Acoustic Sensor

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.
The circuit consists of an electret microphone, 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.

Acoustic Sensor Circuit

Acoustic Sensor Circuit Diagram

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.

Author: Engelbert Göpfert – Copyright: Elektor Electronics

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