Veiwing an ohms change on an oscope????

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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

 

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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

 

[CWatters]

I just want to be able to measure the time it takes from when the resistance
leaves 1355 ohms and returns to 1355.

Try putting the resistor in series with a 1mA current source to produce an
output that varies from 1355mV to 1375mV. Then feed that into the scope.
Adjust the gain and offset of the scope to display 1355mV at the bottom and
1375mV at the top. Most scopes have 10 grid squares vertically so you should
be able to see 1mV (=1 Ohm) changes easily.

If you can't adjust the offset on the scope enough then feed the output into
an opamp configured as a subtractor...
http://www.semiconfareast.com/inamps.htm
Set it up to subtract a constant 1355mV from the input...
1355-1355=0
1375-1355=20
so the resulting output would be 0-20mV where 0mV corresponds to 1355 Ohms
and 20mV = 1375 Ohms.

You might be able to use the scopes trig level and trig output (if it has
one) to control a timer/counter.

Check that the source resisrtor isn't temperature sensitive. If it is then
you might be able to compensate for this by using a similar part (with a
constant resistance) to produce the 1355mV needed for the subtractor.

Colin