Combine Stop and Turn lights
If you have a car with separate stop and turn lights (European style) and you need to use a trailer with combined red stop / turn lights (American style) then you need a circuit that will combine the two into one. All the solutions I find online are complicated electronic circuits which need separate power supply.
Here is a simple solution which only uses relays and does not need any separate supply. It is the same circuit represented two different ways. You need one for the left side and one for the right side so four relays in total.