Can Relay Outputs Directly Drive LEDS ? (for testing)

[DonRotolo]

Originally Posted by de_

All 8 relay ports are in use this year (that's a lot of Spikes & PWM cables !). We like to test our software out first on a table test board as much as possible. We would prefer to not have to dedicate 8 more spikes and pwms just to have visual confirmation of the state of the relay outputs.

If I read Kevin Watsons CPU to RC I/O mapping chart and the CPU spec also posted by KW, it appears each pin might be able to provide 25ma, enough to power a led (with an appropriate dropping resistor). We would probably limit it to 10-15ma (just enough to see reliably).

Anyone one know for certain this is doable or more importantly if its a bad idea ? I would hate to blow our test RC. (We asked Innovation First but did not hear back.)

For the price of an RC I would research the spec myself; however if an LED draws 15 mA and the output can source 15 mA or more, then the answer is yes.

While you are looking at those specs, also see what the output can "sink" - that is, how much current can it allow to flow when it acts as a ground (and not a power 'source'). Most circuits can sink two or more times the current that they can source. If you want to do this, connect the anode of the LED to +5v, and the cathode (through a dropping resistor, maybe 82 to 120 Ohms) to the digital (relay) output pin.

Don

[EDIT]Digital pins can only source 7 mA. I'd test them by allowing them to sink the current instead, maybe ask IFI