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Any of you analog whizzes want to take a crack at this?

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John Sorensen

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Here's what I want to do:

I have a reverse osmosis unit that trips on underpressure. When it trips it sounds an alarm that is at about 2-3 kHz (guessing) and plenty loud. I want to be able to detect the alarm without tampering with the circuit. So I was going to use an electret mic:

mic->amplifier->band pass filter->rectifier->low pass filter->uC.

As I've mentioned before, I'm an analog rock, and I'm stuck back on the amplifier stage. I thought I'd be doing good with an INA122 differential amp, but the DC offset is killing me. I think if I was using split supplies it would be a lot easier, but I want to use a single ended +5. I also don't want to use any exotic parts, I have a good supply of all the garden variety amps and transistors.

Anyone want to take a stab?

j.
 
You don't want to use a circuit which reads DC bias. Use a feedthrough cap to get rid of any DC component.
 
Why not just run the mic->amp->micro. Just do the filtering in the micro, since you are just detecting the signal, the filter is straightforward. Depending on the waveform that you get from the mic, you could just square up the signal and count edges to find the frequency and therefore detect the fundamental of the siren.
 
Why not use as Mic, into an amplifier and than into a "Touch Tone Decoder" such as an LM567. It can trigger an alarm directly from the output.

I Can also Supply you one if needed.
 
Why not just run the mic->amp->micro. Just do the filtering in the micro, since you are just detecting the signal, the filter is straightforward. Depending on the waveform that you get from the mic, you could just square up the signal and count edges to find the frequency and therefore detect the fundamental of the siren.

Because my itty-bitty eight bit micro has other things to do :wink:

Why not use as Mic, into an amplifier and than into a "Touch Tone Decoder" such as an LM567.

Yeah, that looks like the easy way to go. And I see they're less than a buck at Digikey. Hmmmm. Yeah, I'll probably buy a couple, but I'm still going to try my plan for the experience of it.

I Can also Supply you one if needed.

Generous as always, Chemelec, but it would cost as much to post it as it is to buy :)

For some reason, and I don't know how or why this works, but I put my signal from the mic into a high pass filter (so it was about 5mVpk-pk about ground) then to my single ended INA122 (so the input was going below the INA122's rails) and referenced the output to a voltage divider and the input is faithfully amplified and referenced to the midpoint of the rails on the output.

Next step is to research inductorless band pass filters...

j.
 
Why go crazy with all the filtering?

If the alarm is loud, don't use a very sensitive circuit. If the microphone is close to the alarm you shouldn't have to worry about ambient noise and you can probably just rectify/filter the output of a single stage op-amp preamp circuit and use a second op-amp as a comparator.
 
The RO (reverse osmosis) unit is in the mechanical room, which can be noisy when the RO unit is running, the furnace is running, or the water softener is doing its thing. You're right that the alarm is louder than any of them, but it would be marginal.

Thanks for the idea, though,
j.
 
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