"scanner80" wrote in message
...
I just want to be able to measure the time it takes from when the
resistance
leaves 1355 ohms and returns to 1355. I can do it with a meter and a
stopwatch, but I was looking for a more verifiable way to do it. I want to
remove human error. This is for calibration purposes.
Is there any type of simple timer or timer circuit that can will trigger
as
the ohms changes and stop when it returns?
Jeff
"DBLEXPOSURE" wrote in message
...

"scanner80" wrote in message
...
Hello,
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355
ohms.
It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19
seconds.
I
will need to be able to see a change as small as 1 ohm if possible ,
but
the
most important thing is to see the reading change from 1355 and return
to
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but I
need
a way to exactly measure the change.
I will be greatful for any help.
Thank you,
Jeff



Jeff:



To measure resistance you need to apply a voltage and then
measure the current. Another way is to use a "voltage divider", put your
changing resistance in series with a known resistance, apply a voltage to
the circuit and monitor the voltage dropped across the known resistor.
As
your changing resistance decreases current in the circuit will increase
and
the voltage drop across your known resistor will increase. This will all
be
linear so you will be able to make notations on your scope graticule so
that
you can directly readout the display in ohms.



You will want to use a regulated voltage source. A look into Ohms law
will
provide the math for you to make the calculations..





Typically they prefer that you bottom post in this group. Doesn't bother me
but, when in Rome..........


Well, I don't have time at the moment to work out all the details; maybe
someone else will chime in with more help. But I can at least get you
started.



First, use the voltage divider circuit I talked about in the previous post.
Instead of monitoring the voltage across the known resistance with your
scope, use that voltage to drive the Vin of a simple "Op Amp non inverting
comparator" circuit. Here is a link to get you started with the comparator.
http://home.maine.rr.com/randylinscott/learn.htm



The idea is when your voltage across the known resistance of the voltage
divider exceeds the Vref of the comparator the output of the comparator will
go high.



This output voltage can be used to drive a reed relay. You can find reed
relays that will energize with very little coil current. The relay will be
used instead of the transistor depicted in the schematic of the
non-inverting comparator shown on the link provided.



Now get yourself a cheap stopwatch that has one button that will both start
and stop. Wire the switch contacts of the relay across the start/stop
switch of the watch.



There are more eloquent ways to go about it but this will be a pretty easy
and cheap way to get the job done. I have left a lot of the design to you,
resistor sizes, voltages etc.. Newark Electronics might be a good place to
find the relay. The 741 op-amp crosses to a NTE941M, which can be found
here http://www.weisd.com/store2/ntesemi/NTE941M.html



Well, duty calls, gota run for now.........