One source of rotary encoders designed to be manually turned
is found in cast-off, but modern, car radios.

	After a shop accidentally killed our car radio by, I think,
hooking the battery up backwards, I took the now totally dead old
radio and found that the volume encoder was a discrete assembly held
on mechanically with a threaded collar just like a pot.  The detents
are about 32 or so per revolution.  When I unsoldered it from the
board and removed the nut, it came out easily.  A continuity check
showed that the Gray code repeats every two clicks.  On one click,
both the A and B phases are open with respect to the Common terminal.
On the next click, depending upon which direction one is turning it,
both A and B phases short simultaneously to common and then open at
slightly different times on the next click.  If one rotates the knob
in the opposite direction, both phases make at slightly different
times and break simultaneously.  The push switch in the knob is a
momentary on type switch.  In better times, it signaled the CPU to
energize the radio or de-energize it if it was already on.

	The shop we went to did pay for the damage without any fight.
They initially said that someone had bumped the handle of a wrench
against Earth as they were either removing the old battery or
installing the new one.  In this case, Earth was a small metal pipe
assembly on the liquid side of the air conditioning condenser and the
resulting spark burned a pin hole right through the metal and let out
all the refrigerant.

	That certainly explains the burn hole in the pipe.  What that
story doesn't explain is how a fuse got blown and why the radio got
killed.

	The fuse was never exactly identified, but what I really think
happened was that it was a reverse polarity protector on the car
computer.  After all, that's expensive so the manufacturer probably
saw fit to put a reverse polarity diode and fuse on it.  The radio, on
the other hand, is not nearly as expensive and there was no protection
on it at all, not even an en-line fuse.

	How they fried the radio and burned the liquid line in one
operation is a real feat of something, although the exact words escape
me.  I know one is supposed to connect the frame or Earth lead last
and disconnect it first to prevent just such catastrophes from
happening.  Heaven only knows what really happened.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group
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