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need more help with sound trigger

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gvi70000

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hello,
I need to trigger a camera using the sound made by the splash of a drop of water. The sound is picked up using a electret mic placed at about 20-30 cm from the sound source.
The best i could think of is posted here https://www.edaboard.com/viewtopic.php?t=368713&highlight= but with this design i have to place the mic too close at about 2 cm from the sound source
I also need to adjust the level at witch the camera is triggered (for popping balloons or water drops).
Can anyone help me to achieve this?

PS: I've uploaded two picture with the results I've got so far
 

Endolith

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Why do you have to adjust the level? If a water drop triggers it, then a balloon popping will trigger it, too. You're just checking for an event, not recording the sound.
 

gvi70000

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I need to adjust the level to not have false triggers.
Anyway, my problem is that i have to place the mic too close to the sound source and i need the mic placed at 20-30cm away from where the water drops are falling.
 

Endolith

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Honestly, I don't understand this circuit. I don't know if that's because the circuit's wrong or because it's just unfamiliar to me, but if that's just a normal op-amp, it looks like you have no negative feedback at all, so the op-amp's just running at full tilt and acting like a comparator.

Can you look at the output of U1 with a scope?

Try adding a pot between pins 2 and 6 of U1 to vary the gain and a resistor between the transistor and pin 2, so the op-amp is running as an inverting amplifier.

Added after 9 minutes:

In fact, the image on that op-amps datasheet is a inverting amplifier used to amplify an electret, which is exactly what you're doing. Why not just copy that?

 

Audioguru

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Like I said on the other website, the transistor is saturated and is not amplifying because the value of the 500k trimpot as its collector resistor is much too high.

Use a 5k trimpot and adjust it to about 1.4k then the circuit will trigger on faint background noise.

Then there will probably be a problem that the water drip sound will be to short in duration to trigger the camera.
 

Endolith

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It's only 500k at the end. If it's an audio taper 500k pot it will still have a usable range.
 

gvi70000

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Thanks for replaying Audioguru and Endolith,

I've change the pot with one of 10k + ore 10k resistor in parallel(i don't have a 5k right now) and the result was almost the same in sensitivity..so i change the 2n2222 with a bc547 (measured gain is 650) and the sensitivity has increase a little bit. Maybe the mic is not sensitive enough - i think is a Velleman with a sensitivity around -68dB and a nominal voltage of 4.5V - next week i will receive a new mic BCM-9767P from BE-STAR -44±2dB (VS=3V/RL=2,2kΩ) i hope that will do the job.

Do you think that if i make a Darlington pair with 2 BC547 will increase the circuit sensitivity?

Do you think is a good idea to remove the transistor near the mic as i connect the mic preamp with lmv722 input with a stereo shielded cable about 1m long?
 

Audioguru

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You need a preamp with voltage gain and that is biased properly. Your transistors weren't.

Don't use a darlington because its voltage gain is the same as for an ordinary transistor but it has a very high current gain that you do not need.

I already replied to your other thread on the other website so I won't repeat it here.
 

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