Of course, there is no suggestion of originality. But small stepper
motors are widely available and often very cheap, and *this* was the
main attraction. Things like synchros are not so cheap and available
(not everyone has access to the Pentagon's discards). As for the
'missing pulse' problem I found that even if I turned the motor really
slowly I got a reliable output, though you could, if determined, turn it
so slowly that it didn't step. This would not be an issue in most
applications, though.

> ----------
> From:         Mike Keitz[SMTP:mkeitz@JUNO.COM]
> Reply To:     pic microcontroller discussion list
> Sent:         Friday, January 09, 1998 5:45 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [OT] Best way to read a POT without the A/D?
>
> On Thu, 8 Jan 1998 17:28:48 +1300 Andrew Mayo <andrew@GEAC.CO.NZ>
> writes:
>
> >The other day I picked up a small permanent-magnet stepper motor
> >(about
> >1 inch diameter) and twiddled the shaft idly, when suddenly the
> >thought
> >occurred, what sort of transducer does a stepper motor make?.
> >
> >Hooking it up to the scope, the answer was, a very good one.
>
> This has of course been done before.  The major problem is that it
> will
> miss counts if turned very slowly, since it depends on the moving
> magnet
> to induce a voltage.  However there are a lot of places where it will
> work well, especially since the motors usually have ball bearings and
> no
> contacting parts, which could lead to a very long life.
>
> >the output and you have a rotary positioner where the output tracks
> >the
> >input (antenna rotator, for instance). Because the two motors are
> >identical, rotating one by x degrees will cause the other one to
> track
> >exactly.
>
> This has also been done before.  It's called a synchro motor.  The
> motors
> have two or three-phase stator and a single-phase rotor energized with
> AC.  The two motors are connected in parallel, with one's shaft driven
> by
> the antenna or whatever position is to be monitored, and the other
> allowed to turn freely and connected to a dial pointer.  When
> energized,
> the motors will move until the voltages in the stators are in phase.
> Since only the one connected to the readout is able to move, it will
> indicate the position of the other one.  It will always jump to the
> correct position, regardless of the position when the power was turned
> off.
>
> Sometimes an amplifier is added to boost the drive to the receiver
> motor,
> especially if it has a significant load such as moving the deflection
> coil around a radar CRT.  The stator's AC voltage and phase can also
> be
> interpreted electronically.  This circuit is called a resolver.
>
> All variety of synchros have been available as military surplus for a
> long time.  Be prepared for some unusual voltage and frequency
> requirements though.
>