Room Noise Detector

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

Useful to detect too noisy environments

Circuit diagram:

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.


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