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Saturday, October 9, 2010

DIY Simple Car Alarm Flashing LED

Everyone on my block has a flashing light car alarm and I want one too!!! Actually there is more to it than that. I have a car alarm and a good one at that but it doesn't have the Anti-Theft flashing light. At night when I park my car someplace and I'm walking in to where ever it is I am going, all the cars seem to have that flashing light. I started to wonder if the car thieves look at those things... I imagine that if there is a parking lot full of cars and all of them except one has a light that one would be the car that gets stolen. So I decided to add a flashing LED light to my car. Will it make a difference? I don't know but it took less than an hour to make and install it so why not?
Car Alarm Flashing LED Circuit

Above is a picture of the circuit I came up with and the components I used. The trickiest part in getting it to work is finding the wires in the car to hook it up to. The circuit itself is really simple and straightforward and it only took me 20 minutes to build... and another 40 minutes to install it in the car!
As you can see in the schematic there are only three components and one of them is optional. I used a 'flashing LED' that flashes all by itself when you apply power and it has a 12 volt dropping resistor built in. That means you can connect it directly to 12 volts and it will work correctly. The Constant +12 volts is on all the time and the Switched +12 volts only has power when the ignition is on.

There is a small relay that operates on 12 volts and has a Normally Closed contact. Normally Closed means that when there is no power to the relay the connection is closed. Those are the two main parts to the circuit but there is one optional part as well. The was this works is when the ignition is off there is no power on the Switched +12 volts so the relay is off. If the relay is off the Normally Closed contact is closed and the flashing LED is on and blinking. When the car is started the Switched +12 volts turns the relay on which turns the flashing LED off. Some people just connect the LED from the Constant +12 volts to the Switched +12 Volts so the LED comes on when the ignition is off but I don't like that for a couple of reasons.
The first reason is I don't know where the Switched +12 volts is coming from and some electronics don't like to sink current when they are off. The second reason is I have had friends that hooked similar LED's up that way and they somehow drained their batteries. I don't know why so I don't do it that way, but if you have any ideas email be at or leave a comment!
I decided to add one more diode (the one marked optional in the picture) in series with the relay. This diode is not light emitting and is there in case the ground wire comes off. You don't have to put that optional diode in the circuit but if the ground wire were to come off and the Switched +12 Volts is off (meaning it's essentially at ground) current will flow into the circuit from the "Constant +12 volts" down through the relay contact, go right past the "Chassis Ground" (since it's fallen off and not connected) and then go through the relay coil and out the "Switched +12 volts". That will cause the relay to operate when the Flashing LED turns on, turning the flashing LED off, which will then turn back on etc... so not only will the LED be flashing but the relay will be clicking on and off each time it flashes and eventually break.

I decided to mount the LED in one of the blank switch positions on the dashboard of my Highlander. There are 4 positions for switches and one of them has the interior / dash lights dimmer.

The switch location covers snap in place and can be removed by carefully prying on them with something like a guitar pick or credit card. I don't recommend a metal screw driver for prying on car interior stuff. The left most switch cover had a unused wire harness attached to it and in that harness there happened to be a Constant +12 volts. Underneath the switches is a little compartment for putting stuff in and when removed there were a bunch of wires (see below) and one of them had a Switched +12 volts.
The next thing I did was drill a small hole on one of the switch covers and mount the LED in it. On the back of the switch cover I used hot glue to glue the relay in place then soldered all the wires.
Flashing LED and relay mounted in the switch cover

Above is a picture of how it turned out. At this point all I had to do was snap it back into the car and connect the wires. I decided I didn't want to cut any of the existing wires in the car so I soldered some short pieces of solid wire onto the stranded wire in the picture above and heat shrinked them. The solid wire was easy to push into existing unused pins of the plugs I found under the dash and I secured them with tie wraps and electrical tape.

Car dash wires and LED installation
Above is a picture of the installed LED and the wires / plugs that are under the dash. That hole where you can see the wires is where the little compartment removed. Looking closely in the hole on the upper left you can see the single black wire that I added for the Switched +12 Volts. There is a metal frame in there too that has a few screws in it and I attached the Chassis Ground wire to one of them.
As I mentioned this entire project took about an hour from start to finish. I have a car alarm so I wasn't loosing any sleep over not having a flashing light  in my car but now I feel like everyone else who has one - and it's a good feeling!