Blinking Red LED Car Alarm Light

(Click for version I)

*WARNING* This modification may be the cause of a battery drain problem I was having. Use this info at your own risk.

PROBLEM: My Elantra GT has an alarm, but no visual deterrent!

SOLUTION: “Install” a blinking red LED similar to those on other alarm systems.

1 – bright red 5mm Blinking LEDs:
1 – 470 ohm resistor.]
1 – 5mm LED holders from RadioShack: Item #: 276-079
misc. clips & wire.

First I removed one of the dummy switch panel / place holders next to the panel dimmer switch. Use instructions similar to these by southpawboston. Next I soldered some wires to the LED and soldered them to some quick connect clips. Then I drilled a 1/4 inch hole into the center of the switch panel and inserted the LED and clip.

I was going to use a relay, but saw a great post on CandlePowerForums about a better / easier way to do this. A fellow CPFer, CNC Dan said, “Find a source of +12v that is on when the car is off.(clock power?) Then find a source of +12v that is only on when the car is running and always uses power.(daytime running lights? ignition?) Wire your blink led/circuit using the first wire as your POS. connection and the second wire as your NEG. connection. When your car is on both sides of your circuit get +12v so no current flows. Whe the car is off, the switched +12v goes to 0 volts and your circuit sees +12v and 0v and begins to flash.”

What I didn’t know was that the fuses could act as ground when they were not on. I did some tests with my multimeter and saw that everything would work as mentioned in the forum.

Here is my schematic:

Next I needed to tie into the fuses. I was about to run down to Radio Shack for some fuse taps, but decided to try simple wire wrapping… this proved to work very well. I soldered on the other quick connects and put everything together. I made sure everything was insulated with electrical tape and shrink tubing.

Here’s the final product:

Notes:The blinking red LED was designed to focus the light into a tight viewing angle (15 deg). This made the light very bright in one spot, but not very visible from the sides. I took it over to the grinder and removed the cone and flattened the top off the LED to make it diffuse the light. This worked GREAT.


This modification may be the cause of a battery drain problem I was having. Use this info at your own risk.

43 thoughts on “Blinking Red LED Car Alarm Light”

  1. Barry, I really with I had an answer for you. Unfortunately I wasn’t able to track down the issue and I haven’t had the guts to attempt it again. I’ll try to schedule some time to give it another go when I’m sure I won’t be needing my car… just in case I drain the battery. ๐Ÿ˜‰

  2. Thanks for the quick reply.

    I had ordered a “blinking” LED light with a red and black leads that had no instructions other than it was for an add-on to an existing alarm. I thought I could wire it the same as yours, but was wondering why you added a resistor.

    Also, it seems that a light with little current draw would take forever to wear down the battery.

    Thanks for any help you can provide.


  3. Barry, LEDs will blow if you feed them the full 11 – 14 volts from your car battery. The red LEDs usually like to run at around 2 volts, so the resistor is necessary to reduce the flow.

    The drain on the battery (if even related to this project) couldn’t have come from the LED alone, but would have come from the way I had this wired to the fuses.

  4. I’ve had a similar problem with an LED alarm light I hooked up to my daughter’s Scion XB The “fake” alarm LED light kit was purchased on eBay. I don’t recall it having any resistors, just three wires.

    I have been having dead battery issues since I’ve installed it. Wouldn’t think that the LED would drain the battery after a couple of weeks, but it appear to be doing just that. Battery and alternator have tested good.

    Has Rob determined if the LED was the source of his battery problem?

  5. Gerardo: Do a search on ebay for “blinking red led” or something similar and you should find them.

    Peter: I haven’t narrowed the power drain down to this modification yet. I rely on my car too much right now to even attempt it. I’d love to have someone with a bit more car electronics experience review my diagram to give feedback on if the way I have this configured is viable.

