10.5.3 Example

Let's say the voltage swing (of the phototransistor output) is between 1.8V and 4V. We want to make the switch-over point centered at 2.9V with 0.5V of hysteresis. This means that we want the comparator to output high when the voltage drops below 2.4V, and output low when the voltage exceeds 3.4V. There should be no change when the voltage is between 2.4V and 3.4V.

This can be achieved by 130k for R2, 200k for R3 and 330k for R5. Once one of the three resistor values is fixed (R5, for example), the other two can be solved in closed form. It is the ratio among the resistor values that is important.