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...