  6. The LED shouldn’t drain the car battery that fast, maybe after a couple of weeks continuous use, think about cars actually with alarms in them how come their batteries don’t drain down when they have the exact same LED constantly flashing, there must be another problem with your battery

  7. James, you are spot on. A car battery has on average 45 amp hours (45,000 milliamp hours) and the little red LED uses at most 20 mA that means it could run for 2,250 hours straight, or 93 days, or 3 months!

    My thought is that there is a problem with how the circuit is laid out specifically my use of the fuses to power and turn off the LED. I think something other than just the tiny LED is draining power.

  8. I was just reading these posts. I think the batter drain problem is caused by the way you have the circuit wired. You decided NOT to use a relay. Instead you went two existing circuits. While this approach may work, you don’t know what else is on those circuits and if their components are able to “handle” the fake ground you are causing them to imitate.

    I had a similar blinking LED in one of my cars. I used a relay to power the LED when the car was off. It lasted for 5 years with no problems.

    Bottom line is USE A RELAY!!!!! This way you are only tapping off of the battery’s power and not adding any other circuitry into the mix.

  9. I wanted the same fake alarm blinking led, went to Radio Shack and they fixed me up with the following parts and a basic wiring diagram:
    blinking led 3.0-3.8 volt part no. 276-312
    led holder part no. 276-079
    270-Ohm resistor part no. 271-1112
    12vdc relay part no. 275-241

    soldered connections, used some heat shrink and all is working. Lights off when ignition switch is on and starts binking when ignition is off. Not sure how to post a picture on this site but if someone could tell me I could attach the diagram.

  10. ok the proper way to do this is to buy a 12volt rated flashing led. Then mount it and run the positive lead to a constant power. Run the neg lead to prong 87a on a 5 pin relay. Run power with ignition to prong 85 run ground to prong 96 and 30. you now have a led that flashes with the car off and not while the car is on. Not you will have a ground on prong 87 only when the car is on.

  11. I bought a blinking light with two wires attached, one goes to a push on/off switch and the other to a ground, from the switch one side to a power source and the other side is from the light as mentioned above, I have done this for years and have had no problems and no battery drain problem, maybe all the relays and resistors are your problem………

  12. Michael,

    Can you send me an email describing how to connect the wires to the 12vdc relay and where to put the resistor(s)? If I look at the bottom view of the relay I have two prongs on the upper left, side by side, and two on the lower left, side by side, and then one single prong on the lower right. Can you describe how to wire this up?


  13. Just an update. I took a chance to see if this circuit would drain my battery overnight. I just couldn’t understand how an LED with such a small drain could quickly cause a dead battery. Maybe after a few weeks it would but my car doesn’t sit that long. Anyway, the car started fine the next day and has ever since. I used the wire for the cigarette lighter (the unswitched +) and the electric window wire (the switched +) for the neg. lead of the LED. It works great!

    Thanks for the info,

  14. anyone have a wiring diagram for this? I have all the parts from Radioshack but haven’t seen anyone be specific about how to actually hook it up. if so can you please, please, please email it to THANKS

  15. Hey so i got everything from radio shack also . i couldn’t figure out the relay so im just using a switch but can some one pls send me the wiring diagram so i can get it to work with a relay?

  16. All good but you can go as low as 100 ohm on the res and you should add a signal diode to catch any spikes I always connect the red wire direct to a fused 12v supply (direct from the fuse box) The other wire needs to be connected to the ignition feed so that when the ignition is off the LED will back feed earth most likely via the coil of a relay. Often hear a soft click after installing a dummy alarm never bothered to cheek never had any complaints if device does not work change other wire of LED (EARTH) to the accessory feed. Better solution is to get an insurance policy.

  17. I have an insurance policy but I’m getting sick of the stomach in my throat feeling every time i go to open my door and it’s unlocked with my seat kicked all the way back and all the items from my glove box strewn about the car….then seeing the open space where my deck used to be. I’d love to catch someone in the act. Anyone with me on this one?

