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Hairy Ears
member

Joined: 06/09/03
Posts: 495
Loc: UK
Creating a digital pulse from a piezo input
#988484 - 20/05/12 10:25 AM
Can anyone point towards a circuit that would take an input from a piezo crystal and create a +5v pulse from it?

I had been trying to do it with a simple op-amp circuit, with the legs of the piezo inputs connected either side of a 5M pot, with one leg of the piezo connected to the - input of the op-amp through a 150k resister and also tied to 0v. The wiper of the pot connected to the + input of the op-amp (also through 150k resister) thus relying on the theoretical infinite gain of the op-amp to produce the pulse.

Supply to the op-amp is +5V and 0V.

Even with no signal present I am measuring 75mV across the + and - inputs of the op-amp which is enough to swing the output to +5V.

All I want is a 5 volt pulse, no velocity sensitivity is required.

This is the first circuit I have fiddled with in about 20 years, so I am definitely rusty on my circuit design! Any help would be appreciated.

(I do know now that I should be rectifying the input from the piezo).

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Hairy Ears
member

Joined: 06/09/03
Posts: 495
Loc: UK
Re: Creating a digital pulse from a piezo input [Re: Hairy Ears]
#988486 - 20/05/12 10:41 AM
Here's a drawing of my naive input circuit.

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James Perrett

Joined: 10/09/01
Posts: 9654
Loc: The wilds of Hampshire
Re: Creating a digital pulse from a piezo input [Re: Hairy Ears]
#988643 - 21/05/12 09:41 AM
Op-amps have a really high gain so any imbalance on the inputs will result in full scale output. You need to add negative feedback to reduce this gain. A standard non-inverting configuration should work.

If you are triggering samples from drums you should be aware that successful drum triggering isn't simple. A simple circuit will often give multiple triggers so you may need some way of creating a hold off time before the circuit can be triggered again.

James.

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Martin Walker
Watcher Of The Skies

Joined: 28/02/01
Posts: 16381
Loc: Cornwall, UK
Re: Creating a digital pulse from a piezo input [Re: Hairy Ears]
#988660 - 21/05/12 10:51 AM
Hi Hairy Ears!

No point re-inventing the wheel – a quick Google threw up quite a few circuits for you - this one is the simplest:

Oh, and part from rectifying the pulse, that capacitor after the diode will 'hold onto' the pulse and let it decay slowly courtesy of the bleed resistor – this should help with the mis-triggering that James mentions by holding the rectified pulse high until any multiple trigger input signals die down, although you may need to tweak values depending on your piezo element.

Hope this helps!

Martin

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