This mechanical vibrations detector is based on a small piezoelectric disc-type converter. It can be used in security systems.
The idea for the circuit came up when somebody decided to put an end to the action of a bad driver who caused damages to many cars in a private parking area. The sensor gave an appropriate alarm and the bad driver "arrested" in no time.
Mechanical vibrations are converted into electrical signals by a small piezoelectric transducer disk. The voltage generated by the piezoelectric converter is around 20mV and is further amplified by an operational amplifier, the IC1A. The amplified signal is rectified from the DS1 diode, then is smoothed by the C5 – R9 network and it is then applied to the inverting input of the IC1B. IC1B acts as a Schmitt trigger comparator. Its threshold can be adjusted from the R6 trimmer. When the vibration signal exceeds the comparison threshold, the comparator output goes to a logic zero and activates a monostable circuit, based on a 555 (IC2) timer.
When activated, IC2 produces a pulse with a specific duration that can be adjusted by the R14 trimmer. With the recommended component values, the duration of the pulse can be from 2 seconds to about 2 minutes. The pulse activates the relay via the TR2. Thus, the relay is activated for a specific duration each time vibrations are detected. The relay is switched off again when the mechanical vibrations ceise, after the timer elapsed.
LED DL1 glows when vibrations are detected while LED DL2 stays on when the relay is on.
The sensitivity of the mechanical vibration detector can be adjusted via the trimmer R6 which, as mentioned above, essentially adjusts the threshold level of the comparator.
There is also a small delay stage consisting of the TR1 and the C10, DS5, R21, R20 and R15. The purpose of this stage is to turn off the circuit for the first 10 seconds after applying the power supply. This is done for two reasons. Firstly, in order not to have incorrect activation of the relay due to transient phenomena and secondly, for some applications the time delay may be useful: for example if we connect the power supply to the circuit when we close a door, and we wish not to have a false alarm due to the door vibrations.
The vibration detector can be easily assembled on the circuit board provided below. The board is double-sided and all components are mounted on it according to the assembly guide of Figure 2.
The body of the piezoelectric disk is actually the one contact of the piezoelectric transducer and must be soldered to the board, in the slot provided for this purpose. The other contact of the transducer is connected to the board with a thin cable.
For better sensitivity, the use of a counterweight is also required. The counterweight can be made of a rigid wire (copper wire 2mm thick or thicker, about 7cm long) that must be soldered to the body of the piezoelectric disk (as shown in Figure 3) and a small weight, such as a pcb terminal connector, (as in Figure 3) to be fitted to the free end of the wire.
All resistors in the circuit are of 1/4W type and of 5% tolerance or better. Care must also be taken with regard to the polarity of the electrolytic capacitors. The circuit requires a 12V power supply, so it can be used in the car as well.