At 03:11 PM 10/03/97 -0500, you wrote:
>Well, The schematic is in Autocad DWG and its kind of ad-hoc, so I hesitate
>to tarnish my rep by sending this nasty thing out...  but if you can deal
>with DWG or DXF I'll be glad to Email it to ya!  Yeah, that sensor problem
>was a different system which I plan to optically isolate, this system is
>interfaced to a device which holds the I/O line high via a separate power
>supply (in the PC)  and that always puts a few extra millivolts on my
>power-off.  Should I perhaps use a diode/relay/analog switch to turn off
>the I/O when power is off?  I hope the optical isolation helps with the
>other problem... mabye thats what I could do here as well.... But I DO need
>something extra, because even with everything unplugged, the system STILL
>holds enough voltage in the filter caps to screw it up even over-night!

Sorry can't read .DWG or .DXF files any longer - I deleted AutoSketch...

I think if you go for the reset clamping device it should be OK, you get the
extra benefit of a cleaner power up reset anyway. So the extra stuff with
relays, diodes etc should not be necessary.

The other thing which might help is put a series resistor on that IO line
from the PC - high enough so there's little power from it but not too high
so it will upset operation. If you are driving a CMOS input from this IO
line then somehwere around 100K should be fine. Go down in division by
2 each time if 100K is too high in practice.

Did you measure the current through a 1K resistor between Vcc and Gnd after
the power was off ? How big are those filter caps anyway ?

What happens if you replace the 10K with a 1K mentioned earlier ?

What is the operating voltage on your filter caps when the system is ON ?

Rgds

Mike

Social Scientist - "A person who thinks that a statistical
correlation is proof of cause and effect"