Noise Detector for Isolated Room

Figure 1 

Parts List

The circuit was designed to provide an indication of an environment or room noise that exceeds a fixed threshold through the use of a flashing LED.

• LM358 – a dual operation amplifier consisting of two independent, high gain, internally frequency compensated op-amps which were designed specifically to operate from a single power supply over a wide range of voltages having advantages of eliminating the need for dual supplies, compatibility with all forms of logic, and power drain suitable for battery operation, and with features of low input offset voltage, large output voltage swing, and very low supply current drain
• BC327 – a PNP general purpose transistor in a TO-92 package, used for general purpose switching and amplifier applications which is suitable for AF driver stages and low power output stages of audio amplifiers due to its features of high current at 500 mA maximum and low voltage at 45 V maximum

Only a single red LED is used during the operation of the circuit that flashes when the noise goes beyond the fixed levels of either 50 dB, 70 dB, or 85 dB. The LED is being driven by two op-amps that provide the required gain of the circuit for the audio signal that is being perceived by a miniature electrets microphone. The first position of switch SW1 sets the circuit in the OFF state. The second at 85 dB, third at 70 dB, and fourth at 50 dB positions start to power the circuit while the threshold sensitivity input is being set. The current consumption of the LED at OFF state is less than 1 mA while at steadily ON position, it draws a current of 12 mA to 15 mA.

The circuit may be placed in a small box and be situated in the room where the ambient noise is intended to be measured. During night time, the noise in the bedroom is monitored by setting the circuit at 50 dB. The room can be concluded to be too noisy for sleeping purpose if the LED is often flashing brightly or steadily ON. For living rooms, the 70 dB setting is preferred and can provide a conclusion that the apartment is rather uncomfortable if the level is often exceeded during the day of monitoring. Finally, a noise level monitored 8 hours a day. is continuously exceeding 85 dB, it can be concluded that the place is not safe enough for the tenants.

To illustrate, here are some examples of noise levels and their corresponding sources:

Figure 2 

One of the advanced usages of this noise detector circuit is on mobile phones where the ringing volume is dependent on the ambient noise. The ring tone volume is adjusting depending on the background noise. An example is a nice and quiet or vibrating ring when inside a quiet library and a loud or full volume when inside a noisy bar. This is possible by the ambient noise level measurement via microphone.

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