Arduino + Relay Board Additional LEDS

Hello to everyone,

I am a newbee in electronics and I am trying to control some lights using a 4 socket relay Board and an arduino uno. I have "gutted" an old power bar in order to get the electronics into it. What I would like to do is to have leds over each socket turning on when the circuit is active. I have some surface mounted mini leds on the relay board. My question: Can I just connect the new leds using the mini leds (parallel - like when using the blinky sketch and an external Led on the arduino 13 pin...)? If yes will I need a resistor? If not how do I do it?

Thank you very much for your time.

Theoretically, yes you could. You would calculate the series resistors for the new leds just as though the relay board leds were not there. But you should do one extra calculation just to be safe. An Arduino pin is rated for a maximum of 40mA, which means you should avoid exceeding 25~30mA in practice. What you need to check is that driving two leds plus the transistor that drives the relay coil does not exceed 30mA in total. You can calculate the current that your new leds will draw knowing the led's forward voltage and the series resistor value. For the existing led and transistor, you may be able to work out the current from the schematic of the relay board but it will probably be easier to simply measure the current with a multimeter.

Thank you very much for your time and input!