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Lofthouse 06-29-2012 03:35 PM

Hacking my home alarm system (LED stuff)
Hi Everyone,

I have read some good information here on this subject but I am afraid my skill level just calls for an answer that is more specific to my situation. Thanks in advance for your help.

My home alarm only indicates that is is disarmed by an audible response to the remote key fob button. There is a light on the panel that says it is armed or disarmed but the light is not visible from outside the house.

I located the LED which stays on when armed. It is red in color and is mounted to the circuit board. It is rectangular and measures maybe 4 mm across. I have already tapped a wire into the connectors and it measures 1.62v when lit up.

My intention is to run a thin wire all the way to the outside of the house where I can mount another LED so that I can see it when I drive up. If my remote button is working from this distance, I should be able to turn off the system and see that the light has gone off.

Here is the problem:

I purchased a green LED for $2 at my local electronics supply store. Spoke with the fellow there about what I was doing and he was not much help. The packaging on the LED said only that it was rated for 2v. I figured that with 1.62v available it should glow enough for me to see it. The store did not have any 1.5v LED's. I tested polarity and connected the LED properly considering that it is a diode and will only work when the electricity flows one way. It will not light up. For that matter, when I hooked it up to a 1.5v dry cell just to test, it still does not light up. Not having a power supply I chose not to try a higher voltage as my next option is 3v.

I have also scoured the internet looking for a board mounted LED that looks like this (low output, rectangular and flat)but no luck. This unit seems quite common in various toys and electronic devices around the house but all I find when I look for it is the dome shaped LEDs. I figured if I could match it exactly then it would work but nothing is printed on it to help me figure it out.

I realize that there will be those who wish to warn me about voiding warrantees and all that stuff - duly noted.

Can someone give an electronic newb some advice? Many thanks.

zappa 06-29-2012 04:40 PM

If you plan on keeping the inside LED in the circuit, and have enough power supply voltage, both LED's could be wired in series with one current limiting resistor. Otherwise you will need to attach your wire for the second LED from the supply side (or ground side) of the already installed resistor for the inside LED and use a second resistor on the outside LED. You won't be able to just attach the wires across the inside LED like you described.

Don't test without a resistor in series like you did or you will blow the LED if the voltage is high enough.

Lofthouse 06-29-2012 05:36 PM


Okay I think I understand. So I should look for the supply to the board, measure it, calculate the resistance needed to get the current down to the 2v I need for this LED (Already read about this on here) and connect it in series. Should I tap in at the Neg side of the existing LED?

zappa 06-29-2012 06:00 PM

It's difficult to describe what to do without a schematic in front of me as there are a few different ways to operate an LED. There should be a current limiting resistor connected to one leg of the existing LED. IF the LED circuit has enough voltage (2 volts + 1.62 and a little bit more) you could wire it up in series. Try measuring voltage from one leg of the LED and the far end of the resistor so you are measuring both the LED and resistor in series.

It doesn't matter which leg of the existing LED you tap into for your series circuit. Of course the second LED must be installed with the correct polarity.

zappa 06-29-2012 06:12 PM


Originally Posted by zappa (Post 954222)
Try measuring voltage from one leg of the LED and the far end of the resistor so you are measuring both the LED and resistor in series.

BTW....these 2 points are where you would connect the second LED and second resistor if you choose to use a parallel circuit for the second LED.

curiousB 06-30-2012 06:48 AM

A couple thoughts.

1) Extending this circuit could be hazardous to the alarm due to potential to pick up electrical surges such as static electricity and the like. Fortunately this is likely pretty low impedance as it is driving an LED but beware this could have side effects. PCB lines which are meant to go into external wiring usually have some extra protection devices on board for this reason.
2) You are better off to run the LEDs in series. That means removing one leg of the existing LED and wiring the new one in same loop and with diode cathodes in same direction. This will also mean lowering the series resistor value on the PCB. You can start just getting the LED working with the existing resistor and then experiment with values to bring the brightness up. Don't go crazy though as if you exceed the current the driver is capable of you may permanently damage the driver stage of an IC.
3) If you modify the alarm electronics you give your insurance company an out to deny a claim later on due to the equipment being monkeyed with. Is an LED indicator worth it?

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