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Please help with Electret Op Amp LED circuit

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Luminaux

New Member
Hello, I'm trying to design a sound to light circuit.
I found this:
http://www.electro-tech-online.com/threads/simple-sound-to-light.93961/?highlight=opamp
which is probably what you all will tell me I should use, but I was trying to make this as compact and cheap as possible, and that seems a little more complicated then what I was hoping for. And I don't even know what some of those parts are... like the "U1OPTO".
I have a working circuit, but I'm new to electronics and I'm confused why my circuit isn't working as well as I'd like it to.

Here's the circuit I've been using.
http://terraspira.com/ElectretOpAmpLED.bmp

I had it working better at one point, but I was stupid and didn't write down the details. So the values I found to be best this time around are:
Rm = 1000Ω, C1 = 1 µF, Rin = 100Ω, Rf = 10^6Ω

It works, but the voltage outputting from the op amp seems low, and the mic isn't sensitive enough, which I suppose means the op amp isn't applying enough gain, right?
According to my understanding if I make Rin 100Ω and Rf 10^6Ω the gain should be 10000, and if I make Rf = 10^9Ω the gain should be 10000000, but it seems to be maxing out at around 10^6Ω.
I tried putting a NPN transistor on the output and it didn't seem to do diddly! Can someone tell me what I would need to do to use a transistor? If I ever get the mic sensitive enough I would really like to be able to run more than 10 LEDs (the max output of this op amp is 200mA and the LEDs run at 20mA max).

I originally had a resistor in line with the LEDs too, like this:
http://terraspira.com/ElectretOpAmpLEDwRd.bmp
(which I thought was necessary since I don't want to kill my LEDs with the "gained" output of the op amp) but when I removed the resistor the LED was brighter, which I still don't really understand... something about the op amp and the mic taking voltage?

I know there are probably many things I'm doing wrong, as I haven't had any training is circuit building, other than what I've given myself. I would very much appreciate any help anyone has to offer. But hey, be nice, I'm a newbie.

Thanks in advance, ~Paul.
 

BrownOut

Banned
It's hard to say without knowing the spec on your microphone. If you were able to measure output current and resistance, we might be able to help more. One think I though of... make sure you have the connections to your op-amp correct! Many people get turned around. I think the inverting input (-) is pin2, and the non-inverting input (+) is pin 3. If that doesn't work, try connecting the op-amp in the non-inverting configuration ( basically, connect the signal to the (+) input, and connect the resistor labeled "Rin" to ground using the lead that is presently connect to C1, leaving the other lead connect to the (-) input.

Good luck!
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
You don't need the spec's for the microphone because electret mics have almost the same sensitivity. It is easy to set the gain of the preamp to whatever you want.

You are powering the mic with a 1k resistor from 5V which should be a 10k resistor from 8.5V.
Your opamp is inverting with a 100 ohm input resistor that is killing the output from the microphone. The opamp should be connected as non-inverting that has 100k biasing resistors for a high input impedance. Then the mic will not be loaded down.

Notice that I said, "biasing resistors? Your opamp has its input without any bias and most opamps will not work without an input bias voltage.
You forgot to say which opamp number you used.

Here is a proper preamp circuit for an electret mic:
 

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