Room Noise Detector Schematic Circuit

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.

Parts List :

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

Use :

• 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.