constant voltage to single pulse

Like Baer said:

If it has a good power rating, one could use a capacitor to couple to the relay, and maybe a reverse diode for re-charging.

An additional relay (call it RL2), a big capacitor and a resistor.

Here's how you wire the additional relay coil:

Alarm output------RL2----gnd

And here's how you wire RL2's contacts: Below shows alarm armed state:

gnd--cap---o o-------100ohms------ +12 \\ o---door_relay_coil---gnd

Below shows alarm disarmed state:

gnd--cap---o_o--------100ohms------ +12

o---door_relay_coil---gnd

Here's how it works: With the alarm disabled, RL2 is denergized, and the capacitor is connected across the battery via the relay closed point. When the alarm is armed, RL2 is energized, and the capacitor is switched to the door lock relay coil. The cap discharges through the coil, energizing the relay momentarily.

The lower the resistance of the door relay coil, the larger the cap must be. You want to size it so that the door relay energizes for roughly 1/2 second or so.

Optional RL2 circuit to press a button and lock the doors while driving (alarm disabled):

Alarm output--------diode--+--RL2coil---gnd |

Don't forget backwards diodes across the relay coils.

Ed

It occurred to me that you probably want more than what I posted. My first response gives you the relay based "one-shot" that you asked for, but does not give you "direction" (ie lock or unlock). You need to be able to both lock *and* unlock the doors with your alarm system.

Here's a better approach:

D2 R1 +--->|---+ +-/\\/\\/\\-+ D1 | | | | Alarm output--+--||--+--RL2---+---+---||---+-- gnd | | | | +---||---+ +-/\\/\\/\\-+ D1 | | | | +--||--+--RL2---+---+---||---+-- gnd | | | | +---|