The rest of the dial

// The rest of the dial.
//
// There is no supply in this circuit, on purpose. Ohms, diode, continuity and
// capacitance all work by pushing the meter's own small current through
// whatever is between the leads, so they only mean anything when nothing else
// is driving it. On a live circuit the meter lights IN CIRCUIT and says so.
//
// Four things to measure, and where the leads go:
//
//   D  diode.  Red lead on the LED's anode (column 8), black on its cathode.
//      1.42 V. That is the drop at the 1 mA the meter pushes, worked out from
//      the same Shockley equation the LED itself runs on, not a table. Turn
//      the leads around and it reads OL, because a diode blocks one way, which
//      is how you find which end is which on a part with no markings.
//
//      Worth knowing: a real meter on a real red LED reads nearer 1.6 V. The
//      LED card here is fitted to 1.8 V at 20 mA with an emission coefficient
//      of 2, and that makes its curve too soft a decade down. It is written up
//      in docs/accuracy-audit.md rather than quietly rounded.
//
//   O  ohms.   Across the 4.7 k resistor: 4.70 k. Across the resistor and the
//      10 k in parallel with it: 3.20 k, which is the right answer for what is
//      between the leads and the wrong answer for either resistor. The meter
//      cannot see a parallel path and neither can a real one; lift a leg.
//
//   -|- capacitance. Across the 1 uF: 1.00 uF. The meter charges it with a
//      known current and solves the first-order response, so the number does
//      not depend on the ramp being straight. From about a nanofarad to a
//      millifarad.
//
//   .)))  continuity. Across the wire link at columns 20 to 24 it beeps and
//      reads under an ohm; across the 4.7 k it does not, because 4.7 k is a
//      connection and continuity is not. The threshold is 30 ohms. The beeper
//      mark beside the digits mutes it.
//
// And the two buttons that are not a range: HOLD freezes the display so you
// can get the probes out of somewhere awkward before you read it, and MIN/MAX
// records the lowest and highest reading since you pressed it. RANGE steps
// off auto onto a fixed full scale, and the display reads OL above it, which
// is the point of a manual range, and something auto will never show you.