  18. Got my flashing light from bay. Attached to a ‘lighter plug’ from a cell phone car charger, plugged in and light flashed for 5 – 10 minutes then stayed on. Do I need a resistor and what is the size? If I can get it working seems like a simple solution , no switches or heavy wiring just pull the plug when not in use. Too easy?

  19. LED Lights does not drain the car battery for it uses a very small amount of current. I tested a Red Blinking Light on a 9V Duracell battery and left it blinking for 1 year (Approximately the duration of replacing my home smoke alarm battery) …
    A resistor is a MUST HAVE. You need a 470 Ohm to reduce the 12V current to a feasible 2.0-2.5 Volts or else you will burn out the LED (These parts can be bought at your local Radio Shack and they are not expensive to build. I built mine using a 12 volt relay, so when the car is off, there’s no juice to the Relay and my 9V battery powers up the LED, and when I start the car, it activates the relay and it turns off my LED. I have not changed the 9V batter for the past 9 months. Another way to get fancy is to build a small circuit board useing a 555 IC Timer and a Blue LED however this will use a little more current so you will get at least 6 months of use from a 9V battery.
    Hope this helps.

  20. Dan, that is most interesting , I am still trying to connect this light and after reading your comments will try again. So far I have connected the light with two 270 Ohm resistors to the 12V. It still blinks for a time, then stays on. I will now try three 270 Ohms and see if that works, I would have thought that 540 Ohms would have lowered the power sufficiently, but no. This is the last shot at it, after this it is 2 1.5V Duracell. batteries to power it up, I know they work OK, it just seemed to be more reasonable to hook up to the car battery. I really hate to give up. Your reply was most helpful

  21. I forgot to say that I am using a BLUE flshing light which may have a different rating. Any comments,Thanks

  22. The blue LED can take a little higher voltage but no more than 3.5 volts. Another good idea is to use a +5 Voltage regulator using 78L05 or 7805 IC to drop the 12V to a 5 volt. A simple circuit can be built from this website: the input voltage range can be from 7V and 20V and the output voltage is between 4.75V รขโ‚ฌโ€œ 5.25V. It’s a very simple circuit to build, just add a small heat sink to the IC. On thing that you have to remember, a car battery has a higher Amp than a standard battery which it’s probably causing your LED lights to fail. I’m in the process of building another one myself this weekend using a different design and if I’m successful then I will not only post it here but also have a simple schematic available for free if anyone wants it.

  23. go to they have all kinds of hookup diagrams. let me know what you think about the site, this should help you wire just about any kind of auto circuit

  24. Hi guys. Just wandered into this thread and thought i’d share a little relevant story re: led power drain.

    About 15 years ago, i installed a small dummy security camera outside my mothers house. It had a small flashing led powered by the small squareish 9v battery with the poles beside each other that used to be very common. Think it might have been called a pp3 irc. Anyway, that little led finally quit flashing about 3 years ago…..Thats 12 years flashing on that one little battery !!!

    Best example of the miniscule power consumption of one led that i can think off anyway.

  25. A friend of mine wanted a flashing LED for his car and just hooked up a low voltage 5mm flashing red LED to a 9V battery. Drained the 9V in about 5 days or so, which is sort of expected since the LED consumed about 20mA at 3V or so (using resistor) and the 9V battery provided about 5000mAh.

    I copied this circuit and put it on a custom PCB for him

    Runs on 2xAA batteries and can use any LED. Using the TLC555 series of 555 timer ICs which is very low power and apparently according to that site should last for months so we’ll see how it goes.

  26. After months of playing around with a connection from the car accessory outlet (blowing the fuse in the meantime) I finally gave up with that idea. I went to a local surpluss store and bought a holder for 2- C type batteries at $0.75, the batteries were two for a dollar from Dollarmar. I then hooked up one of the flashing blue lights that I bought on Ebay $2.50. Total cost $4.25, time to connect 10 minutes, no wiring diagrams. This was at the beginning of October and the lights are still flashing. I put the batteries in the sunglass holder above the windshield with the bulb hanging down where it is quite visible. Some times I turn it off for daylight mostly I do not. These batteries could last a year or so, at which time I will buy another pair. Thank you Darren for the idea I changed it a little bit.

