There are a series of PIC that have built in A/D converters - I'm using the
PIC 16F877, for example. The A/D converter is a sample and hold converter
and is quite fast. It has a resolution of 1 part in 1024 (0.1 %).

You could use op amps to increase the two voltages to the range 0 to 5
volts, then use two channels of A/D to measure them. The A/D conversions
take only microseconds, so there is no need to hold the voltage for 3 seconds.

All you would do is read the two A/D channels in sequence; when the first
one gives a voltage above the threshold, you process the second one -
display it, or whatever. There are inexpensive LCD displays available ($10
or less).

Larry


At 07:55 AM 1/7/2003 -0800, you wrote:
>I want to know if I could build a simple sample and hold circuit with just
>a couple op amps on a small bread board with a +5 volt DC power supply
>that would do this:
>
>I am watching 2 voltages. When one voltage crosses +.412 Volts DC, I want
>to be able to hold the other voltage for about 3 seconds so that I can
>read it with a DVM. The held voltage would be anywhere from about +.255
>volts to +.500 volts dc, and I need the .001 volt resolution.
>
>Is this do-able? Practical? Is there an inexpensive DVM that has this
>ability built-in? Any other ideas as to how I might do this?
>
>Thanks
>John
>
>--
>http://www.piclist.com hint: PICList Posts must start with ONE topic:
>[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads