LM324 four-LED bar graph

// An LM324 bar graph: four comparators in one chip, four LEDs.
//
// The LM324 is four of the LM358's op-amps in a 14-pin chip, on the same
// single supply. Here each one is a comparator with no feedback. All four +
// inputs are on the knob's wiper. The four - inputs are on a ladder of five
// 10 k resistors from 5 V to ground, which taps off 1 V, 2 V, 3 V and 4 V:
//
//   amplifier 1 (pin 2)   1 V      amplifier 3 (pin 9)    3 V
//   amplifier 2 (pin 6)   2 V      amplifier 4 (pin 13)   4 V
//
// Each output goes high, and lights its LED, once the knob is above its tap.
// Turn the knob up from 0 V and the LEDs come on one by one: one past 1 V,
// two past 2 V, three past 3 V and all four past 4 V. This is how the level
// meters on old stereos were made.
//
// Mind the supply pins. On an LM324 they are not on the corners: VCC is
// pin 4 and GND is pin 11.