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Resistance comparison

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MikeMl

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Please check and suggest if any enhancement has to be implement.

If resistor R10 is more than 149Mohm then the test is pass. A green LED will glow. (Now I changed the requirement from 100Mohm to 149Mohm).
If R10 is less then 149Mohm, the test will fail and RED LED will glow. Buzzer will also ON.
This is reasonable.

IF R10 is not connected to the test-point, then ORANGE LED will glow. Buzzer will also ON.
...
Totally naive and unrealistic! First, there is no reliable way to discriminate between air in the test fixture (no device under test between the probes) and a good sample which happens to have almost no leakage. Second, having the buzzer sounding continuously while there is no sample in the test fixture will drive everybody nuts, and will lead to having the buzzer being ignored.

As I mentioned previously, the only way to know for sure that there is a sample in the fixture is by using some other method of sensing, not measuring conductivity. Use a microswitch, or use an optical method of detecting the sample.

Sound the buzzer only if (there is a sample in the fixture) AND (sample conductivity <= 6.7nS).

If you think that there is a way of determining that a sample is in the fixture based on a conductivity measurement, what would be the effective resistance (or conductance) that you would use as a threshold? 1GΩ, 10GΩ, 100GΩ? Good luck making a circuit to detect that!
 
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