Everything for Electronics

From the Q&A

With TJ Byers

Trailer Light Monitor


I want to create a device that you can connect to a trailer hitch wire harness that will tell the driver if the trailer lights are burned out or not. I am looking for a unit with a few probes that I could connect to the trailer hitch wires that would monitor for 12 volts and current flow at the same time. I will then develop some software to act on this input and display something like: Left trailer turn signal light is burned out or left trailer brake light is burned out, etc.

— Dave DiMenichi


Can you live with an “ignition-on” lamp check? If so, I can really simplify your problem. That is, you only check the lights at start up with the lights turned off, and don’t monitor them 24/7.

The circuit is shown in Figure 8. In this circuit, a very small current is applied to the lamps when the “Press To Test” button is pushed. If all is well — no open bulbs — all four OK lamps will light. Why? Because if the filament — which has a resistance of less than 100 ohms — is open, it will have a logic high instead of a logic low, and that LED won’t glow. This logic is fed into four 1N4148 diodes, which are the equivalent of a four-input AND logic gate where any fault lights the Alert LED. If you have a defective bulb, the Alert lamp will light. Should you hardwire the push-button switch closed, you can follow the action of the trailer lights as you change turn switches and brake pedal — but it will take some visual translation.