Re[2]: digitising arm XTALS
Posted by
Alan Marconett KM6VV
on 2000-11-16 11:32:09 UTC
Ian and the list,
I'm a little confused. There are two types of crystal (yeah, 3+, also a
"xtal diode"), one is typically an XY cut used in oscillator circuits
(xtal osc.), and then there are piezo-electric crystals, used in the
phono pickup you describe (I prefer dynamic). Did I understand that the
commercial equipment uses osc. xtals? And you suggest making the phono
pickup oscillate? What freq? What about an FT-243 xtal holder, with a
"whisker" (not to be confused with xtal detector), and then the whisker
touches the stock? The piezo (sp) effect could just generate a pulse
without any oscillation, if that's what is wanted.
Alan
Ian Wright wrote:
I'm a little confused. There are two types of crystal (yeah, 3+, also a
"xtal diode"), one is typically an XY cut used in oscillator circuits
(xtal osc.), and then there are piezo-electric crystals, used in the
phono pickup you describe (I prefer dynamic). Did I understand that the
commercial equipment uses osc. xtals? And you suggest making the phono
pickup oscillate? What freq? What about an FT-243 xtal holder, with a
"whisker" (not to be confused with xtal detector), and then the whisker
touches the stock? The piezo (sp) effect could just generate a pulse
without any oscillation, if that's what is wanted.
Alan
Ian Wright wrote:
>
> HI,
>
> I think I might have a play about with this sometime soon as it seems
> interesting but I think I would be inclined to take a slightly different
> approach. How would it be if we used a light but stiff wire (piece of music
> wire) glued onto one end of the Xtal out of a phonograph needle assembly and
> glued the other end to a stout mount. Then we could use the Xtal as the
> resonant element in a simple oscillator and monitor the change in frequency
> as the Xtal deformed under side loading and have a logic circuit to trigger
> at a certain deviation thus indicating contact with the surface. In
> principle it looks as though this could be very accurate as a very small
> change in frequency could be detected. The Xtal would need a shielding tube
> with just a small opening for the sensing wire to protect it from excessive
> flexing (ham-fisted use!). These Xtals are really sensitive and easily
> obtained cheaply - a few years ago I built myself a couple of microphones
> using them to listen to watch ticks. All I did was to glue just one end of
> the Xtal element to the watch movement holder leaving the other end free to
> vibrate in air, and coupled up the wires to a simple amplifier. Even the
> smallest ladies watch produces enough vibration through the mass of the
> movement holder to give a loud tick at the loudspeaker simply by shaking the
> Xtal.
>
> Perhaps this is a daft idea - perhaps it would be an ideal 'PIC' project -
> what do you think?
>
> Ian