Folks
I have problem that looks simple to correct but the obvious isn't working
or its the infamous "tree in the forest scenario".
NOTE: Logic is CMOS
The circuit is a PNP transistor switch being driven by a band switch. In
parallel the +V from the band switch is also sent via a diode (isolation)
to an OR gate input.
Position 1 works fine.
Position 2 opens the +V to the PNP base and the OR gate input. The line is
floating but is controlled as follows. The OR gate has a resistor on its
input (to ground) to ensure its input remains at "0" and doesn't "dance
around" driving the OR gate to an unknown output. Since the PNP input has a
base resistor to ground the PNP turns ON as expected but it looks like the
voltage at its base is finding its way to the OR gate input. I put in an
additional steering diode in the PNP base circuit to halt the voltage from
backing into the paralled OR gate input. It doesn't stop it.
I removed the PNP from the board and its the circuit causing the voltage to
feedback to the OR gate even with the additional steering diode in the PNP
base circuit. So it looks like there is no sneak path
If there is anyone out there that may see what I'm missing, I'll make a pdf
file and send it to you.
I have updated the circuit schematic with an additional inverting circuit
which will do the job.
I would hate to scrap this R&D board (over 100 components) if a not so
obvious fix can be "white wired" into the board.
Thanks
Jay
W2IJ
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