tekHPoynt

T
he sound generator shown here uses its built-in loudspeaker as a microphone when it is in the stand-by state. As soon as it detects a noise that exceeds an adjustable threshold level, it becomes active. This small, clever circuit can be used as an alarm generator that reacts to noises. You can also use it to help you locate an object in response to a loud sound, such as clapping your hands or whistling loudly.

The gate at the bottom of the schematic diagram acts a as linera low-frequency amplifier, due to the negative feedback resistor R3. It receives its input signal from a loud speaker LS1, which acts as a microphone when T1 is cut-off. The amplified output signal from this gate passes via C1 to a second 4011 gate, which triggers a monostable formed by the two gates located before and after C5 and R5. The DC threshold level is applied to pin2 of IC1 via R2. This DC voltage is superimposed on the signal from C1. Retriggering of the monostable is prevented by the combination of R4, C4 and the first gate, in addition to which the sound generator (IC2) prevents the loudspeaker from acting as a microphone once it has been activated.