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artist needs help- difficult mic circuit

Before I begin I should say that we are pretty familiar with most basic electronics but are really stumped here.

What we are looking for is a circuit that would use very little power- be battery operated and as small as possible. The idea is to a mic that would be "listening" to the room it would store a charge relative to the mic signal- or ideally from the small charge generated by the mic itself- when this value crossed a certain threshold it would trigger a serperate circuit- lets say a buzzer. the triggering charge would be reduced to zero and it would all start over again.

In practice what this would mean is that put on a shelf in an a closed office- the buzzer might go off once or twice a day assuming the telephone was used - and if there was an office party it would be going off all the time.

Now the trick here is that this thing should ideally be able to be "on" for a few months without draining the battery.

We were thinking of a simple biased capacitor charged by a mic- with the cap discharging at a certain level- but are having problems.

Any ideas? Thanks Raphael and Peter

Reply to
raphihell
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why don't you steal the guts from one of those singing bass fish things;

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Reply to
no_one

One approach would be an analog 'integrator' circuit to keep track of the charge. Unfortunately, integrators are usually built with capacitors, and its pretty hard to keep a capacitor charged with tiny amounts of charge over a long period of time like hours or days; the cap will leak.

Another way would be to count how many times the microphone went over a particular input excitation, and use that count as the trigger. You could do this with CMOS logic chips; perhaps a counter chip activated by some kind of analog level meter circuit on the microphone. Once a particular bit on the output went high, fire off the buzzer for a bit using a C555.

A tiny microcontroller, like a PIC 12F675 or ATTiny would also be a good choice for this kind of circuit; you could use the internal comparator to wake it up when the mic was active, and keep track of the count in its eeprom. It could output the 'buzz' as well, so you could use a cheap piezo disk rather than a buzzer. Thus, it might be a single chip solution. It would be very low power, but would require you to write the software for it. Keeping the microphone powered would probably take the most of your energy budget. However, you might be able to power the thing using a small solar panel charging a little rechargable cell. Offices usually have pretty good light.

This is going to be a fairly complex circuit to pull off, I'm guessing. Good luck!

Regards, Bob Monsen

Reply to
Robert C Monsen

I'm sure they're using simple level triggers.

I think Robert is on track in his suggestion of a low power microcontroller. A mic, a rectifier and a cap before the AD of the uC. The uC should wake up once in a while to measure the level and integrate. Probably it'll prove OK to do a discrete mesaurements once or twice per second - allowing the mic to be powered down.

/Anders

Reply to
Anders F

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