This Moosquito repellat circuit uses two BC547 transnistor as astable multibibrator. The Astable Multivibrator, which is generally used as a signal generator, is once again used here to generate the desired frequencies. It is an excellent example of the fact, how versatile simple basic electronic circuit can be.
Astable Multivibrator Circuit Operation
When T1 is conducting T2 is off and when T2 is conducting, T1 is off. The capacitors C1 and C2 contributes decisively to this ON/OFF cycles for the transistors T1 and T2. The time taken by C1 and C2 to charge and discharge decides the shapeof the output waveform. Another important factor in the operation of the circuit is the fact that the transistor goes into conduction only when the base-emitter voltage exceeds 0.7 volts (for silicon transistors). From this basic knowledge we can visualise how the transistors exchange their roles and how the voltage on the collector of each transistor jumbs between the lower and upper level, producing a rectangular waveform. If you take a close look atcircuit, you will notice that C1 and C2 are not equal. They differ in their values by a factor of four.
The output signal will thus be a non symmetrical waveform. Such a non symmetrical signal contains more high frequency harminics compered to the normal square wave signal. The output of our circuit will have the basic frequency of 5 KHz along with harminics of 10, 15 and 20 KHz. If some insects are deaf to frequencies upto 5 KHz, they may react to 10 KHz or 15 KHz or even 20 KHz, one never knows ...
The piezo buzzer used should not have an internal oscillator built into it. The circuit consumes 0.3 ma current, and can give about 1500 hours of nonstop operation.
Parts List:
R1,R4 - 10 K Ohm
R2,R3 - 560 K Ohm
C1 - 82 PF
C2 - 330 PF
T1,T2 - BC547
Piezo Buzzer (Without internal oscillator)
Source: http://www.flashwebhost.com/circuit/mosquito.php
