Resistor Ohm Meter

[T.Jackson]

I think an interesting OHM meter project would be something that goes slight beyond the traditional OHM meter as we know it. Something that's purpose specific, for say; quickly determining resistor values. Instead of showing a readout of 2178Ω for a 2K2 resistor, it would display 2K2 (which is ultimately what the user is looking for) A 2K2 resistor with 1% tolerance, on a traditional meter could measure in anywhere between; 2178Ω - 2222Ω. It takes our brain some slight extra time to compute that, that's a 2K2 resistor as apposed to the display directly showing 2K2. This would significantly speed things up if the user had intentions of measuring a considerable quantity of resistors.

Another feature might include averaging! The user samples 100 or so resistors and system spits out an average. So if you're going to do an OHM meter project - think different and it might all be worth while. Otherwise, that $5 meter on eBay looks like the go.

[Pic_User]

Hi Trent and Aubrey,

I like the idea of a PIC resistor sorting display.

I have unskilled (young) student help (paid), sort 5% resistors back into single value bins.

There are 169 standard values (1 Ohm to 10 Meg-Ohm 5%).

Either they have to sort by color code or use a digital meter.

Digital meter:
When they get a value, they have to calculate which standard value is within 5% (prone to errors!).

And, in the teaching lab.
I could even place one at each student (learning) lab station to avoid the same human error problem Aubrey (TheMadMan) mentioned for the auto-placing robot. It would help the students building projects.


A nice “go / no go” method would be great, too.
I have visions of 169 LEDs. One over each bin.
The student labor could “swipe” the resistor (from the pile to be sorted), an LED over a bin would light. Bingo, less error, more speed. The LED would stay lit till the next value is “swiped”.

Now all I have to do is teach them not to touch both leads on the higher resistance value.

I do not have the PIC skills or the programming skills to know how to begin either of these interesting projects.

Any Ideas?

I have occasion to sort several thousand resistors. Every few months.

-Adam-

[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]

<table width="100%" border="0" cellpadding="2">
<tr>
<td>
<img src="http://us1.webpublications.com.au/static/images/articles/i307/30755_3lo.jpg" align="left" border="1">
</td>
<td>
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>
</td>
</tr>
</table>

[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">
</br>
If I was going to do a project like this, this is probably the road I'd take.
</br>

[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

[TheMadMan]

Hi Adam
Take it you are a teacher/lecturer or something similar.
The majority of the reply to your comments is covered in my previous post.

There are 169 standard values (1 Ohm to 10 Meg-Ohm 5%).

Didnt know that - you live and you learn!

The screen display idea may be just the ticket for the lab top units you need. Maybe you could get the students to build them as a project for point credits. Get the local cash register company to "donate" some screens from old cash register units. As a plus, the students would unknowingly "learn" the color code for each value without even knowing it.

But first there must be a circuit diagram and in my experience, that is normally where things fade away. Fact of life I guess.

I have visions of 169 LEDs. One over each bin.

About 21 serial to parallel converters would give you a pin for each of the 165 bins so we are probably talking about a 40 pin unit with a fair amount of memory for the LED Selection array/s.

A nice “go / no go” method would be great, too.

Would probably need a keypad to set the "base" value to be tested against.
An easyer way would be a unit where you put a (example) 1K 1% resistor between 2 crocodile clips as a master and when you touch the 5% resistor onto the 2 test contacts, it works out whether the test piece is within 5% of the master. If it is, flash green led, if not, flash red led.
This will probably be far easyer to design and build.

I do not have the PIC skills or the programming skills to know how to begin either of these interesting projects.

I probably hav'nt either - YET! But a small detail like that should not stop us trying.

I have occasion to sort several thousand resistors. Every few months.

LUCKY!!!
Every time I run out of something, it means waiting for Saturday and then making a 150km round trip to re-fill the drawers. Very frustrating sometimes.
Sometimes I "substitute" but that cost me 4 opto-isolators and 8 leds this week. Bad News.

Best,

[TheMadMan]

Instead of showing a readout of 2178Ω for a 2K2 resistor, it would display 2K2 (which is ultimately what the user is looking for) A 2K2 resistor with 1% tolerance, on a traditional meter could measure in anywhere between; 2178Ω - 2222Ω. It takes our brain some slight extra time to compute that, that's a 2K2 resistor as apposed to the display directly showing 2K2. This would significantly speed things up if the user had intentions of measuring a considerable quantity of resistors.

Another feature might include averaging! The user samples 100 or so resistors and system spits out an average. So if you're going to do an OHM meter project - think different and it might all be worth while. Otherwise, that $5 meter on eBay looks like the go.

Nice out the box idea!!! Like the "2K2" readout.

Here's an even more "out the box" idea:
Somewhere I've got an article saved where an interface to a normal PC screen is shown.
So how about using one of the small screens (like those on cash registers) as the output device.
If it is a color screen, you could also display what the band colors are.
The human brain will probably start to associate the patern (as opposed to the individual bands) with the value and very soon will associate the patern with the value.
Unfortunately, if you then ask them "what are the band colors", they probably wont be able to tell you but if you put a resistor in front of them, they will recognise the color patern and tell you "2K2" or whatever.
This is why people can read but some are terrible at spelling. They recognise the patern of letters in the word and know the meaning but dont ask them what individual letters make up the word.
Best,

[T.Jackson]

Nice out the box idea!!! Like the "2K2" readout.

Here's an even more "out the box" idea:
Somewhere I've got an article saved where an interface to a normal PC screen is shown.
So how about using one of the small screens (like those on cash registers) as the output device.
If it is a color screen, you could also display what the band colors are.
The human brain will probably start to associate the patern (as opposed to the individual bands) with the value and very soon will associate the patern with the value.
Unfortunately, if you then ask them "what are the band colors", they probably wont be able to tell you but if you put a resistor in front of them, they will recognise the color patern and tell you "2K2" or whatever.
This is why people can read but some are terrible at spelling. They recognise the patern of letters in the word and know the meaning but dont ask them what individual letters make up the word.
Best,

Well that's an interesting, yet rather scary thought.