Basic power related question - Pulse or Constant?

**Tical84** · 11-03-2009, 11:20 AM

I'm sure the answer is out there but I just can't seem to find it.

I'm looking to purchase an alarm, and I want it to be able to boot my truckputer when I unlock the doors and/or remote start the truck. This is the question I've been asked by the person selling it to me:

"Quick question. How does you truckputer boot up? Does it take a pulse or something or does it require that a constant power source is turned on to boot the computer when you hit the unlock or when you remote started the vehicle?"

I've read people doing this but I can't seem to find that thread anywhere. Can anyone answer this question for me?

**Bugbyte** · 11-03-2009, 11:41 AM

If you are using an intelligent DC-DC power supply like an M2-ATX, that unit requires constant power to be applied to the accessory line to initiate a startup. When it detects a loss of power to the accessory line, it commands a shutdown.

Some units have a boot lockout feature so that once it detects power, it won't check again until some number of seconds later. This is to allow the computer to boot completely before shutting down. It keeps voltage cutouts on the accessory line (like when you turn the key to start the car) from causing a shutdown.

**Tical84** · 11-03-2009, 12:56 PM

Ok thanks. That's what I wanted to know.

How does it work to have the alarm/door unlocker boot the PC since it just sends a pulse does it not? I'm not that familiar with how that part of it works, I'm assuming a relay of some kind would be used but I only vaguely even know what a relay does.

**justchat_1** · 11-03-2009, 01:04 PM

You can use a latching relay to hold power on from a single pulse...or some users with less complex setups will use the pulse to trigger their power button.

**Tical84** · 11-03-2009, 01:26 PM

Originally Posted by justchat_1

...or some users with less complex setups will use the pulse to trigger their power button.

Ah, because when you push the button on the front of a PC it's really just sending a pulse to tell the mobo and everything to "turn on" right? Does that mean I actually can just use the pulse from the starter to turn everything on as long as the PSU has constant power?

**soundman98** · 11-03-2009, 01:27 PM

yes, the problem with a constantly on power supply is that it has more potential to drain your battery.

just be careful using the door lock/unlock pulse to start the computer-- on some cars, you need to press the unlock twice--first to unlock the drivers door and twice to open the other doors-- if you hooked this up to the computer, the second pulse would shut the computer off (it would also apply for impatient people like me who press the button more than once

).

**Tical84** · 11-03-2009, 01:32 PM

Right.
I've seen some people rig up the tailgate unlocker on my gen F-150 and they did it so first push unlocks driver, second unlocks the rest, and third unlocks the tailgate. I'm sure if that's doable I could program it so it only turns on after pushing it twice (no need to make it three for me)

**Bugbyte** · 11-03-2009, 04:22 PM

If it is a laptop, you can do this without the relay. The pulse will turn it on.

If it is a desktop, doing this will require your power supply to always be 'on'. That drains your battery even when your desktop is asleep. Some of the power supplies will respond to a pulse on (I think the Carnetix ones will).

**Curiosity** · 11-03-2009, 06:10 PM

Most of the PSUs have a pretty long debounce. Maybe 1-2 seconds because the key goes from off to acc, then acc+ign, crank (acc off), and back on so the pulse may need to be extended. I think what people are doing is using the AUX out as constant and just remembering it's on. The range gets pretty short on superhetrodyne receivers when the PC is on so you might have to go out and manually turn it off. Try to find the manuals online. Mine has 4 AUX outs and some are programable as constant or momentary, and the momentary is as long as you hold the button so it can be decoded as tap, double-tap, long press and such with a PIC. Anyway, as far as I know a long enough pulse should start it up and then shut it back down again a couple minutes later.