Room Noise Detector

Posted in Sound sensor, on 2015-04-14

Circuit diagram:

Room Noise Detector


  • R1____________10K 1/4W Resistor
  • R2,R3_________22K 1/4W Resistors
  • R4___________100K 1/4W Resistor
  • R5,R9,R10_____56K 1/4W Resistors
  • R6_____________5K6 1/4W Resistor
  • R7___________560R 1/4W Resistor
  • R8_____________2K2 1/4W Resistor
  • R11____________1K 1/4W Resistor
  • R12___________33K 1/4W Resistor
  • R13__________330R 1/4W Resistor
  • C1___________100nF 63V Polyester Capacitor
  • C2____________10µF 25V Electrolytic Capacitor
  • C3___________470µF 25V Electrolytic Capacitor
  • C4____________47µF 25V Electrolytic Capacitor
  • D1_____________5mm. Red LED
  • IC1__________LM358 Low Power Dual Op-amp
  • Q1___________BC327 45V 800mA PNP Transistor
  • MIC1_________Miniature electret microphone
  • SW1__________2 poles 4 ways rotary switch
  • B1___________9V PP3 Battery
  • Clip for PP3 Battery

Device purpose:

This circuit is intended to signal, through a flashing LED, the exceeding of a fixed threshold in room noise, chosen from three fixed levels, namely 50, 70 & 85 dB. Two Op-amps provide the necessary circuit gain for sounds picked-up by a miniature electret microphone to drive a LED. With SW1 in the first position the circuit is off. Second, third and fourth positions power the circuit and set the input sensitivity threshold to 85, 70 & 50 dB respectively.

Current drawing is <1mA with LED off and 12-15mA when the LED is steady on.


Place the small box containing the circuit in the room where you intend to measure ambient noise.

The 50 dB setting is provided to monitor the noise in the bedroom at night. If the LED is steady on, or flashes bright often, then your bedroom is inadequate and too noisy for sleep.

The 70 dB setting is for living-rooms. If this level is often exceeded during the day, your apartment is rather uncomfortable.
If noise level is constantly over 85 dB, 8 hours a day, then you are living in a dangerous environment.

dB  Example of sound sources
20 Quiet garden, electric-clock ticking, drizzling rain
30 Blast of wind, whisper @ 1 m
40 Countryside areas, quiet apartment, wrinkling paper @ 1 m.
50 Residential areas, quiet streets, fridges, conversation @ 1 m.
55 Offices, air-conditioners
60 Alarm-clocks, radio & TV sets at normal volume
64 Washing machines, quiet typewriters
67 Hair-dryers, crowded restaurants
69 Dish-washers, floor-polishers
70 Loud conversation, noisy street, radio & TV sets at high volume
72 Vacuum cleaners
78 Telephone ring, mechanical workshop
80 Passing trucks, noisy hall or plant, shuffling @ 1 m.
90 Passing train, pneumatic hammer, car hooter @ 1 m.
95 Mega "disco", circular saw
100 Motorcycle without silencer


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