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constant voltage to single pulse

Here is my situation: I have this rather cheap car alarm system which I am looking to install, basically just for the keyless entry. It has a two wire output for the "keyless entry module" - the two wires produces

12v when disarmed, and reverses polarity when the alarm is armed.

I think I can create a relay system to tap into my power door lock switch, but since the alarm's output is constant 12v, the door lock motors will constantly be activated (i believe, correct me if I am wrong). Therefore, I need to turn that constant voltage into something like a 1 second pulse.

All help is greatly appreciated~~Thanks, Jon

PS. - I would buy the "keyless entry module", but its a cheap KMart alarm from about 4 years ago and the part isn't carried anymore.

Reply to
jdsmn
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Like Baer said:

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Reply to
JeffM

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.

Reply to
Robert Baer

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 |

+12----MomentarySwitch-----+

Don't forget backwards diodes across the relay coils.

Ed

Reply to
ehsjr

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 | | | | +---|

Reply to
ehsjr

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