Circuit #28 : Driving Multiple LEDís
To turn on multiple LEDís using a single digital output, it is recommended to put all the LEDís in series so that they all have the same current and all appear equally bright. Begin with the circuit below.
Q1 has to be a logic-level MOSFET, like the part number suggested in the figure, though there are lots of other logic-level MOSFETís to choose from. The supply voltage V+ has to be larger than the forward voltage of each LED (Vf), multiplied by N, the number of LEDís in series:
The current limit resistor Rlimit can be estimated according to:
where is the desired amount of current in the LED string.