I would suggest trying this circuit so the strip will dim with the dome light.
View attachment 308306

I realized after posting that peak detection may not be needed. But, as a suggestion, you might want to add a PMOS stage so the TS can connect the strip directly to chassis ground.

 

I would suggest trying this circuit so the strip will dim with the dome light.
View attachment 308306

TS was originally concerned about damaging the unknown PWM signal output so I added an op amp and push-pull gate driver. This way the input draws no current directly and the FET's inrush current is limited to 100mA plus the beefy flyback diode protects the FET.

My reasoning for doing this is allow for a fairly high PWM frequency without stressing the FET.

In your circuit, the turn on time will be instant but the turn off time will be slow due to the pull down resistor. This will lead to extra heating if the frequency is too high because the FET will drop volts during the slow turn off period. Since he is switching 6A, I can see this being a big problem.

I realized after posting that peak detection may not be needed. But, as a suggestion, you might want to add a PMOS stage so the TS can connect the strip directly to chassis ground.

What difference does it make?

 

In your circuit, the turn on time will be instant but the turn off time will be slow due to the pull down resistor.

I don't see that as an issue as the PWM frequencies are typically less then 5K.

 

I don't see that as an issue as the PWM frequencies are typically less then 5K.

Agreed but we don't know the frequency and TS doesn't have a tester. It's one of those situations where I tried to imagine everything that could go wrong. I'm glad you guys jumped in because no one else was offering solutions!

 

Agreed but we don't know the frequency and TS doesn't have a tester

He might have a tester as a lot of meters have frequency counters. Just need to know the model number.

 

What difference does it make?

it permits the use of a convenient chassis ground.

 

This way the input draws no current directly and the FET's inrush current is limited to 100mA plus the beefy flyback diode protects the FET.

I’m still unsure how to proceed. I’m thinking the schematic you came up with seems the safest to prevent possible damage to the bcm.

I guess all that is left is to build it?

 

I’m still unsure how to proceed. I’m thinking the schematic you came up with seems the safest to prevent possible damage to the bcm.

I guess all that is left is to build it?

It's a pretty easy project so I think you'll be fine with either circuit but I will stress that you keep a close eye on it for the first week. The main concern is excess heating but that shouldn't be a problem unless the PWM frequency is very high. If the costs are moot to you, I recommend a large heatsink which costs a few bucks. Keeping things cool = longer lifetime.

Also, I didn't get around to testing the circuit but I will tonight. I'll post the results soon.

 

It's a pretty easy project so I think you'll be fine with either circuit

I assume the best way for now is to mount to a breadboard and use wires to connect. Then scale up to possibly a premade PCB for additional trucks so it’s easier for my coworkers.

If the costs are moot to you, I recommend a large heatsink which costs a few bucks.

Which components need the heatsink? I am not concerned about costs per se and it offers more protection so I’m good with that.

 

Hey guys,

I am trying to achieve the same thing just on a ford. I want to add 4 LED lights in the cabin because the factory lights just don't do the job. My goal is to have them mimic the factory lighting. I can easy tap into the rear dome light and already have connectors to make a jumper. Using my scope i was able to record some info to show what I'm dealing with. The dome light is a 3 pin connector.
In the pictures i have Channel 1 (yellow) in the Control? wire, Channel 2 (green) in the constant 12v and my ground in the connector ground. This is a little over my head so any help would be awesome