LM339 window comparator

// An LM339 window comparator: one LED lights when the knob is outside 1.67 V
// to 3.33 V.
//
// Three 10 k resistors from 5 V to ground make 3.33 V and 1.67 V. Two of the
// LM339's four comparators watch the knob:
//
//   comparator 1: knob on 1IN- (pin 6), 3.33 V on 1IN+ (pin 7). Its output
//                 pulls down while the knob is above 3.33 V.
//   comparator 2: knob on 2IN+ (pin 5), 1.67 V on 2IN- (pin 4). Its output
//                 pulls down while the knob is below 1.67 V.
//
// The two outputs, 1OUT (pin 1) and 2OUT (pin 2), are wired together. That
// is only allowed because they are open collector: each can only pull down,
// so either one pulling down pulls the pair down, and neither fights the
// other. One LED and a 470 ohm resistor from 5 V into the pair is their
// pull-up and their lamp: it lights while either output pulls down, outside
// the window, and goes out inside it.
//
// Two op-amp outputs wired together like this would fight, one driving high
// while the other drives low. That is the difference.
//
// The LM339's pinout is its own: outputs on pins 1, 2, 13 and 14, VCC on
// pin 3, GND on pin 12. Comparators 3 and 4 are not used.