DIY Automotive Shutdown Controller

From Fozzee.net

So I considered the automatic solution to powering a laptop on and off, and have decided that I dont really like it. For starters, I can be troubled to push the button when I want it. Second, I really want the manual control. (I know, Seth will call me a short-haired hippie or some crap, for not even trusting my own circuit) Anyhow, the real issue is that I have to be able to power off and on the external air/auto power supply, because it sucks almost 250ma, just idling, with no laptop attached.

I whipped up the following mess of relays and whatnot to power the external brick. The switch powers both the brick and the laptop, (via an unseen second pole and some wires soldered into the laptop ;) ) [the right side of the diagram], and the 5v from the USB/PS2 port hold the brick on. When the laptop shuts down, 5v goes away, and the relay drops, powering down the external brick.

I'm well aware that I could do this with a little PIC or something, but it's a whole lot easier (overall) to just hack out an analog solution. I did find, in testing that I had to make a slight deviation from the schematic, in that I added capacitors across the coils for the push button relay, (left, blue) and the output relay (center, black) because it was misbehaving during startup.

The only issue remaining now is that, because the DC-DC power supply (not pictured) starts so slowly, you have to hold the button in for a second or two. The capacitor across the push button relay shortens how long you have to hold, as it "holds" for you for a second or so, but I couldnt cram a large enough capacitor on the board to keep the relay latched long enough to have the laptop power switch held in by the time the power supply was making juice.

Things I'd change (and still might):

• The relays could really use diodes across the coils. I can see this when I drop the USB power, the green LED blinks a few times.
• The USB input really should be current limited. USB is pretty beefy (500ma) but the PS/2 port (if I go that route) isnt so forgiving (like 100ma, I think)
• More Blinkenlights (duh)
• The capacitor "latching" issue could be averted (some) if I wasn't powering the DC-DC PSU off the same "bus" as the relays themselves. A diode or two would fix this, though it might take some rerouting as I was pretty lazy with my wiring...

In conclusion, this is basically ready for primetime. Just waiting to install it, until I can wire up the dash switch.