Room Noise Indicator - Online Article


Now days we are living in very Noise polluted Environment therefore it is very important to know, what is the maximum level of Noise that would be tolerated. Here is a Sensor that detects the Noise Level in the Environment.

Sensor detects the three levels of Noise 50, 70 and 85 dB.

dB: Decibel - Unit of Sound/Noise

Noise Level

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

Circuit Diagram


Circuit Description


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 ----------------10F 25V Electrolytic Capacitor
C3 ----------------470F 25V Electrolytic Capacitor
C4 ----------------47F 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


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. The IC LM358 is the low power dual op-amp IC. The first Op-Amp of IC i.e. IC1A which is used as Op-Amp as an adder and another one is used op-amp as amplifier. With SW1 in the first position the circuit is off. In Second, third and fourth positions power the circuit and set the input sensitivity threshold to 85, 70 & 50 dB respectively. We use a slider switch SW1 which has two parts SW1A & SW1B. It is used to define the three levels of noise and respective input at pin 2 of IC1A. The transistor Q1 is the BC327 45V 800mA PNP Transistor which is used here as collector -base -emitter configuration.

From source B1 current i divided into two parts the first part i1 flows from SW1B to SW1A (dotted line) which is i/p1 of IC1A at pin 2 which work as Op- Amp as an adder. The current i2 flows in the branch of circuit which joined through slider switch SW1B. This current again divided into two parts i3 and i4. The current i3 flows into transistor Q1 which is a bipolar junction PNP transistor and the configuration of this transistor is collector-base-emitter (C-B-E). The other part of current i4 passes through R11 to R2 which adds to the i/p 1 of IC1A but the major part of this current i4 passes through R11 to R1 and to MIC1 where it is an i/p for MIC1 i.e. noise. This point becomes short circuited and the current passes through MIC1 that becomes the I/p of IC1B at pin 6. The current which passes from R1 also passes through C1 and becomes i/p2 for IC1A at pin 3. The output of IC1A at pin 1 becomes i/p of IC1B at pin 5. Here IC1B work as Op-Amp as an Amplifier which amplifies the o/p signals of IC1A. The output of IC1B now passes through capacitor C4. When this capacitor fills up then it passes the current to the base of transistor Q1. Now the base-emitter junction of transistor is activated and we get the output at diode D1 with respect to the given i/p level of noise that can be set using SW1.

Current drawing is <1mA with LED off and 12-15mA when the LED is steady on.

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