You could possibly take two low-cost ultrasonic transducers and mount
one on a fixed base and the other on the sliding part, then pulse the
transmitter and measure the delay to the receiver. From memory the speed
of sound is approx 300m/s so 1.5 inches will provide a delay of around
100uS if my quick math is correct. This method is, however, sensitive to
variations in temperature as the air density changes, but I don't know
how significant this would be.

> ----------
> From:         XYGAX[SMTP:XYGAX@AOL.COM]
> Reply To:     pic microcontroller discussion list
> Sent:         Saturday, January 10, 1998 9:37 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Linear sensor input for PIC16F84
>
> I need a very accurate linear position sensor, that can measure
> verticle
> object travel of approx 1.5 inches. Cost is an issue. It will be an
> input for
> a PIC16F84, so a digital signal would be less work.
>
> Can anyone help?
>
> Try this....
> Mouse ball princable and a fixed sensor to check end of travel......
> a slotted (laser etched) strip and a slotted opto switch with an
> additional
> switch opto or mechanical to indicate 'home'
> Alternatively if the mirco is controling the movement a single switch
> to
> signify end of travel and a steppermotor driving a worm gear to move
> the
> object sorting out the position is a matter of counting steps.
> Alternitivly read the thread on 'reading a pot without a-d and use a
> pot
>
> Just a couple of ideas....
>
> Cheers Steve............
>