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footstep detector

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confounded

New Member
Hi, i thought i'd experiment with a couple of lm386 op amps and build a footstep detector by connecting 1 to a speaker from my hifi then the other to that op amp as shown in my pathetic diagram!
when i put just 1 to the speaker it was fine, i detected voltage change when tapping the floor near speaker :) about 50mv max on top of aprox 3.8v dc
But when i connect the next op amp to it and turn on power the speaker makes a noise. Can someone please tell me why its making a noise?

I have 200 gain in op amp 1 and 20 gain in op amp 2. I am using a 9v battery. The speaker has impedance of 6 Ω. I hope i have supplied all necessary information for you guys.
 

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Leftyretro

New Member
I don't think a speaker would be a very effective sensor for detecting foot steps. A microphone element coupled to the floor might work better.

Lefty
 

Sceadwian

Banned
What kind of noise? Could be noise coupled through the volt meter from the 2nd stage ouput. I know 4000 gain is a LOT of gain. I'm not that good with the analog stuff though. With the multi meter disconnected does it still make noise?
 

confounded

New Member
thx for reply!
yes it still makes with voltmeter disconnected
the noise is like a puft puft (only flicked on power and then turned off quick after hearing noise as not sure if im damaging anything) it is quite loud.
 

Sceadwian

Banned
A speaker should act like a decent low frequency microphone and it will barely if at all pick up high frequency noises so it's like putting a built in lowpass circuit. Simply place the speaker face down on the floor. But Nigels right, he should use a 'real' opamp not a power amp. They're high noise and not meant for ultra high gain, just power amplification.
 

Mikebits

Well-Known Member
A geophone would make the best sensor. Might be hard to find though.
 

confounded

New Member
thank you all for your help, but i'm really interested in why the speaker makes noise, and how to stop this.
Nigel mentioned above i havnt connected my power amps correctly, could someone please tell me what i have done wrong.
I notice i havnt put polarity of conection on my drawing, the speaker and 2nd op amps inputs are both connected to positive input incase that helps diagnosis?
when sceadwian says they are high noise, what does this mean? Does it mean the common mode rejection ratio is small?
I built this to test out op amps after reading about them, so if you could explain any answer i would be gratefull as opposed to just recomend a certain part here etc.
Thanks again
 

audioguru

Well-Known Member
Most Helpful Member
Your circuit is "motorboating". It makes a putt, putt, putt sound like a little motorboat.

I think it is because your circuit does not have an important supply bypass capacitor (use 100uF) so the 9V is jumping up and down which causes low frequency oscillation.

The 9V jumps up and down because the weak little battery has internal resistance.
 

Boncuk

New Member
in addition to audiuguru's reply I guess there is acoustic coupling between input and output.

You are using one speaker as microphone and another one for AF-output, aren't you?

Some speakers start screaming, others will just "put put". :)

Boncuk
 
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