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Topic: need some assistant on the op-amp circuit attached in the thread. (Read 18245 times) previous topic - next topic

yaantey

#30
Jan 02, 2012, 06:21 pm Last Edit: Jan 02, 2012, 06:29 pm by yaantey Reason: 1
Hi,
Can I directly connect the electret microphone to oscilloscope to estimate the maximum output voltage from it so that I would know how much amplification is needed. I know noise and stuff will effect and I am going to test considering it. I'm going to scream at the microphone and try some loud music too. Attach is the sort of microphone i am going to use to giv u an idea (but not exactly the same model). Do u think I can hook it directly to oscilloscope to test?

Thanks.

Grumpy_Mike

It depends on two things:-
1) The maximum sensitivity of the oscilloscope input.
2) If the microphone requires a voltage to make it work.

yaantey

Attached is the microphone type i am using. You think it can work if i connect to oscilloscope?

dc42

If you connect a 10k or so resistor from the +ve side of the mic to the +ve side of a 9v battery, and connect the -ve side of the 9v battery to the -ve side of the mic, and switch your oscilloscope input to AC, then you should be able to see the mic signal on the scope and thereby estimate the signal strength.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Techone

@yaantey

I happen to have a few of those mics, so I did this experiment using my scope I have. Well I set my voltage setting at 2 mV/div and I did get something, ( I was blowing on it ) and I place close to the speaker of my DIY AM radio, I did get something ( a sound waveform ) and try to measure it, it between 2 mV to 5 mV ( peak-to-peak ). If you feel the op-amp circuit do not amplify enough, you can still add another amp.  Usualy, when I design this type of system, I add  a transistor for the fisrt stage and the second stage a op-amp. Then I add a filter ( in your case a band pass filter set around the baby cry frequency ) and amplyfy and buffer to going into a analog input of the arduino.

And I will use a electronic circuit simulator to check the design and change some values ( resistors and capacitors ) if needed - if it improve or not.

 

yaantey

#35
Jan 03, 2012, 06:30 pm Last Edit: Jan 03, 2012, 06:54 pm by yaantey Reason: 1
I manage to test the voltage output from the electret mic and attach are the output from the mic when I shouted at it, played music and did nothing. All outputs were produced when I shouted and played music in very closed proximity to the electret mic, almost touching it. But unlinke Techone I found the my electret mic produce 23 mVpp. I got 8 mVpp without music and shouting, and by shouting I got 31 mVpp, hence 23 mVpp.

Attached are the output and the simple connection used, does the reading seem good?

Techone

My experiment simply measure the output of the mic without voltage and resistor.  After net surfing, search : electret microphone ,I come across this site : http://www.openmusiclabs.com/learning/sensors/electret-microphones
Which I did know about the inside JFET, so DC42 is correct, it need a resistor and a voltage supply. I wonder if you will get better result by changing the resistor value ( lower or higher ) ? Hum , maybe...

@yaantey

The reading seem right.

ajofscott

Electret mics make voltage by the effects of altering dielectric thickness by sound waves. There has to be a charge across the capacitance for dielectric changes to manifest as voltage changes. C=Q/V, where C= capacitance in Farads, Q= charge in coulombs, and V = plate differential in Volts. Rearranged we get V=Q/C, thus anything that increases C reduces V and visa versa. If V=0 because the cap was never charged we will never see that C changed at all.

yaantey

Hi,

I have downloaded a baby crying voice from (http://www.soundjay.com/baby-crying-sound-effect.html) and used audacity to analyse the frequency. I have got the graph but I am not quite sure how I can find the fundamental frequency. I have attached the .txt file and the graph. Can someone who know the stuff point me to the fundamental frequency. Thanks.

Grumpy_Mike

It's a baby crying, it doesn't have a fundamental frequency.

yaantey

Atleast there should be a frequency of baby's voice so that I will be able to make a band pass filter for the circuit. Or else how will i be able to do it.

Grumpy_Mike

Quote
At least there should be a frequency of baby's voice

No it is all the frequencies in that graph.

Quote
Or else how will i be able to do it.

You can't on an arduino.

Techone

@yaantey

You need a band pass filter using op-amps. By looking at the graph, from 500 Hz to 11 000 Hz is where the baby is the loundest ( higher db value : -50 db ( lound ) is higher than -95 db ( silence ) ) The graph represented the a fourier analysis responce of a baby craying.  So design a op-amp circuit - could be more than one op-amps at those frequency mention. And I notice the lounder point is around 1 KHz whish make sense.
 

ajofscott

Play it through an 11 band EQ tweak on the controls and find the combination that gives the dryest result. "Dry" loosely meaning harsh and lacking in lower frequencies. And a single passive notch filter could be made using the second amp in the dual package for the filter.

yaantey

I am trying to come up with my own circuit to amplify the signal from mic. I would like to know these terms because when ever I see op-amp amplifying circuits it comes up. I tried going trough somethings on the internet and keep getting confused. So can someone explain to me in a simple way what dc biasing (is it same as bias voltage?), dc offset and dc coupling means.

Thanks

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