LM393 night light with a ULN2003A

// An LM393 night light that switches a 12 V lamp through a ULN2003A.
//
// No microcontroller. The LDR and a 10 k make a divider on 5 V whose middle
// (column 3) rises as the room gets darker; the knob sets a threshold. The
// LM393's comparator 1 has the divider on 1IN+ (pin 3) and the knob on 1IN-
// (pin 2), so when it gets dark enough that IN+ goes above IN- its output
// (1OUT, pin 1) lets go.
//
// An LM393 output is open collector: it can only pull down. The 10 k pull-up
// from 1OUT to 5 V is what makes the high, and that same 10 k is what feeds
// the ULN2003A's input 1B (pin 1). The ULN2003A then pulls 1C (pin 16) down
// and the 12 V lamp between 12 V and 1C lights.
//
// Things to try:
//   - Drag the LDR darker and lighter, past the threshold: the lamp follows.
//   - Delete the 10 k pull-up. The LM393 can no longer make a high at all, so
//     the ULN2003A's input is never driven and the lamp never lights.
//   - Turn the knob: it moves the threshold, so it decides how dark is dark.
//
// Comparator 2 is parked: 2IN+ (pin 5) on ground and 2IN- (pin 6) on 5 V, so
// its output (pin 7) is held on and nothing is left to float.