Need a simple voltage decay circuit

[CutRock]

My goal is to produce a circuit that will pulse a high voltage for about a second before dropping down to a lower voltage once the device turns on (the simpler the better!). I assumed this would be relatively easy, but all the intuitive solutions that I've come up with haven't really worked out.

I'd appreciate any feedback or suggestions, or even just some key words to start searching for when trying to find a solution.



One thing to note: I drew the drop-off as a linear decline in voltage for V_out, but it could potentially be exponential decay or discrete, dropping almost instantly from the high voltage to the low voltage. [/img]

[PHOBoS]

what current/voltage are we talking about here ?
(something with an LM317 comes to mind or a simple timer controlled relay)

[fonik]

so a simple passive RC network (time constant) didn't work for you? the decay would not be linear, of course.
(i used this as part of an ignition system for the ADC in my sequencer)

[richardc64]

[JingleJoe]

I made a similar thing once from a 40106 schmitt trigger based monostable circuit, of course one would need a buffer/voltage follower to get the decaying voltage from the cap without affecting the charge/discharge time.
Hmm thinking about it, if using that method one may also need a trigger pulse generator, not too hard to make from a cap and another schmitt trigger.
Circuit diagram of what I'm rambling about provided on request.

[CutRock]

Thanks for all the input, guys. I'm only slightly EE literate, so I'm sorry if the solution was extremely obvious. I had seen a couple circuits with a voltage source that charged a capacitor, which was then discharged through a resistor to give a decaying voltage, but these all seemed to require a physical switch of some sort. This application will need to be completely automated so that when the voltage is provided, that voltage peaks for a second or so, then decays to a lower voltage.

It looks like the circuit proposed by richardc64 (and also fonik?) will be fine. I'll play around with it when I have some free time.

Thanks for all the help!

[fonik]

under certain circumstances the ADC i used in my VCPS-8 needed a momentary connection of one pin to GND to start working properly.
i just used a cap fed by the 5V line and a resistor to ground to create a short pulse whenever the power is switched on. this short pulse then triggers a transistor to switch the ICs pin to GND. works perfectly.

[CutRock]

Thanks all! Got it working relatively quickly - turns out you have to type "Meg" for millions of ohms and not just "M" in LTSpice (which makes it milliohms). After that little snaffu, it turned out to be relatively straightforward. I ended up using Fonik's ADC ignition setup and a couple pots to tune the peak height and the voltage that it eventually decays to.

Despite the simplicity, I figured I should still post my final results:

[beep]

[beep]

some active design. the resistor and the capacitor to ground determine decay time. needs short pulses for input.
http://www.falstad.com/circuit/#%24+1+5.0E-6+78.57719942274176+50+5.0+50%0Aa+368+176+480+176+0+9.0+0.0+1000000.0%0Aw+368+160+368+128+0%0Aw+368+128+480+128+0%0Aw+480+128+480+176+0%0Ac+368+192+368+256+0+2.2000000000000002E-8+0.1986380111320897%0Ag+368+256+368+288+0%0Ar+480+176+544+176+0+1000.0%0AO+544+176+592+176+0%0AR+256+272+128+224+0+2+96.0+5.0+0.0+0.0+0.01%0Ar+368+192+304+336+0+100000.0%0Ag+304+336+304+368+0%0Ad+256+272+368+192+1+0.805904783%0Ao+7+64+0+34+9.353610478917778+9.765625E-55+0+-1%0A