This acoustic sensor was originally developed for an industrial
application (monitoring a siren), but will also find many domestic
applications. Note that the sensor is designed with safety of operation
as the top priority: this means that if it fails then in the worst-case
scenario it will not itself generate a false indication that a sound is
detected. Also, the sensor connections are protected against polarity
reversal and short-circuits. The supply voltage of 24 V is suitable for
industrial use, and the output of the sensor swings over the supply
voltage range.
Circuit diagram :
Simple Acoustic Sensor Circuit Diagram
The circuit consists of an electret micro-phone, an amplifier,
attenuator, rectifier and a switching stage. MIC1 is supplied with a
current of 1 mA by R9. T1 amplifies the signal, decoupled from the
supply by C1, to about 1 Vpp. R7 sets the collector current of T1 to a
maximum of 0.5 mA. The operating point is set by feedback resistor R8.
The sensitivity of the circuit can be adjusted using potentiometer P1 so
that it does not respond to ambient noise levels. Diodes D1 and D2
recitfy the signal and C4 provides smoothing. As soon as the voltage
across C4 rises above 0.5 V, T2 turns on and the LED connected to the
collector of the transistor lights. T3 inverts this signal.
If the microphone receives no sound, T3 turns on and the output will be
at ground. If a signal is detected, T3 turns off and the output is
pulled to +24 V by R4 and R5. In order to allow for an output current of
10 mA, T3’s collector resistor needs to be 2.4 kΩ. If 0.25 W resistors
are to be used, then to be on the safe side this should be made up of
two 4.7 kΩ resistors wired in parallel. Diode D4 protects the circuit
from reverse polarity connection, and D3 protects the output from damage
if it is inadvertently connected to the supply.
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