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.