How to Compute Resistor Value?
September 9, 2009
I want to use this phototransistor in an Arduino project: http://www.sparkfun.com/datasheets/BOT/QRD1114.pdf
Based on the data sheet, I believe pins 2 and 4 go to ground. Pin 3 connects to +5 with a 66 ohms resistor in between. I got this value by the following computation:
R1 = (Vs - Ev) / If
Vs = supply voltage, which is 5V on Arduino board
Ev = emitter forward voltage, which the data sheet states as 1.7V
If = emitter continuous forward current in Amperes, which the data sheet states as 50mA (so that converts to 0.05A)
Applying the formula, we get the following:
R1 = (5V - 1.7V) / 0.05A
R1 = 3.3V / 0.05A
R1 = 66 ohms
I think I need a "bias resistor", but do not know how to compute that.
So, my questions are:
1. Did I compute R1 correctly?
2. How do I compute bias resistor?