Adding LEDs to alarm output

Well, decided to look further at how this is set up instead of trying to guess- From the manual of the Viper 300 ESP, which I'll assume may use the same LED drive setup, it says: The super bright LED operates at 2V DC. Make sure the LED wires are not shorted to ground as the LED will be damaged. Multiple LED's can be used, but they must be wired in series. Sounds like the output uses a current driver, so the series connection will work, since the alarm should adjust the output voltage differential to push the specified current, as long as you don't have too many hooked up. If you use the higher voltage LEDs you'll hit the voltage limit sooner. It might be kind of a pain to hook all your LED's in series with a ring pattern, because you can't use a common ground, which is the advantage of doing it like Kevin outlined, which lets you wire in together with single wires radiating out from the controller. The hookup to the transistor may have to be tweaked though, because it doesn't sound like the alarm module output switches to ground when it lights up the LED, but instead drops it to 10V, when a single LED is connected. That's something you'll have to verify. When you measured the output of the alarm and saw the 2V, were you measuring between the two wires going to the LED? If you measure from the wires to ground, I think you may see closer to 12V when it's off.

The +5V source makes sense and would match up with bobk's info that only a maximum of 3 LEDs could be connected in series- Any more and there wouldn't be enough voltage per LED. The control side must be configured for a constant current, to maintain the same brightness regardless of the number of LED's connected (up to the limit anyway). Since he's planning on using LEDs with a higher Vf, it'd probably only be good for maybe one blue along with the original red LED, before dropping in brightness too much. I'd guess the string of LEDs could be powered with 12V instead of the 5V from the alarm, with the alarm's control side output used to sink the 20mA current from the other end, but that might be risky without first doing some verification. Even so, the PNP transistor still sounds like the best way to go about it-