I’m trying calculate resistor required to drive a single led from A2 on Gemma MO.
Looking for the brightest LED solution that will last a while, as LED is glued in place.
Options are all white LEDs ADA778, ADA846 or FIT0372
Question A.
What voltage if on pad A2 (powered by LiPo battey 3.7V)?
Question B.
When calculate for ADA778 LED, I’m confused by the adafruit assembly info which shows 51 ohm ?
I have www.quickar.com
So say supply 3.7V, Vf 3.0-3.4, 20mA max gives 35-15 ohm… which is much less that tutorial 51 ohm ?
Hence my confusion.
Any advice please ?
Is my understanding correct … that the output of the Gemma MO pads (A0,A1,A2) of ~7mA is the limiting factor with driving my led ? not the max current capacity of the LEDs.
Hi Gregory,
I think you are overthinking this a bit. The pin A2 will output 3.3v. Use a 330 ohm resistor and you will be fine. Most white LEDs are equivalent brightness so I don’t think you will find much difference between the different brands. 
Just as an FYI the limiting factor is the output of the GPIO. You will always choose the resistor so the the current does not exceed the minimum of Maximum output current and the Maximum load current.
Thank you. Agreed…Gemma Mo is stated at 7mA…so limiting resistor calc based on 3.3v and 6mA.(to be safe)…about 50ohm for led ada778 Vf of 3.0v
1 Like
