Simple Sensitive Vibration Detector Alarm Circuit

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 I amplified in A1 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 W 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 adjusted by preset potentiometer Pl. The oscillator output (pin 8) is fed to amplifier stage Tl which drives the loudspeaker. The oscillator will continue to run however, so C3 charges steadily and will keep the output at pin 7 of A2 negative. As this is not the purpose of the circuit, the incoming signal must be interrupted somewhere in the chain. To do this, an FET, T2, is used as a switch. As soon as the output of the comparator becomes negative, D3 conducts, T2 is cut off and the incoming signal is interrupted. When C3 has discharged to the extent that the voltage across it drops to below 0.7 V, the output of A2 (pin 7) becomes positive, D3 is cut off and T2 conducts.

This should, however, not happen too rapidly, otherwise there is the risk that a false alarm may be given. Therefore, the gate (drive input) of T2 is connected to earth via capacitors C2 and C8. The consequent delay ensures that the circuit is not reactivated before half a second after the loudspeaker has gone quiet. The earth potential is fixed by the voltage divider R9/R10 and im- pedance converter A4, which derive a symmetrical supply of i 4.5 V from the 9 V battery. When T1 conducts, the supply voltage will drop a little because a battery cannot deliver energy as well as a mains power supply. It can therefore happen that the output signal of A3 is superimposed on the supply voltage. This undesired feed- back should be prevented by C5 and C6. lf in spite of these capacitors diffi- culties are encountered, it may be beneficial to increase the values of R5, C2 and C8 by trial and error. lf that fails to improve matters, increase the value of capacitors C5 and C6.