Resistor Ohm Meter

[Melanie]

2K2 Example...

2K2 +/- 5% = 2090-2310R

oh... 2310R... is that a 2K4 (+/-5%) Resistor?

and... 2090R... is that a 2K0 (+/-5%) Resistor?

Your students have now sorted all your Resistors into their respective bins. Next lesson, a student dips his hand into the 2K0 Resistor bin and pulls one out marked Red-Red-Red. Now, is it a Resistor that's been put into the wrong bin, or a really poor 2K2 that's closer to 2K0? Does he then learn from his experience that Red-Red-Red means 2K0?

I don't think you could encompass the entire Resistor selection, but certainly the basic 82/84 (12 step) range (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2).

[T.Jackson]

I guess one possible solution would be to have roundup & round down buttons. Pressing these buttons would effectively remove the tolerance from the reading by either adding or subtracting 0.25 to 10% (user selectable) - of the resistor's value. From memory there's about 180 different values in the 5 band 1% range. This is the E? something range (totally forgotten) The more obvious solution would be to database the entire range of resistors. When a reading is taken, the system would basically look for a closest match and display the results. Lot of resistors to catalog though.

[T.Jackson]

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This discussion reminds me of a project that I did sometime ago. There's no microcontroller or anything real fancy in this, but it can give an accurate (go / no go) indication, both audible and visual - for many resistive components. The user simply selects an acceptable maximum level of resistance (below or equal to this level is the pass level for a good component) - then this gizmo just returns the results as valid or invalid via a LED & buzzer.

However, it's very limited. At the time I just wanted something done real quick and dead cheap. The unit can be built for less than the cost of a bus ticket, but it's range is limited to a few hundred ohms only. Another restriction is that, there's no provision for setting a minimum level of acceptable resistance. It has its place, bet auto electricians love it!

The published article can be seen here: <a href="http://siliconchip.com.au/cms/A_30755/article.html" target="_blank">Programmable Continuity Probe</a>
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[TheMadMan]

Hi Trent.

The more obvious solution would be to database the entire range of resistors. When a reading is taken, the system would basically look for a closest match and display the results.

There will have to be a database (array) present to get it working. (use a PIC with a big enough program area)
Once the tested value is known, it should display ALL the "standard" values where the tested value is between the upper and lower of said "standard" value.

I guess one possible solution would be to have roundup & round down buttons. Pressing these buttons would effectively remove the tolerance from the reading by either adding or subtracting 0.25 to 10% (user selectable) - of the resistor's value. From memory there's about 180 different values in the 5 band 1% range. This is the E? something range (totally forgotten) Lot of resistors to catalog though.

Problem is that a 10% may only be 5% off target. If you subtract 10% you wouldnt get the correct value anyway.
But your post gave me another idea: A rotary selector for the tolerance value to use when the PIC has to figure out what to show. The lower you set the tolerance, the less overlapping there will be.

You forgot the "E? range"! I didnt even know!

BTW. What are the standard tolerance ranges?
I've got 0.05% (grey), 0.1% (violet), 0.25% (blue), 0.5% (green), 1% (brown), 2% (red), 5% (gold) and 10% (silver).

If that is it, we'll need 3 pins to drive the tolerance rotary selector.
Rotary selectors are EXPENSIVE, maybe a dipswitch would be less expensive, especially in an environment where you only deal with one specific tolerance.

All I've ever used is 5% (grey) which sometimes looks like silver.

I swear I spend more time figuring out what the value of a resistor is than I do soldering the thing into place. Even then I often get it messed up as I simply cant make out the colors properly on some of the small ones.

Did you know that a 3mm led makes an audible "pop" when it goes?

All the best,

[Luciano]

Hi,

Are you a resistors manufacturer?

From page 3: (THEORY of OPERATION)
http://www.harris-irt.com/anonymousftp/irtman5012xp.pdf

The 16F877 Microprocessor based circuit includes a dual limit comparator which compares
the measured resistance with upper and lower tolerance values entered on front panel thumbwheels.
Red and green front panel light emitting diodes indicate whether or not the resistance is within tolerance.
An output relay permits the resistance tester to be converted to a rejection device.
For “fail-safe” operation the contacts are normally open. The contacts close if the part is
within tolerance. The Microprocessor based circuit also provides for electronic range switching
and system control.



Best regards,

Luciano

[T.Jackson]

Problem is that a 10% may only be 5% off target. If you subtract 10% you wouldnt get the correct value anyway.

Yes, that did dawn on me shortly after I posted that. A half-baked thought that didn't have a real world scenario attached to it.

[T.Jackson]

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2074&stc=1&d=119177155 3" border="1">
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If I was going to do a project like this, this is probably the road I'd take.
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[Luciano]

Hi Trent,

(Maybe one stage of your circuit).

Resistance measurement with Current Source

http://www.delabs-circuits.com/cirdi.../del20015.html
(Click on the link "PDF circuit" and then click on the Acrobat Reader save button).

Best regards,

Luciano

[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.

[TheMadMan]

2K2 Example...

Thats why placing the results on a color screen makes good sense.
If a value falls into the overlap of 2 (or more) values, show them all with the correct color code for each.
A look at the resistor and then the screen to pick up the "correct" value, should work well.

BTW. Have you had any experience interfacing a PIC to a color CRT monitor?

Best