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Room Noise Detector Circuit Schematic

One LED monitors three levels: 50, 70 & 85 dB, Useful to detect too noisy environments

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.

Circuit diagram:
 Room Noise Detector Schematic Circuit Diagram
Room Noise Detector Circuit Diagram


R1 = 10K
R2 = 22K
R3 = 22K
R4 = 100K
R5 = 56K
R6 = 5.6K
R7 = 560R
R8 = 2.2K
R9 = 56K
R10 = 56K
R11 = 1K
R12 = 33K
R13 = 330R

C1 = 100nF-63V
C2 = 10µF-25V
C3 = 470µF-25V
C4 = 47µF-25V
D1 = 5mm. Red LED
Q1 = BC327
B1 = 9V PP3 Battery

SW1 = 2 poles 4 ways rotary switch
IC1 = LM358 Low Power Dual Op-amp
MIC1 = Miniature electret microphone


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