can you use several connections to a single output?

I am adding ardunio projects to a Model Railroad. I am first connecting the Digital output to a terminal block. My questions are this.

  1. If two flickering LED's have the same script can they be connected to one terminal?
  2. If I connect two LED's to one terminal do I need two resistors?
  3. Instead of two resistor can a go to a larger resistor such as 1k?
  4. Am I correct that I can connect all LED Cathodes to one Ground terminal?

Stay Safe


  1. Generally, yes provided you follow some rules, e.g. no more than 20mA total current.
  2. It depends if they are in series, parallel, or just powered from the same source. If powered from the same source, then yes two resistors. That is the best way.
  3. You have it backwards. Smaller resistances make more current, not less. It isn't good to parallel LEDs using only one current limiting resistor because the current usually won't balance equally in the LEDs.
  4. Usually.

It is generally best to think of the arduino (uC) as a logic component and not a power source. Aarg has answered your questions but if you don't make the mental divide between the logic and the 'doing work' part of your circuits you are likely to get into trouble at some point. Generally a few leds here and there for prototyping on a breadboard are ok but as soon as you want to make a real world project you are normally better separating the uC from the things that need power. Your power supply can often be used to power the arduino and the various other parts of your circuit although sometimes you need to step it down for the uC. Look at things like logic level transistors for switching larger loads. These act as switches. The arduino controls the switches which allow the power to go to your loads.