contant to momentary

k so i did my own power trunk release and have it all wired to my alarm and everything.  works fine.  the only thing that bugs me is that the output for my alarm has a slight delay to it.  as in when you release the button it doesn't cut off for another half second or so.  with the power trunk release what happens is you hear the solenoid engage an no matter when you release the button there's still about a 1sec or so pause before you hear it disengage.  call me anal but this bugs me.  i want it to disenage right after the trunk is popped.  i'm gonna add a cap and resistior to the negative side of the coil for the relay that controls the trunk. this will cut off the output when the cap become fully charged.  i just need to know about what size to go for to give me a very very brief output from the relay.  basically only as long as is necessary to pop the trunk.  it should only take a split second of output or so - just enough for the solenoid to fully engage.  any suggestions on cap/resistor size? i kinda forgot how you figure out the charge time in relation to capacitance and voltage and everything..  thanks!

I'm not sure the Resistor & Cap(RC)is the solution as usually this combination is to give you a delay so it might actually energize your solenoid for even longer. Usually the cap get's charged up when it gets power and when power is gone, the cap will discharge through the resistor. The more resistance or higer value of capacitor, the longer the time it takes to discharge. Sounds like what you need is called a one-shot. So regardless of wether you have a long pulse or even a constant output, the one-shot circuit will output a pulse of X seconds (depends RC Values).

no the cap is being put on the ground side of the coil of the relay.  initally there will be a potential across the coil and thus engage the coil.  when the cap becomes charged, that potential will be lost (both sides of the coil will have a +12v charge) and the relay will disengage.  the length of ouput will be dependant on the time it takes the cap to charge to a level at which the voltage difference between the +12v  and the cap is no longer enough to engage the relay.  i'm not sure what the release voltage is on most relays but i think it's about 6-7v so once the cap charges to about 5-6v the relay will disengage.

check the special apps relay page.  it's the first example.  the resistor is used to bleed off the charge on the cap once the input ceases.  i'm prolly actually gonna hafta mod my setup with an additonal relay b/c my alarm uses a (-) output... this setup requires the +12 side of the coil to be switched, not the ground.  but that's beside the point.  this setup works - jsut need to know some specifics on what caps an stuff to use.

https://www.the12volt.com/relays/page5.asp

Interesting, Haven't checked out the relay example pages in a while. But I see what your saying. Looks like the relay coil is getting it's ground through the 10K resistor which sure cuts down the current to the coil. Then the cap get's charged through the positive flow through the coil till the potential on the "Ground" side of the coil is too high for it to get the necessary ground. Interesting idea...I'll have to try it out one of these days. As far as the one-shot, I wouldn't say it's as easy or as cheap (about $5 to build), as it is an actual circuit with an IC and several resistors & capacitors in addition to the relay but it is a lot more customizeable as far as timing. If your interested, I can post a schematic, but for cheap & simple, I guess use the relay, resistor & cap.

hehe cool i think i'm gonna go w/ the resistor cap idea then..  always good to see new ideas though.