How to build Room Noise Detector

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

Circuit diagram

Parts:
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.

Use:
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