  27. I’m going to grab some relays and play around a bit. I like David’s approach above with wiring to a 12v. relay and I think it is a good idea. My only question is this: Where is the best place to find a ground in my fuse box? I looked all over and couldn’t find anything accessible to ground to.

    I’m pretty inexperienced with relays, so when David said, “run ground to prong 96 and 30” above I’m guessing he actually meant 86 and not 96, right?

  28. Well, yesterday I visited a well respected car audio / alarm company in the area and talked to 2 guys that have been doing installs a LONG time (13 and 25 years). Both of them said they would wire this light exactly the way I originally did and they actually would NOT use a relay (but they gave me a few free ones to play with anyway… SCORE!).

    I told them about my battery problems and they said it was almost guaranteed to be a coincidence. Honestly, the more I think about this, the more I tend to agree with them. My car battery was already about 6 years old and I believe was already having problems independent of this circuit.

    So, in the interest of experimentation and taking one for the team I’ve hooked the circuit back up as originally designed. I’ll keep you posed and let you know if it works… or my wife will let you know if my car blows up. ๐Ÿ˜‰

  29. I can’t believe that you guys are still fooling around with a 12v solution to the flashing light problem.
    after a while I think (actually I know) it gets to be an ego thing. My solution, see 5 Feb above, all over in 10 minutes. Maybe better from the car battery, but life is too short.

  30. Ray, your solution sounds exactly like my earlier solution here:

    The problem with that / your solution is that you have to manually turn the light on and off every time you get in / out of your car. I got tired of doing that after about 10 days. ๐Ÿ˜‰

    So far, the re-implementation of my idea first discussed in this post is working great, and I think my total cost was only about $1.50

  31. Hey Rob. I tried this and it works. But what I did is set up the Negative side to the Window Fuse. It only works when they key is on. And the Positive side to the Lock/unlock Fuse. Thanks for your input!!

  32. The relay solution with the LED has two circuits. There is the current drain of the LED when the relay contacts are closed. There is also the current drain through the relay coil. I suggest using a normally closed relay. That way when the car is off the relay coil will be drawing no current. When the car is on and running the relay coil will be drawing current while the cars battery is being charged.

  33. PEOPLE, why are you ordering blinking LEDs from ebay? Make it easy on yourself, you can order ALL parts, blinking red LEDs, resistors, LED holders, EVERYTHING, from your local radioshack.

  34. My soution with the 2 c type batteries is still working after 16 months. I do not turn off the light in the daytime, so to be accurate it has been blinking non-stop for 12 months. Actually I cannot believe it these batteries came out of the dollar store. Total cost of the installation is $2.00 and only the batteries need to be replaced

  35. didn’t have time for all this so about 5 years ago i found a toy at Dolarama that had a blinking LED, and ran on 9v, so i open it up, cut the LED then added some wires to another LED that i had and put all of it under the ceiling panel and the LED is blinking just over the rearview mirrow, i had to take it down once to change the 9v. it blinks every 32 sec or so, faster blinkers will drain faster and slower well last even longer but have less chance of beeing seen. Total cost $1.15 tax included. For some is the DIY part, for others is just to have it work in as little time as possible and with low cost and non need to get at it every month or soo… i’m very happy with my solution and have been in a parking lot where 7 cars has been broken into and they skiped mine, i was the only one with a blinking light

  36. Mario, my solution is as above,
    I never turn off the light and after 2 years the 2 C batteries are still going strong. Anyway, my question is, what top did you buy from Dollarama? Glad you kept the thieves away.

  37. Hi I have installed 2of these in my xsara picasso after having my wing mirror kicked off has been working now with no problems for over 15 months

  38. I should say got them off eBay less rated 9-12v so came off car supply perm live and switch live really pleased with it

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