Reset timing cap of AR to pull output to zero

fonik

i built an AR generator on breadboard. my idea was to short the timing cap to force the circuit to go down to zero volt in the release phase at some point.
the idea was to use a comparator to create a short pulse that controlls a NPN in parallel to the timing cap (circuit below).

it works in vitro (screenshot of scope in the simulation software), but not in vito (picture of the analoge scope). in real life the 100n cap seems to try to reset when upper limit is reached?

any ideas?

AR_scope2.gif

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Simulation Software

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AR_scope.JPG

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real life scope with circuit on breadboard

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AR_01.gif

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AR gen circuit

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fonik · Posted: Thu Aug 05, 2010 6:45 am Post subject:

anyone?

i thought i had triplechecked the breadboard, so what could cause this behaviour? ideas? should i try a comparator instead a quarter of the TL074?

Blue Hell · Posted: Thu Aug 05, 2010 7:06 am Post subject:

Not sure if its a problem ... but when your comparing-opamp goes negative the switching transistor is reverse biased and that may do some odd things if it goes into zener mode .. which it probably will.

fonik · Posted: Thu Aug 05, 2010 7:12 am Post subject:

Blue Hell · Posted: Thu Aug 05, 2010 7:16 am Post subject:

fonik · Posted: Thu Aug 05, 2010 7:31 am Post subject:

Blue Hell · Posted: Thu Aug 05, 2010 9:40 am Post subject:

BananaPlug · Posted: Fri Aug 06, 2010 7:15 am Post subject:

Rethinking the original problem. Your timing cap discharges through a diode (for separate attack/decay resistors) so I assume that's where the slight residual charge you want to dump comes from. You have two op amps. How about using them one after the other (as inverting op amps) for the output buffer. Now you have a summing point at the first one and can sum in a small negative voltage to null out the effect of the residual charge. You could put a diode in the feedback path of the final op amp to guarantee uni-polar output.
Just a thought.