I have an LED controller which, for my purposes, is a sealed black box. The LEDs strips are the typical 5050 SMD on a roll. They have a 12v positive and 3 ground signal wires. If would be great if I could feed the 3 grounds and the positive into the single color roll which just has a negative/positive wire.. But it is not that simple.

I tested the circuit against the battery ground. The 3 ground outputs read +6volts when the unit is 'off' and they are under a quarter volt when the unit is on. The 12volt line is always on. So I'm now wondering if I can have a circuit that waits for any of those lines to go low to then turn on the 12v to the additional LEDs.

I'm also puzzled why the 'grounds' would become positive 6Volts when the unit is off. That seems strange.

 

Here you go :

The diode type will depend on the LED current per string. 1N4148 will do up to ~100mA, or 1N400x beyond that.

I'm also puzzled why the 'grounds' would become positive 6Volts when the unit is off.

Your meter draws some current through the LED strings, which drops the voltage below the supply voltage.

 

Here you go :
View attachment 135797
The diode type will depend on the LED current per string. 1N4148 will do up to ~100mA, or 1N400x beyond that.

Your meter draws some current through the LED strings, which drops the voltage below the supply voltage.

So If I'm reading right I just need 1 diode off each line all to the negative side of the single color LED strip?

 

Yes. The diode voltage drop will reduce the LED string voltage by about 0.7V, but that probably won't make much visible difference to the LEDs' brightness.

 

Yes. The diode voltage drop will reduce the LED string voltage by about 0.7V, but that probably won't make much visible difference to the LEDs' brightness.

Thank you. I purchased some 1N4004. I hope to report back later this week.

 

My colleague bought few rgb led strips. They all were common anode and 12V. Sounds like ones you have.
The draw was 500mA per strip per channel. I suggest doing same measurements, that will allow you to use them any way you like.