Resistor Ohm Meter

[TheMadMan]

Hi All
Really having a problem with the tiny blue resistors.
Black, Brown, Blue, Violet all took the same to my tired old eyes.
Results - various amounts of smoke - not good!

Was wondering if someone has a circuit and source for an Ohm meter using a PIC that has a multiple 7 segment display output.

Idea is to <B>ACCURATELY</B> measure the resistor before soldering it in place. May prevent placing a 10 ohm resistor where a 10 M ohm one should go - caused big smoke and popping and I was rewarded with a flash burn.

Any takers?
Best wishes

[Melanie]

Go buy a $5 DVM from your local hardware store or even eBay if all you want to do is check Resistor values. The Voltmeter functions will come in useful for checking your PIC circuits later.

By the time you get all the parts for a PIC, the display, build a circuit, add the software, put it in a neat box with some leads, and factor-in twenty hours of your time, you could have budgeted for a Fluke.

[Luciano]

Hi,

Which came first, the chicken or the egg?

How can you build an ohmmeter if you can’t see the
colors on the resistors you will need to build one?

* * *

If you buy a multimeter invest at least 50 USD.
Cheap multimeters are not accurate and in some
cases these multimeters have a poor insulation.
(Dangerous when you measure voltages > 50V).

Automatic power-off helps conserve the batteries of
your multimeter. AAA alkaline cells are cheaper
than a 9V alkaline battery.

Best regards,

Luciano

[TheMadMan]

Hi guys
Thanks for the input. I agree that buying one would probably be the cheapest, quickest option and I think I'll get one on Saturday.

<B><U><FONT COLOR = RED>HOWEVER</FONT></U></B>

Has anyone got any idea how to do it using a PIC?

<B><U><FONT COLOR = BLUE>Ulterior motive : </FONT></U></B>

Been thinking of how to build a CNC driven PCB assembly robot.

Would be nice to check that the (error prone) human put the right components in the right magazine and to make sure that the component was not faulty or out of spec.

I'm not talking about lightning speed here, but the bragging rights might be impressive. :>o

I know I'm probably mad, hence the name, but I think up all sorts of weird things while I'm stuck in traffic. Helps keep the blood pressure at an acceptable level.

Go Well

[TheMadMan]

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

[mackrackit]

Have you read the PBP manual yet?

Look at RCTIME, POT, and the ADCIN commands.

[T.Jackson]

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2073&stc=1&d=119164380 4" align="left" border="1">
I have used this relatively simple, "poor man's" schema in a few past projects. Never tried to do all the maths with just the PIC though, instead I just spat out 10 BIT samples to a host computer and performed all the calculations there. Worked O.K.
<br/>

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

[TheMadMan]

Hi All

Have you read the PBP manual yet?
Look at RCTIME, POT, and the ADCIN commands.

Although I have written major apps using GWBasic back in the DOS days, I'm still a total newcomer to PICBasic. The PBP manual is required in-bed reading at this time (Wife bitches like a drain!!!!) and I'm still busy absorbing the info.
My gut feeling after reading up on the 3 commands mentioned is that the range that will be able to be handled is not wide enough to cover from 100ohm to 10K (in my opinion the range things fall into for "normal" projects without having "extra components" to the PIC) measurement range limits.
After all, the PIC range is a general solution that has tremendous versatility but is not able to be all things to all men or women in Melanie's case.
Would someone who has the experience and knowledge at thier fingertips care to comment on whether a PIC's RCTIME, POT or ADCIN commands would be able to do the job in the 100ohm to 10K range?
Best

[TheMadMan]

I have used this relatively simple, "poor man's" schema in a few past projects. Never tried to do all the maths with just the PIC though, instead I just spat out 10 BIT samples to a host computer and performed all the calculations there. Worked O.K.
<br/>

This method should be able to work quite well but possibly will not be too accurate if the test piece DUT is too big or too small.
As far as I can remember, the ADC converts the voltage received (between 0v and 5v) to a number between 0 and 255. You will have to adjust the value of R1 to get a reading that is accurate for high value resistors and another R1 value for low value resistors and probably a number of times in between.
This method will probably work very well in cases where you are trying to get a number of resistors as close as possible to identical, closer than 1%.
Must add this to my library.
All the 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]

