Auto-Range Capacitance Meter

Hi!

I want to build a capacitance meter, following this link: http://arduino.cc/en/Tutorial/CapacitanceMeter

Since the charging resistor is crucial for the time it takes for the measurement, I want to have a auto-range feature. Here is the schematic of my idea:

Where charging happens from the left, decharging with the right lower one and measurement on the upper right pin. R1-R3 are different resistor-values. I would charge through the smallest, and if the time is too short for an accurate measurement, do it again with a higher value and so on.

Does this work or will I have inaccuracies because of the changes to the original schematic?

Are you looking for a resistor who's value you can control from the Arduino?.

Mark

holmes4:
Are you looking for a resistor who's value you can control from the Arduino?.

Well, this would solve the problem too, didn't think about this yet.. Thanks!

got the following grouchy message when I tried to open your link

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You don't have permission to access /en/Tutorial/CapacitanceMeter on this server.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

the circuit is simple enough, maybe the easiest thing is to bread board it and see how it works if you have the script to calculate the timing.

Yes, but too complicated.

The simplest requires two pins: one to charge and the other to discharge / adc. No diodes and no dedicated discharge / adc pins.

A better approach is to use the analog comparator:

  1. discharge the capacitor.
  2. charge up with the appropriate energizing pin;
  3. When the comparator flips, capture the timer count -> that indicates the capacitance.

Unless you have a way to measure the internal reference, I would use an external precision voltage reference as one of the source for the comparator.

The example given here requires just one wire! Well I was going to give a link to the capacitive sensing section of the playground but the main site is playing up.

Mark

Yes, but too complicated.

The simplest requires two pins: one to charge and the other to discharge / adc. No diodes and no dedicated discharge / adc pins.

A better approach is to use the analog comparator:

  1. discharge the capacitor.
  2. charge up with the appropriate energizing pin;
  3. When the comparator flips, capture the timer count -> that indicates the capacitance.

Unless you have a way to measure the internal reference, I would use an external precision voltage reference as one of the source for the comparator.

That is too complicated... WAAAY too complicated.. but typical of dhenry.
Build the circuit and use the internal ADC reference (1.1V) and calibrate the circuit with a known capacitor.
Better yet use (borrow) a capacitance meter and use it as the comparison standard... It can't be any worse than the measurement required to find the "Exact" value of the ADC reference voltage. The only thing important is that the reference voltage must be a constant and not vary. The rest is all number crunching.

Bob