keyboard contact rubber fun

[Grant Richter]

I believe I actually saw this circuit applied in an old organ manual.
The single transistor oscillator was tuned just below oscillation without
the slug. When the slug was inside the coil the oscillator started up, was
rectified and the voltage used as a key gate signal.

If the single transistor (Colpitts?) oscillator were tuned more broadly,
the frequency would be proportional to key (slug) distance, this could be
decode by a "frequency discriminator" (a tuned circuit with low Q such that
the output amplitude increases slowly as the frequency gets closer to the
tuned point, that and a diode was an early frequency to voltage converter).

The whole thing would take one transistor, a diode and some caps and
inductors and could probably fit in under 1" x 1" of PCB if done in surface
mount. An ARRL manual from the 1960s should have all the schematics you need
and most libraries keep them forever (Hams on staff?).



>
>> :::    One thing I always wanted to try was using an iron plunger going into
an
>> :::inductor.  The only thing that stopped me was it seemed like an awful lot
of
>> :::work, both mechanical and electronic.  But this should be able to give you
a
>> :::signal that is proportional to how far the key is pressed down.  It has
some
>> :::very interesting posibilities.
>>
>> But how do you ensure that the pulse comes in the very moment the svanner
looks
>> at it?
>>
>> m.c.
>