Re: Larson Scanner Kit with 27 LEDS

Windell Oskay

You probably can’t really drive 3 LEDs in series directly from each output.   

The absolute maximum voltage for the microcontroller is 5.5 V; do not exceed that value.    The actual voltage output may be up to 0.8 V below the input voltage, so you may get as low as 4.2 V output– only 1.4 V/LED.  If this is the case, the LEDs may not light up at all, even without a resistor.  On the other hand, the voltage drop on the output may be lower, so it’s possible that you’d get up to 1.83 V/LED, which may be enough to overdrive them.  So, you are trying to match the 3X the forward voltage of the LEDs– a not very specific spec –to be less than or equal to the power supply, less the voltage drop of the microcontroller– a not very specific spec.   
You’re welcome to try, of course, but I wouldn’t guarantee that it would work.