Resistor Ohm Meter

[T.Jackson]

Hi Luciano,

Have you built & tested that circuit? ... (looks comprehensive at first glance)

I usually do most of my projects from scratch. But I do get ideas from other people's circuitry. I know that there's a lot of people around that just slap together odds & ends that they find in magazines, from books & on the internet. Some of them even try claiming it as their own. This is one of the reasons why I procrastinate about doing anymore projects for publication.

[Luciano]

Hi Trent,

Here is a circuit which was published in a magazine back in 1994.
The circuit was connected to the serial port of a PC via a parallel
to serial interface circuit. A MS-DOS program (see picture) was
used to display the measured resistance value and show the color
code of the measured resistor.

10mA was used for resistors from 1 to 100 ohm.
1mA was used for resistors from 100 to 1k ohm.
100µA was used for resistors from 1k to 10k ohm.
10µA was used for resistors from 10k to 100k ohm.
1µA was used for resistors from 100k to 1 Mega ohm.

TS27M2CN datasheet:
http://www.st.com/stonline/books/pdf/docs/2306.pdf

LM334 datasheet:
http://www.national.com/ds/LM/LM134.pdf

Here is the schematic. (I have added the values).

(Click to enlarge)

How the current source is used:

(Click to enlarge)

The DOS program:

(Click to enlarge)


Best regards,

Luciano

[T.Jackson]

That's top info Luciano!

National Semiconductor's LM334 is quite good. Seems a bit of a waste to have one for each range though. I think a lot of magazines tend to step & repeat sections sometimes, it is a much easier read. And I also think the project is a bit over the top.

Who would use it and how would they benefit from it?

That's the first question I tend to ask myself these days.

[Luciano]

Seems a bit of a waste to have one LM334 for each range though.

Hi Trent,

The price of a LM334 is USD 1.20.

If you have just one LM334 you will have to switch the resistors and this
will reduce the accuracy of the measurement system. Just supply the +5V
to each LM334 with an I/O of the microcontroller and you are done. The
transistor BC547 is not needed because the I/O of a PIC can supply more
than 10mA.

Best regards,

Luciano

[T.Jackson]

Bet I could work something for a quarter of the cost with the help of a 4066 quad bi-lateral switch. The LM334 is around $2.50 in Australia.

[TheMadMan]

Hi All
Way back I seem to remember seeing a circuit in some book or other that acts like a resistor but can be changed using only electronic means. Was supposed to replace the volume pot in a amplifier I think.
Question 1 : Is it possible to "emulate" a resistor in this way?
Question 2 : Is it possible to "emulate" a capacitor as well?

Now the big question : If both the above are possible, does anyone have a circuit for each that is nice and simple (and stable).

Reason: I still like the 555 timer idea but dont want to have a whole bunch if resistors and caps to get it working. If there is a way to "create" the values I need, lets get the PIC to do the hard work. I'll worry about the display later.

Best

[T.Jackson]

Emulating a resistor? - grab yourself a BJT (bipolar junction transistor) - and there's your emulator. As for a capacitor? - just find other parts that exhibit good quantities of capacitive like characteristics and model a circuit around them.