any takers for this project?

[Bohdan.L.Bodnar at att.com]

ASCII's lousy (takes more time than I have).  Here's a description:

Take an N-channel JFET (e.g., 2N3819, MPF102...) and connect its source to
ground.  From ground to the gate, connect a high value resistor (I'd try 1
Meg).  Shunt this resistor with a switching diode (1N914 or equivalent) so
that the diode's anode is at the gate and cathode's at ground.  Take a
moderately high value resistor (I'd try 10k) and connect the drain to Vdd
(positive voltage).  Stick a scope from drain to ground and observe the output
as the pickup pulses this amplifier.  Incidently, the pickup is nothing more
than an AC amp probe (Allied Electronics sells the one OTC uses -- for
something like $35 and, of course, it doesn't have the OTC label).  If the
output voltage swing doesn't look good, reverse the pickup's leads.

What you have now is a simple high impedance amplifier.  The voltage swing may
not be what you want, but that's something which can be easily shaped using a
plethora of ways;  e.g., feed the amplifier's output into a common-emitter
class-C amplifier via a zener diode -- this will act as a High Threshold Logic
(HTL) inverter and feed the output of this inverter into your logic circuit.

This is only one way of doing what you need.  You could try using an
enhancement mode MOSFET (this will probably be the simplest approach) as a
high impedance/high gain saturating amplifier (be sure to protect the gate
from overvoltage).

Cordially,

Bohdan Bodnar
bohdan.l.bodnar at att.com