noise and vibration detector

11 noise and vibration detector Whether it is the hi-fi next door, the cat purring quietly, or a knock at the door, the detector described here does not miss a thing. Whenever it picks up a sound or vibration, it emits an ear-piercing tone. The circuit is based on the use of an 8 S2 loudspeaker as microphone/ loudspeaker. As the signals from this microphone are very small, they are amplified in Al and rectified. The resulting DC signal is then compared with a reference voltage in A2. When a noise or vibration is picked up by the microphone, the voltage at the inverting input of A2 (pin 6), rises suddenly to about 4 V and then slowly decays to 0 V. The decay time depends on the time constant R6/C3. The voltage at the non-inverting input of A2 (pin 5) is held constant at 0.7 V by R3/R4. When the input at pin 6 rises above 0.7 V, the output of A2 (pin 7) instantly switches to —4 V, which causes the squarewave oscillator A3 to start. The frequency (tone) of the oscillator can be adj...