More Inductive Pickup

[dn]

> Inexpensive pen-size detectors are available from (gasp) J.C. Whitney
> and Snap-on Tools and other automobile tool suppliers.  Holding these
> detectors near any spark plug wire will show the presence of high
> voltage as a flashing light inside the detector.

Hey, a light bulb just went on in my head (sorry).  These pen size high
voltage detectors consist simply of a standard NE2 type neon light bulb.
One lead of the bulb is grounded (or tied through the metal case to your
body, which acts as an antenna).  The presence of high voltage nearby is 
enough to ionize the neon in the bulb and light it up... so why not 
optically couple a phototransistor to a standard neon bulb by heat-
shrinking the two together, and, voila, instant pickup.  One lead of the 
neon would have to be tied to circuit ground to provide a return path for
the ionization current, the other lead is simply cut off. The photo-
transistor could be simply wired as an open collector switch, tied via
pullup resistor to your input.  2 wires, simple as hell (if it works)
Haven't actually tried this, but it should work, and for about 25 cents
worth of parts.

Well, that's my two bits worth...
dn


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