Headlight Alarm Circuit

Posted on Feb 5, 2014

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.

Headlight Alarm Circuit
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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. 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...

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