Answer 5
This circuit is actually a simple form of delta-sigma modulator being used as an A/D converter. The value of variable c represents the voltage at the wiper of the potentiometer.

Negative feedback is used to make the voltage at the output of the RC filter track the voltage coming from the potentiometer. If the negative input of the comparator is too high, then the digital output goes low to pull it lower. If it's too low, then the digital output goes high to pull it up. Overall, the duty cycle of the digital output will match the voltage on the potentiometer expressed as a percentage of the power supply voltage.

The software measures the duty cycle of the digital output by counting the intervals that it's high within a period of 256 intervals. This creates a simple kind of down-sampling low-pass filter in the digital domain. The output sample rate is 2560/256 = 10 Hz.

Contributor: Dave Tweed

Published May 2003