Hi All

3:21am local - couldnt sleep so I was thinking about the problem.

How About This:

Using a 555 timer which has a nice formula for working out the pulse timing.
(t (time) = R (resistance in ohms) * C (Capacitance in Farads)
We can easily calculate what the resistance is if the capacitance is known and we can measure the frequency (or time) being emitted by the 555.

The circuit:
Some sort of PIC.
An array of capacitors that can be individually connected to the 555 by the PIC using transistors.
Of course, a bunch of 7 segment display units.

The program:

The PIC then starts by connecting the highest value cap and counts the pulses coming from the 555.

If it is below a certain frequency, the next cap is connected and the pulses re-counted.

If it is above a pre-determined frequency, reset and try again.

This goes on until the pulse count is within a pre-determined set of values.

At this time, we know what the cap value is, we know what the pulse count value is and we can therefore work out what the resistor value is.

Take the calculated resistor value and send it to the display.

Pause for a second (so that the display does not constantly flicker if it is a low value resistor and the resistance across the fingers is stuffing things around) and then start again.

Only thing we need now is the upper and lower limits of the ohm range we will want to measure and using this, the number of caps needed and the values.

If we use serial to parallel convertors, we should be able to cram in up to 16 caps using 2 pins and the display can be 9 digits long using another 2 pins. Add another pin for the 555 output makes 5 so a PIC12 should be able to do it quite comfortably and we still have another pin available if we need more caps.

I do realise that this approach may not give the ultimate accurate reading but at least you will be able to quickly tell the difference between a 470 and a 4K7. Apart from the fact that I have difficulty in making out what color the little stripes are, the table confuses the heck out of me.<img class="inlineimg" src="images/icons/icon5.gif" alt="Question" border="0" />

Using similar methology, it should not be too difficult to build a unit that can measure capacitor values as well. Mmmmm.....

Ok, concept done... maybe my brain will let me get some sleep now.

What do you people think?

Best