As I was building my Blue LED Radio Faceplate mod, I needed to find something to do with one of the old LEDs that comes all the way out to the faceplate near the volume control knob. I decided to turn it into a red blinking LED “theft deterrent device.” Whether it actually deters theft or not is a whole separate issue and I honestly don’t care. I just think it looks cool.
I first jumped to thinking of dropping in a 555 timer with a customizable duty cycle and all that. In the end, I ended up at Radio Shack buying a single integrated blinking LED (part number 276-036.) It’s not as flexible, but it’s definitely the lowest power solution. Since this will only be blinking when the ignition is off, power consumption is very important. I finally settled on using a 680 ohm resistor in series with the LED, which results in only a 10mA draw on the battery. Here you can find a website to help you calculate the resistor value.
Normally, you can determine the polarity of an LED by finding the longer lead. That’s the positive one. However, a lot of my LEDs have the leads trimmed, so when I determine the polarity of an LED, I tend to look inside at the junction. The junction on these blinking LEDs appears reversed than normal LEDs, so pay attention to the lead length.
Also with the blinking LED, the resistor used to control the current does not appear to affect the blink rate. So, you’re pretty much stuck with whatever blink rate the LED is made for. In this case it was fine. The one I have is about 0.5 – 1Hz.
The second component to this is a normally closed (NC) relay (Radio Shack part number 275-241.) The concept is to allow power flow from the battery to the LED when ignition is off. When ignition is powered on, it energizes the relay coil and opens the circuit, turning the LED off. The NC qualification is very important because if you are energizing the coil of the relay with ignition off, you’re consuming an extra 38 mA of power when the vehicle is not running.
The picture below shows the relay taped to the inside of the radio. As you can see, the leads are leads are soldered directly to the connector. The other picture is where the LED comes out to the faceplate. I didn’t snap the picture when it was on, but you get the idea.
So, this was a really fun and very simple project, but I’m guessing it won’t be much of a deterrent. Enjoy!