I'm trying to copy a build from someone. He was controlling an rc car with an arduino with the use of optocouplers. He used them to simulate a switch. He had a very nice clear instruction of how to make it yourself, but I could not figure out what types of resistors I would need to use.
He posted this schematic:
Anything between 180 - 330 Ohm would be OK.
My friend told me I had to use a 1k ohm transistor. And some places I read 47 ohm resistors 
My friend told me I had to use a 1k ohm transistor
Anything between 180 Ohm and 1k would be OK.
This might also depend on the current through the RC controller switch. Optocouplers don't work as amplifiers. If you measure 1mA on the switch side, allow for 3..5mA on the Arduino side. (= 1k Resistor)
47 Ohm is too small for an Arduino output pin.