Road train lighting

[ricm]

you've got the right approach - you'll need 3 opto-couplers as you've correctly identified, you can buy a single chip that has 4 of them all bundled together.

Each LED might only need 20mA so the unit you've got might be OK, but I would agree to buying a higher capacity unit to install. Maybe use the ones you've bought for R&D.

So you need to splice the 'input' side of each coupler into your trailer wiring loom...the coupler needs to be in series with the relevant LED circuit so it can 'grab' the signal. The 'output' side then also needs a power supply for subsequent LEDs.

You will need at least 5V supply because the tail/ brake lamp LEDs are connected in series with around 2.4V each. If using higher voltage supply, and when powering only 1 LED (eg. turn signal), make sure you have the right protection resistors where required. I use 7.2V supply because I find it easier to have same battery packs throughout my truck/ trailer combos. I have wireless control for my trailer lights so I've had to install a battery in each trailer. In your case if you plan to retain the standard hard wired system, I would suggest that you have a 'power supply' bus in each trailer (ie. a +ve and GND line) with the source being the truck. This way, you can avoid having to fit batteries everywhere.

Whilst technically you can put the optocouplers in the first trailer to switch LEDs in the other trailers, it will mean that you always have to couple your trailers together in the same sequence. I've put optocouplers in all my trailers so that ANY trailer can be hooked up to ANY of my prime movers, in ANY sequence. A bit more effort, but easier to work with once you're done...

[ricm]

for single opto, I used 4N25 everywhere.

[ricm]

Originally Posted by ricm

the tail/ brake lamp LEDs are connected in series

might have got that bit wrong - you'll figure it out if you look at the way they're hooked up....either way, I would suggest 5V minimum with protective resistors. I think the MFU actually provides 6V output if you measure it with a multimeter. The voltage that the LED sees is then dropped down through a protective resistor, once the LED lead is plugged in and the circuit is completed