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lm324 and 7809 problem

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jacob.zurasky

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Hello, I have a problem that I cant seem to figure out yet. I have an electret mic hooked up to a lm324 to give my pic a voltage 0-5v depending on sound level. The circuit works perfectly when running from a 9v battery. I switched to a AC adapter and a 7809 to supply the 9 volts. Now the output fluctuates up and down several volts, but sound still causes the level to rise. There are several capacitors around the 7809 and a 0.1uf cap right next to the lm324. I will post some schematics later so you can see exactly how it is configured. Im more of a digital person, so this is not my specialty. -Thanks jacob
 
It sounds like the input to the LM7809 doesn't have adequate filtering.

Are you sure the mains adaptor outputs DC and not AC?

Try adding a 470:mu:F capacitor to the input of the LM7809.
 
Hero, thanks for your advice. Yes the AC adaptor is providing 12v DC, I only have a 10uf at the 7809 input and output, I will try a higher value when I get home. Would a larger capacitor at the output be beneficial at all? I hope that works... -Jacob
 
Post the schematic of your mic preamp and audio rectifier.
I think it works when the supply has an internal resistance like from the battery but it works poorly when the supply has a very low internal resistance from the regulator.
 
Here is the schematic. I got it semi-stable tonight by changing R2 from 220K to 2.2M, but the response wasnt the same. Removing R2 altogether made it completely stable but very little response to sound. Thanks for taking the time to look at this. -Jacob

**broken link removed**
 
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Your circuit has a voltage gain of 10,200 and the mic is powered directly from the power supply which will have some ripple on it. So the chance of feedback is high.

The value of R3 is so low that it is shorting a lot of the mic's signal. Combined with C1 it cuts low frequencies.

C4 is backwards.
 
In addition to Audio's comments, you might want to bypass the supply to the mic bias circuit. This is the most vulnerable place for hum to get into the circuit.
 

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I was thinking that the unbypassed supply for the mic is the cause of the voltage in the circuit varying up and down due to positive feedback of the output opamp modulating the supply voltage.

Usually it is worse with the high internal resistance of an old battery. Then the result is "motorboating".
 
Thanks for your suggestions! I got the circuit working much better now. Bypassing the supply to the mic and changing the 10k to a higher value helped out. Thanks again! -Jacob
 
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Em, Audioguru how did you get voltage gain of 10,200? I'm getting something like 10,000 (100*100).

The 0.1uF cap b/w the MIC and the 324 is for filtering out the dc components of the sound? What of the other capacitors? I see 10uf capacitors in the feedback loop of both 324's. Can any tell me what they are doing there?

Capacitors sure are the most difficult part of Analog Circuit design!
Thanks :)
 
Yes, the component tolerances will give a greater error.
 
The old LM324 opamp has a gain of about 100,000 at 2 Hz so there will be an extremely small error when its gain is reduced to 101 by negative feedback.
But at 20kHz its gain is only about 30 so the error is huge. It hardly has any output at 20kHz anyway because its output can't slew that quickly.
 
But since he only wants a simple 0-5V according to the sound level the higher frequencies are unimportant so an LM324 will do.
 
20kHz is sound. Maybe he wants to pickup only the low frequency sound from his neighbour's sub-woofer with the lousy old LM324.
 
O sure, sure I now see how:
Vo/Vin = 1+(100k/1k) = 101. Is there just any other reason why C3 and C2 are there? At low frequencies it seems our feedback loop is completely cut off from ground?
 
aibelectronics said:
Is there just any other reason why C3 and C2 are there? At low frequencies it seems our feedback loop is completely cut off from ground?

C2 and C3 cut the frequencies below 16Hz so the DC input offset voltage (max 7mV) of the 1st opamp isn't amplified 10,201 times which could cause the 2nd opamp's output to be welded to a supply rail.
 
16Hz? What about 16Hz? Is it a magic number? :)
A quick check on the datasheet shows the LM 324 as an offset of 2mV.
 
aibelectronics said:
O sure, sure I now see how:
Vo/Vin = 1+(100k/1k) = 101. Is there just any other reason why C3 and C2 are there? At low frequencies it seems our feedback loop is completely cut off from ground?

At low frequencies the op amps act like unity-gain followers. Feed-back is provided by R5 and R7. The DC bias voltage you apply to the non-inverting input and DC offset voltages are not amplified (Av=1).
 
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