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Noise Detector

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Jaydu1904

New Member
Hi Friends,I am Building a Room Noise Detector....
But I am Facing some problems with it, can anyone please suggest the corrections that should be done to work circuit properly????

The list of components and ckt diagram is here....

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____________10΅F 25V Electrolytic Capacitor
C3___________470΅F 25V Electrolytic Capacitor
C4____________47΅F 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. NoiseDet.gif
 

crutschow

Well-Known Member
Most Helpful Member
Yes, it's difficult to know what your "problem" is without you telling us. There are very few psychics in this forum.
 

audioguru

Well-Known Member
Most Helpful Member
The transistor should have a diode from base (anode) to collector(cathode) so that C4 can discharge.
 

audioguru

Well-Known Member
Most Helpful Member
C4 is quickly charged by the base-emitter diode of the transistor which is about 20 ohms. Then it is only partially discharged very slowly by R4 which is 100k ohms. So C4 keeps charging and charging until the transistor does not work anymore.
The added diode discharges C4 on each half-cycle.
 

marcbarker

New Member
Hmmm :)

[DejaVousMode=ON]
I think that part of the circuit works OK to me.
I think the transistor is a class c switch!
[/off]

I'm not 100% sure though.
 
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audioguru

Well-Known Member
Most Helpful Member
When there is loud sound then C4 charges through the base-emitter diode of the transistor which turns on the transistor and the led.
But C4 cannot discharge quick enough into R12 so C4 charges to a higher and higher voltage on each half-cycle of the sound until the transistor does not conduct anymore.

Adding a reversed diode across the base-emitter of the transistor will discharge C4 on each cycle of sound, or changing the circuit so that it is DC-coupled.

When the circuit is DC-coupled then it can cancel the forward voltage of the base-emitter voltage of the transistor which changes with temperature changes.
 

marcbarker

New Member
C4 charges to a higher and higher voltage on each half-cycle of the sound until the transistor does not conduct anymore.
Is this all that was wrong with it? That's good analysis, and without the OP saying anything too :)

Adding a reversed diode across the base-emitter of the transistor will discharge C4 on each cycle of sound
And the LED conduct on half-cycles (half brightness)?

, or changing the circuit so that it is DC-coupled. When the circuit is DC-coupled then it can cancel the forward voltage of the base-emitter voltage of the transistor which changes with temperature changes.
Because Vbe drop variation over temperature alters the calibration slightly?
 

audioguru

Well-Known Member
Most Helpful Member
The LM358 has inputs that work at its negative power supply voltage that is ground in this circuit. It can rectify the input signal and directly drive an LED without the inaccuracy of the base-emitter voltage of a transistor. Then the coupling capacitor in the original circuit is not needed.
 

Jaydu1904

New Member
How to modify it???

Hi, All of u guys, thanks 4 d help....
now dat d circuit is working, can anyone PLEASE tell me how to modify this circuit so that i can have different LEDs shown for different ranges of audio input??? (i.e. LED1 for 20-30dB,
LED2 for 30-40dB and so on...)

please guys help me....
 

marcbarker

New Member
So what did you do to make this (allegedly) non-functioning circuit work?

Other than those problems pointed out above, did you have to do any more corrections to the circuit ?




To make it display 3 ranges at the same time, I think you'd need to replicate some of the circuit 2 more times.
 
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Jaydu1904

New Member
The LM358 has inputs that work at its negative power supply voltage that is ground in this circuit. It can rectify the input signal and directly drive an LED without the inaccuracy of the base-emitter voltage of a transistor. Then the coupling capacitor in the original circuit is not needed.
Hi audioguru, thanks for that solution, now can you please explain me the circuit diagram component by component??? I have stuck on working of IC1A as I am unable to find its exact function....

or you can give the link where u have already explained this circuit.... PLEASE
 

audioguru

Well-Known Member
Most Helpful Member
Hi audioguru, thanks for that solution, now can you please explain me the circuit diagram component by component??? I have stuck on working of IC1A as I am unable to find its exact function....
IC1A is a very simple opamp non-inverting amplifier. Its gain is 1+ (R9/R switched). it is biased at half the supply voltage by R2 and R3.

IC1B is another simple opamp non-inverting amplifier. Its gain is 1+ (R10/R8).
It is biased by the output DC voltage of IC1A.

C2 filters the half the supply voltage bias voltage. R4 is the input resistance of the IC1A amplifier.
 

Jaydu1904

New Member
IC1A is a very simple opamp non-inverting amplifier. Its gain is 1+ (R9/R switched). it is biased at half the supply voltage by R2 and R3.

IC1B is another simple opamp non-inverting amplifier. Its gain is 1+ (R10/R8).
It is biased by the output DC voltage of IC1A.

C2 filters the half the supply voltage bias voltage. R4 is the input resistance of the IC1A amplifier.
Ya i understand that but how do we get the exact noise level at the output???
what exactly happens in IC1A and IC1B?? How do they they compare our input signal(equivalent electrical signal) with the biasing voltage and give us the output???
 

Mikebits

Well-Known Member
C4 is quickly charged by the base-emitter diode of the transistor which is about 20 ohms. Then it is only partially discharged very slowly by R4 which is 100k ohms. So C4 keeps charging and charging until the transistor does not work anymore.
The added diode discharges C4 on each half-cycle.

Ag, you are a brain stud muffin, I think I want your baby... :D
 

marcbarker

New Member
1. how do we get the exact noise level at the output???

2. what exactly happens in IC1A and IC1B??

3. How do they they compare our input signal(equivalent electrical signal) with the biasing voltage and give us the output???
1. I presume you mean "how does this NoiseMeter know how loud a 'noise' is"? A: Because it just does, OK! I think you want to know how it works? If so, maybe we've just gone full circle after the last 12 postings!...

2. I presume you've never seen an opamp before. I presume you're already quite familar with OhmsLaw, if you're not, then you need to be! ICA/B is a opamp. All it does is react to the voltage on its + & - pins. It doesn't like seeing different voltages on its inputs. If it ever sees a difference, it'll move the output to whatever voltage is required to balance the inputs within a few mV of each other. Opamps are really cool, they make good amplifiers, like the circuits based on IC1A and IC1B.

3-A Some text can be written, explaining detailed operation, i.e. suitable for cutting and pasting into an assignment. But you might not know confidently enough if the information is correct or not.

or...

3-B We can work together to help you understand the circuit, then together write and check as we go, ending up with a detailed explanation.

Then if anyone cuts and pastes that it into their assignment, hopefully the smart plagarism police will come here with their 'text search' thingy, hopefully see that you were the one who'd done lots of work on this, and maybe you'll get the credit!
 

sivasankar

New Member
room noise detector corrections

hi friends iam building a room noise detector circuit the circuit does not working properly please tell me the modifications to it i have seen that it is missing a diode what is the diode number and specification where it should be placed
 

corzy

New Member
i do this project... how this circuit know the level of db? how it calculate?
 
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