Dark detector

// A dark detector: an op-amp comparator, an LDR and a pot. No board.
//
// The 10 k from 5 V and the LDR to ground make a divider. In the light the
// cell is a few hundred ohms and the middle sits near 0 V; covered, the cell
// is tens of kilohms and the middle rises towards 5 V.
//
// The op-amp compares that middle (IN+) with the pot's wiper (IN-). With no
// feedback its gain is the whole 100 000, so the output slams to one rail or
// the other: up when the middle is above the wiper, down when it is below.
//
// It does not quite reach the rail, and that is the op-amp being honest. It
// drives through 100 ohms of its own output resistance, so with 220 ohms and
// the LED hanging on it the loop is 100 + 220 + the LED's own 15, and
// (5 - 1.8) / 335 is 9.6 mA. The output sits at 4.0 V rather than 5, the LED
// lights at about half brightness, and the probe on the resistor says so.
//
// Things to try. Drag the slider on the cell towards dark and find the point
// where the LED comes on; turn the knob and the point moves. Swap IN+ and IN-
// and it becomes a light detector. Add a 1 M from OUT back to IN+ and it
// stops flickering at the threshold: that is hysteresis, and it is what the
// night light sketch does in software.