This sensitive sound operated switch can be used with a dynamic microphone insert as above, or be used with an electret (ECM) microphone. If an ECM is used then R1 (shown dotted) will need to be included. A suitable value would be between 2.2k and 10kohms.
The two BC109C transistors form an audio preamp, the gain of which is controlled by the 10k preset. The output is further amplified by a BC182B transistor. To prevent instability the preamp is decoupled with a 100u capacitor and 1k resistor. The audio voltage at the collector of the BC182B is rectified by the two 1N4148 diodes and 4.7u capacitor. This dc voltage will directly drive the BC212B transistor and operate the relay and LED. It should be noted that this circuit does not “latch”. The relay and LED operate momentarily in response to audio peaks.
The gain of the circuit and sensitivity is controlled by the 10k variable resistor on the emitter of the first (left hand side) transistor. A preset may be used if gain is fixed, a potentiometer should be used to trigger at different sound levels.The relay contacts close and then open (momentary action) in response to audio peaks, these can be used to switch other circuit. The diode across the relay is the usual back emf diode and a 1N4003 or 1N4004 will work well here, preventing damage to the transistor.
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4 comments :
Here are a couple of technical faults with the circuit:
The 10k base resistor to the first transistor is far too low. It should be at least 100k.
A low-value resistor will reduce the incoming waveform from the microphone by about 50%.
The electret microphone will produce a waveform of about 2mV to 20mV. The first transistor will have a gain of about 100. The result will be a maximum of 1,000mV
The second transistor is an emitter-follower and will not increase the waveform.
The third transistor has a fixed gain of 10 due to the ratio of the 10k collector resistor to the 1k emitter resistor.
The circuit will produce sufficient waveform (about 1,000mV x 10) to turn on the fourth transistor but the 10u feeding the diodes is around the wrong way and the 4u7 will have a greater effect if it is away from the base by adding a 4k7 resistor.
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