An archive of community.esquilo.io as of Saturday January 26, 2019.

Current on digital pins, a bit confused

JefferyS

Reading the documents it says 8-9 ma is the max if you have the high drive mode enabled.

But looking at the Esquilo schematics for the on board LEDs they have a 220 ohm resister and led connected to 3.3v. That's about 15ma of current.

So I'm a bit confused because that is a lot more than the 9ma shown in the documents for a low digital out with high drive mode enabled?

Jeffery

robertjensen

The voltage drop across the LED can be upwards of 1.5 V, so the current will be smaller than what you calculated. 15 ma would result from putting a 220 ohm resister between 3.3 V and ground, but the current in the LED circuit will be less than of half that if the drop is even 1.7 V. The drop depends on the specific LED.

JefferyS

Thanks, I totally forgot about the voltage drop :frowning:

JefferyS

It seems that most of the regular sized LEDs are using 15ma to 20ma. I know that transistors can be used to make the LEDs work off of a digital pin and I've Googled and found that a HEX Inverter SN74S05 can be used because it can supply 20ma of low level output current.

Does anyone know of any other good solutions, I'm not going to have a ton of LEDs, maybe 4 for indicator lights?

Thanks,

Jeffery

robertjensen

Darlington transistor array. E.g., ULN2803. Good for switching lots of current and even higher voltages. More than you need for an LED, but handy to have in your toolbox for general switching applications.

JefferyS

That's fantastic. I saw a ULN2003 on a board I was trying to fix, it was running relays, didn't even think about using something like that for LEDs.

Thanks,

Jeffery