discharging cycle is very fast and the oscillations generate an Alarm tone through the speaker.