Memoirs of Fls'Zen

Wednesday, January 28, 2009

Headlight Alarm Circuit

Have you ever left you headlights on because your car either does not have an alarm or its existing alarm doesn't work? A friend of mine recently had this problem. I easily found a few existing solutions, however, I believed I could make a simpler circuit (especially that last one).

The circuit design itself has a solitary goal—to sound a buzzer when the headlights are on and the car off.

My first concern was where to get power from. The most outrageous option would be to provide power via a battery pack. A more likely source would be the car's battery. However, since the circuit's goal is to sound the buzzer when the headlights are on, we might as well take the power from the headlights.

In order to sound the buzzer when the car is off, there are two things to consider. First, a signal source needs to be determined. In the case of my circuit, I chose an accessory line. The accessory line is be grounded when the power is cut (car off) and is otherwise on, so we'll have an easy comparison. Second is the problem of determining when the accessory line is low. I first considered a comparator, but I soon realized that all I would need is a PNP transistor to switch the buzzer on and off. With this component, the headlight line is connected to the emitter, the base is connected to the accessory line, and the collector is connected the buzzer. Therefore, when the headlights are on and the accessories are off, the emitter conducts to the collector and the buzzer sounds. Also with this arrangement, if the headlights are off or the accessories and headlights are on, nothing happens. Additionally a resistor is connected between the accessory line and the transistor to limit the current flowing to ground when the buzzer is sounding.

Here's the circuit diagram from Multisim:

As you can see in the diagram, the headlights are on and the accessories are off. There is a little less than battery voltage flowing through the buzzer and ~1mA leaking to ground through the transistor. The transistor in the design was chosen specifically since it was rated for much higher voltage than what the circuit will ever see. It should be possible to use a general purpose transistor, but I did not want to bother with it.

After proving the circuit design in Multisim, I moved on to prototyping. This circuit can be built, complete with enclosure, from the following Mouser components (1 each):
660-CF1/2CT52R103J - 10kΩ Resistor
546-1593KBK - Enclosure
854-PR1593K - Enclosure Circuit Board
254-35C5-ROX - Buzzer
511-D45H11 - Transistor (This component differs slightly from the Multisim diagram—it is rated for higher voltage, and it is cheaper.)

The buzzer could easily be substituted for something else. I chose this one since it was nice and loud. You may want one a little more . . . subtle.

The prototype was constructed and tested with both 3V and 12V. Make note that if the accessory line is floating, the alarm won't sound, it needs to be tied to ground. I connected 10" lead wires to each of the headlight, accessory, and ground lines of the circuit. This length of wire seems to be appropriate for installation to a car's fuse box.

I then installed and tested the circuit in a car, a Honda. The circuit enclosure fit nicely under the dash near the fuse box. The wires fit snugly into the fuse socket, with the fuse being re-installed after the wire was inserted. In this application, I connected the headlight line to the right headlight low-beam fuse, the accessory line was connected to the turn signal fuse, and ground was connected to the chassis.

If you are installing the circuit into a car, I found that it was easiest to use my multimeter to check the voltage of the fuses before I attempted to install the wires into the fuse sockets. In the Honda, the car's circuits were switched before the fuse, so the turn signal fuse was tied to only ground when the car was off and energized otherwise, making it a good spot to connect to. Your car may differ. If need be you could always tap an existing wire.

Have fun!



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