This method should be able to work quite well but possibly will not be too accurate if the test piece DUT is too big or too small.
As far as I can remember, the ADC converts the voltage received (between 0v and 5v) to a number between 0 and 255. You will have to adjust the value of R1 to get a reading that is accurate for high value resistors and another R1 value for low value resistors and probably a number of times in between.
This method will probably work very well in cases where you are trying to get a number of resistors as close as possible to identical, closer than 1%.
Must add this to my library.
All the best,

That schema is relatively orthodox stuff that I was formally taught at TAFE.

[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.

[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]

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 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>

[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.

[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.

[khetanikrunal]

hello friends,

I already obtain the range of 1mohm to 1 tera ohm but i need to go upto 1000 teraohm as per my specification.

so I need Schematic for this solution.

Thanking you,
khetani krunal

[Gusse]

Second time Wow!!

Your application must be something really extreme or not!
1000 Tera = 1000 * 10^12 = 1 Peta

So, you really want to measure up to 1 Peta ohm and with some self-made equipment
Before any hints for schematics. Could you tell a little bit more about you application?

Sorry, but this question make me smile

BR,
-Gusse-

[Dave]

I am also interested in the test voltage used for measuring the resistor?

Dave Purola,
N8NTA

[khetanikrunal]

hello,

These type of measuring devices are used to mesure Transformer oil conductivity.You will find other site too.

Once again i am asking for the schematic related to this topic.

thanking you,

krunal khetani

[Gusse]

I am also interested in the test voltage used for measuring the resistor?

I would be also very curious to know that, because it must be very high. Also how low currents you can measure? Depends on that quite much.

hello,

These type of measuring devices are used to mesure Transformer oil conductivity.You will find other site too.

Once again i am asking for the schematic related to this topic.

thanking you,

krunal khetani

I though that it must be something to do with insulators, obviously. Is there any way that you could boost your existing equipment? I know, 1000x times more is pretty tough requirement (=1000x times high test voltage).

BR,
-Gusse-

[khetanikrunal]

see My test voltage is 500 V DC...

Even i must use amplification of lower current(from Piko amp to Nenoamp)..

system is like this

HV(500v dc)----->Resistance of Oil(1mohm to 2000Tera Ohm)------>LV.

And i have to Take current from LV point and convert it to voltage send to controller via ADC.

[Gusse]

So your current sense capability is limited to ~0.5nA with 1Tera ohm measurement equipment. Definetely it would be much easier to try lower sensing capability than increase test voltage.
Most sensible would be increase test voltage by 2x (or 4x) and try to improve current sensing (what ever it might need).

Pico amper range current sensing is very demanding and any noise from environment will affect your results (even measuring at DC). Most probably you have to do your measurements in shielded rooms, etc..

Even if you could build up measurement system that could measure up to 2 Peta ohm then limiting factor for accuracy would be ADC (even 16bit). One step would be ~30.5G ohm. Probably you should consider some better ADC than in PICs. (This is valid only if you have just one range for input, full range).

BR,
-Gusse-

[khetanikrunal]

hello

I can measure up to 3 nenoamp and 6 uV.
event thats not problem.

I am not able to get linearity.

i.e., if ADC o/p 500mV for 1 teraohm.

then i am not getting 250 mV for 2teraohm.

[Gusse]

hello

I can measure up to 3 nenoamp and 6 uV.
event thats not problem.

I am not able to get linearity.

i.e., if ADC o/p 500mV for 1 teraohm.

then i am not getting 250 mV for 2teraohm.

Even i must use amplification of lower current(from Piko amp to Nenoamp)..

Now I'm a little bit puzzled.

Do you mean that you can measure:

1) 3 nano A (3 * 10^-9)

500V -> Rmax = 170G ohm

or
2) 3 pico A (3 * 10^-12)

500V -> Rmax = 170T ohm

Rmax is maximum resistance that you can measure.

When you try to measure 1T and 2T, how you know that insulator is exactly ~1T and ~2T? This linearity issue is also bit unclear.

BR,
-Gusse-