Veiwing an ohms change on an oscope????

Question

Hello,I'm looking for a circuit I can build or equipment I can buyso I can time how long an ohms change is. I would like to build it ifpossible . I would like to view it on a scope.One example would be a starting ohms reading from a device of 1355 ohms. Itwill increase by approx. 20 ohmsand then return to 1355 ohms. The time it will take is approx. 19 seconds. Iwill need to be able to see a change as small as 1 ohm if possible , but themost important thing is to see the reading change from 1355 and return to1355 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 needa way to exactly measure the change.I will be greatful for any help.Thank you,Jeff

scanner80 · Accepted Answer

I just want to be able to measure the time it takes from when the resistanceleaves 1355 ohms and returns to 1355. I can do it with a meter and astopwatch, but I was looking for a more verifiable way to do it. I want toremove human error. This is for calibration purposes.Is there any type of simple timer or timer circuit that can will trigger asthe ohms changes and stop when it returns?JeffItItheneed

CWatters · Answer

resistanceTry putting the resistor in series with a 1mA current source to produce anoutput 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 and1375mV at the top. Most scopes have 10 grid squares vertically so you shouldbe able to see 1mV (=1 Ohm) changes easily.If you can't adjust the offset on the scope enough then feed the output intoan opamp configured as a subtractor... it up to subtract a constant 1355mV from the input...1355-1355=01375-1355=20so the resulting output would be 0-20mV where 0mV corresponds to 1355 Ohmsand 20mV = 1375 Ohms.You might be able to use the scopes trig level and trig output (if it hasone) to control a timer/counter.Check that the source resisrtor isn't temperature sensitive. If it is thenyou might be able to compensate for this by using a similar part (with aconstant resistance) to produce the 1355mV needed for the subtractor.Colin

Veiwing an ohms change on an oscope????

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