COMPARATOR

COMPARATOR

Show how GPIO can act as comparator.

Demo

1. Potentiometer acting as voltage divider (3.3V/gnd from Pi), connect COMPARE gpio to wiper output
2. Voltage above 1.67V reads as 1, below is 0.

ANALOG TO DIGITAL CONVERSION

Basic idea is to time how long it takes to charge a capacitor.

This technique was used in the original Pong game. http://atariage.com/forums/blog/52/entry-392-paddles/ Woz also used this approach in the Apple 1.

Demo

1. A 2N7000 mosfet is used to discharge the capacitor, control by DISCHARGE gpio. Write 1 to DISCHARGE gpio turns on the mosfet and discharges the capacitor.

2. Connect 10K pot to Pi 3.3V, wiper through 1K resistor to 1uF capacitor. SIGNAL gpio connects to wiper-resistor node.

3. Write 1 to DISCHARGE, wait for capacitor to charge (read SIGNAL until high), measure elapsed time.