I am building a test circuit to connect to my 12v trailer harness and I would like to have an indicator light come on if there is no connect or the circuit is open without me having to go around and check each light. How would I do this? I have searched the forums and can not find a working option. Thanks for the help.

 

Each Light is a separate Circuit,
so You will need at least 4 detection Circuits, Running-Lights, Brake, and L&R-Turn-Signals.
And of course they won't show a connection unless they are supposed to be on at that particular time.

This is not a very practical idea.

It would be better to create the highest possible quality
Wiring, Lighting, and Connector-System on your Trailer,
then test them each time before getting on the Road.

Then, don't worry about any of it,
if You did a top-quality, premium job, nothing will fail to work.

If You want to "cut-corners", your new "test-system" is just as likely to fail as any of the lights are,
so what's the point ?
.
.
.

 

I am not "cutting corners", I live in a remote area and I often am asked to help trouble shoot trailer wiring for others since I am one of the only ones in the area with automotive wiring experience.
Right now I just have to use leads from a battery to check each circuit and so I was going to build a circuit attached to a battery and a trailer plug socket That would allow me to quickly switch between the trailer circuits as I am troubleshooting them.
I was just hopping there would be a way to get a visual feedback from the circuit that the connection was shorted or open without going around and checking the lights or using the meter.

 

This sort of thing was popular years ago.

A reed switch with a coil around it will operate when enough current flows.

Maybe you can work on this?
It can be made to light on a fault too. But the unit will need an isolation switch when no trailer is attached.
There is some info here...
https://www.electroschematics.com/reed-switch-as-a-current-monitor/

 

This sort of thing was popular years ago.

A reed switch with a coil around it will operate when enough current flows.
View attachment 327424
Maybe you can work on this?
It can be made to light on a fault too. But the unit will need an isolation switch when no trailer is attached.
There is some info here...
https://www.electroschematics.com/reed-switch-as-a-current-monitor/

Thanks, this gives me a start.

 

You can create a useful quick-checker using a series 12 volt bulb of s similar rating to the ones on the trailer, a tail-light would be fine, Then the external 12 volt source. A correct circuit will show about half brightness, as bot bulbs in series will light, an open will show nothing, and a short will give full bright on the tester bulb. Then a momentary switch to bypass the tester bulb and light that trailer bulb full-on. That scheme is simple, cheap, and easy and tells which of three conditions in a hurry.

 

You can create a useful quick-checker using a series 12 volt bulb of s similar rating to the ones on the trailer, a tail-light would be fine, Then the external 12 volt source. A correct circuit will show about half brightness, as bot bulbs in series will light, an open will show nothing, and a short will give full bright on the tester bulb. Then a momentary switch to bypass the tester bulb and light that trailer bulb full-on. That scheme is simple, cheap, and easy and tells which of three conditions in a hurry.

I like that idea, thanks I will try that first.

 

You could even assemble a mating connector and have a box with several lights.