Room Noise Detector

One LED monitors three levels: 50, 70 & 85 dB

Useful to detect too noisy environments

Circuit diagram

Room Noise Detector-Circuit diagram


  • 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 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 live 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
author: RED Free Circuit Designs
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