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

• 07-31-2008, 03:41 PM
cMags
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?

http://ourworld.compuserve.com/homep...den/555exp.gif
• 07-31-2008, 08:49 PM
Curiosity
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.
• 08-01-2008, 01:32 AM
cMags
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... ;)
• 08-01-2008, 04:33 AM
Curiosity
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.
• 08-12-2008, 03:16 PM
Beehphy
You could use a schottky. they have lower forward voltages. ~.2 compared to ~.7
• 08-12-2008, 05:42 PM
cMags
^^ Good point, but I've got dozens of 1N4004's and 1N914's lying around so we'll see how it works out.
• 08-14-2008, 07:06 AM
Curiosity
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.
• 08-14-2008, 08:38 AM
cMags
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.