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Octolively load resistor calculations

edited October 2012 in General
Could you provide details as to how you came to 200ohm resistors for RED leds shown on the schematic. Maybe my calculation are incorrect but I come up with 180ohm. I want to make sure I do not exceed the 40ma max pin.


  • As it says on the BOM, the kits with red LEDs actually have 220 ohm resistors; the "200 ohm" figure was a preliminary estimate.  

    The number that you need to be concerned about is normally the total current on the MCU, which needs to be 200 mA or less.  That gives you nearly 40 mA (for one IR LED) plus a little under 8*20 mA for the 8 visible LEDs, plus a few mA to run the MCU. With the standard component set, it runs pretty close to the limit.
  • Thank you. Hungry IR leds.
  • You can run them lower (and then, the other LEDs brighter), but you get better sensor range by running them higher.  
  • Going to have to play with one of the boards. Maybe I will use headers in R pins and see how this work out.
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