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Michael Rosola
11-28-2006, 07:02 AM
Apologies in advance for what I imagine is·a·simple question, but I am very new to electronics and could use a push in the right direction.

I am looking to measure an unknown resistance from my Basic2 MCU.· I have 3 options in mind but am hoping someone can confirm whether these approaches are plausible and what advantages they have over each other and if there are other better options that anyone can suggest.

#1 - Use the RCTime function to measure the time to charge/discharge a known capacitor over an unkown resitance and·extrapolate the resistance.· [ Capacitor Time Constant = R * C ].· I know that RCTime·is not measuring the time constant (63%charge / 37% discharge) but I suspect plotting the times in excel against test resistances·should show me a linear or log relationship that would allow me to figure out the unknown·resistance.

#2 - Connect the MCU to an A/D IC and·use a voltage divider approach. (schematic below)
http://cq.cx/pics/int-measure-r.png··
Vadc = (5 V)×RUT/(RUT + 1k)
RUT = Vadc×(1k)/[(5 V) - Vadc]

#3 - Build a·555 monostable multivibrator and send a pulse from the MCU to trigger the clock and then time the output in the·MCU·where the 555 is governed by a known capacitor and an unkown resistance. [ T = 1.1(R*C) ].· I suspect there are possibly other 555 arrangements to do this more elegantly.

If it is relevant, I anticipate my range of unknown resitances will be·between .01Mohm·and 10Mohm and·I would like to target my acquisition time to be under 2 seconds... but I suspect there will be tradeoffs between accuracy and speed.

Thank you,
Michael

Post Edited (Michael Rosola) : 11/27/2006 11:45:02 PM GMT

PJ Allen
11-28-2006, 09:03 AM
MR,

Look this (RCTIME) up in PBASIC Help, it's fully explained, there's even a recommended circuit.