This 6V battery-operated circuit triggers an acoustic piezobuzzer when a sound is detected. It can also be used as a cheap acoustic-type glass break detector and/or ambient sound level monitor.
The circuit has an ordinary condenser microphone MIC1 as a sound sensor. Sensitivity of this microphone can be changed to some extent by changing the value of the bias resistor R1. When the circuit is powered by a 6V battery through switch S1, it goes into standby mode and the red LED1 lights up to indicate that the circuit is ready for use.
When microphone (MIC1) detects a sound, electrical signal from the microphone is amplified and processed by a small-signal amplifier wired around transistor T1 (BC547). Amplified signal from the collector of T1 is passed to electrolytic capacitor C4 through diode D1 (1N4148). Transistor T2 (BC547) conducts and triggers the monostable built around the timer IC NE555 (IC1). As a result, piezobuzzer (PZ1) at the output of IC1 starts sounding for a fixed duration, determined by the values of resistor R7 and capacitor C5. PZ1 can be replaced with an electromagnetic relay to drive heavy external electrical loads such as power sirens.
Assemble the circuit on a general-purpose PCB and enclose it (including battery) in a tamper-proof cabinet. Glue the condenser microphone at the rear side of the window/door glass to be protected, and connect the microphone to the sensor circuit using a short length of transparent screened cable.
EFY note. Using a glass break sensor may cause false alarms by confusing the breaking of glass such as cookware, or the sound of bells, with the sound of breaking windows.
