How Can You Build a Zero Voltage Detector Circuit for a Burned-Out Tail Light?

[jssamp]

I'm trying to build a simple circuit to detect when a lamp (tail light) is burned out, and light an LED in the dash. I think I can use a couple op amps but not sure how.

My plan was to connect non-inv input to the (-) lead of the lamp socket and the other input to chassis ground. But I think this is the opposite of what I want. When the bulb burns out no current runs through it to the input line so it should be nearly the same as input 2. But this will result in linear mode and I want positive saturation to power the LED.

If anybody can guide me in the right direction I would appreciate it.

 

[Simon Bridge]

Hint: When the lamp is burned out, that part of the circuit will be open, like opening a switch.

 

[NascentOxygen]

I'm trying to build a simple circuit to detect when a lamp (tail light) is burned out, and light an LED in the dash. I think I can use a couple op amps but not sure how.

It's probably not as easy as you'd think, especially if you set a goal of not modifying the vehicle's original wiring. You could do something like adding a resistor in series with each lamp's electrical wire, and look for a voltage across that when power is applied to the lighting circuits.

Or, if you don't want to modify the wiring at all, look into using a Hall Effect sensor. It detects the magnetic field near a cable carrying current. If the bulb is not working, it will draw no current through its wires.