# Thread: Circuit help: Short ground pulse -> long +12V pulse

1. ## Circuit help: Short ground pulse -> long +12V pulse

My car came with the common remote unlock system where one button press unlocks the driver's door, and the second unlocks the rest. I modified this so that all the doors unlock from the first button press, and now I have a spare wire that pulses a GND when I press my unlock button a second time.

I would like to come up with a circuit that will take this GND pulse, and give me a ~5sec positive 12V pulse I can diode into my M2-ATX's ACC sense line. I've noticed that when I start the car or apply 12V to the ACC sense, it takes almost 5sec for the pc to start booting. At this point, I can drop the 12V so that the timer will shut it down again if I don't get in the car for some reason or I hit it accidentally, I don't drain the battery.

I was thinking of some type of 555 ciruit, but have never used these before. I found this schematic for a 555 monostable mode, seemingly for a negative pulse (I assume that means GND):

Supposedly, here, T = 1.1 x R x C. Does the 555 support wide voltage input? Do I need any specific type of resistor and cap, or can I just use a standard 1/4W resistor and electrolytic cap?

Offhand, I remember the other night noticing a 330uF eletrolytic in my parts bin - probably around 25V-50V rating. Could I just take that 555 circuit, toss in that and a pair of 10K 1/4W resistors in series for R (or dig for a 20K or close thereto), and get T = 1.1 x 20K x 330uF = 7.26sec? Is it that simple?

Edit: Just found this schematic (left most), and it answers the input question. Does this look like a better option? Could I just connect where they show the switch to ground to my switched ground pulse?

2. You got it. The one on the left would be perfect. Google "555 calculator" and you'll find some easy web based calcs, but a 20K would be fine. 22K is easier to find I think.

I'd use a diode on the input though. Check your alarm output. It may float at 12V or may just pull to gorund in which case it doesn't need it.

3. I thought about that diode, but would the 0.7v drop over the diode mess with the RC circuit?

And I did try calculator, which is how I found that schematic, but all the calculators I could find were for astable circuits, not monostable. They'd take R1, R2, C1, and C2, and spit out T1 and T2 for the high and low times on a square wave. If you could find a monostable one-shot calculator that'd be handy, tho T=1.1RC isn't too difficult an equation to crunch by hand...

4. Yeah, it's pretty easy to calculate. The diode should work fine. It just needs to drop below a threshold voltage to trigger a flip-flop or something.

5. You could use a schottky. they have lower forward voltages. ~.2 compared to ~.7

6. ^^ Good point, but I've got dozens of 1N4004's and 1N914's lying around so we'll see how it works out.

7. Use a cheap one. Really. As I said, it needs to drop below 1/3rd source voltage so it's really not that big of a deal.

8. Oh that makes more sense. Didn't know what the threshold was, and in this case it would be a 12V system, so it should be fine.

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