arduinoswitches
I want to trigger a LED with both an Arduino and a hardware switch without coding any button logic in the Arduino.
simulate this circuit – Schematic created using CircuitLab
When the button is pressed there is a direct connection between the 5V power supply and Arduino pin 17. Can this do any harm? Is there a better solution?
You can add a parallel path with a transistor for current to flow from your 5V rail through the LED to ground. Effectively you're building an OR-gate, so the LED will be on if the button is pushed OR your micro-controller decides it wants the LED on.
Here's an example schematic using a N-channel MOSFET. The MOSFET is on when the IO pin is driven high, and it's off when the IO pin is driven low.
I can see why you'd wand this since it'd be really nice to be able to reprogram the arduino at every time although having separate grounds while still connected to the same PC is going to get a little hairy.
The USB and the Molex connector share a ground internal to the computer so if you wanted to separate that, you'd have to modify the USB board to use a different ground. This gets into heavy PC modding and can get pretty hairy quickly.
Best Answer
What you have drawn is a bad idea. If the Arduino output is low when the button is pressed, the 5V bus will be shorted. That's really bad. Instead, do something like this.
simulate this circuit – Schematic created using CircuitLab