Electronics experts: Trying to make a simple Resistance (Ohm) Meter with LCD readout

[jivetrky]

Can someone please help me figure out how to do this?

I want to make a circuit that will read resistance (Ohm) measurements and display them on a 4 space LCD display. (To read as high as 5000 ohms, or 5k ohms)
Basically, this is taking the place of what the multimeter would do. And will be mounted in a project box, with the LCD obviously mounted on the box. This way I could use a potenimeter to change the resistance, and the LCD would display the ohm value just as my DMM would.

I've been googling and reading and now my eyes are bleeding, but I still can't figure this out. Please help :)


So the requirements are:

Power will be 9V or 5V 300mah DC
LCD to display as high as "5000"
If possible, I'd like it to be small enough to sit under the LCD with plastic bezel



EDIT: BTW I have found a chip called the ICL7106 by Maxim. But it does 3.5 digits (which I think would only display up to 1999 if I'm thinking right) and I need up to 5000 (or I guess 9999)

[Adak]

Hey! I'd start with two places and be sure to take notes:

howstuffworks
Wikipedia

First, be sure to write down just how an Ohm measurement is made and displayed in a digital format (analog if digital not available).

With that knowledge, then look at how you want to do this with your project. I don't believe 3.5 digits is enough display for Ohm's. For volts, or current - maybe ok, but not for Ohm's - where the range is very broad going into at least 5 digits. For measuring Ohm's in a speaker however, 3.5 digits is more than enough. I just wouldn't make a project that was that limited.

At this stage of the planning, I wouldn't get locked into any specific chip or whatever. Your question should be "How do I WANT to have this project turn out?"

Anything (like this chip), that you run across that looks interesting, jot it down along with where you got that info, in a little notepad for the project. You may want to refer to it later.

You'll then want to sketch out a diagram of the components you intend to use. You can snap a pic of it and load it up so it can be seen on the board, by others.
Here, a pic really is worth a lot more than 1,000 words, sometimes. :)

Then ask specific questions if you have any. I'm certainly not an electronics buff, but I've built a few projects. Hopefully, you'll get answered by someone much more experienced and knowledgeable.

Good luck, have fun, and remember to always design your project with safety in mind. If a component is rated for a max voltage of 12 volts, and you plan to give it 12 volts - it's the wrong part. You want the component to be rated for either 18 or 24 volts max, and then you give it 12 volts, etc. Overdesigning is used in all fields, as well; architecture, civil, mechanical, electrical, electronic engineering, etc.

That's why your house doesn't fall down despite a little termite or dry rot damage, and why car's keep running despite less than perfect timing, etc.

Adak

[Borisw37]

Is your goal to have a working ohm-meter or to build it?
If you just need the finished product it will be MUCH easier and cheaper to just buy one of those simple "made in china" multimeters. Something along the lines of
http://cgi.ebay.com/EW-Mini-DIGITAL-MULTIMETER-VOLTMETER-DMM-E21_W0QQitemZ290060001876QQihZ019QQcategoryZ25412Q QrdZ1QQcmdZViewItem

[jivetrky]

Well, my goal is actually to have an ohm meter that has a footprint as small as the LCD w/bezel. So that way the circuitry can fit under the bezel and not take a lot of space.

I've still been searching and have found some "digital panel meters" that read volts and/or amps. So I think they could probably be modified to display ohms...but I just can't figure out how to do that.

I'm starting to think that just getting one based on a 7106 chip is going to be my best and only real option. This will limit me to 3.5 digits on the LCD, but If I can setup a small switch (maybe a DIP switch bank or something) I can adjust the range to still display from 0-5000.

Many or most of those cheapy DMM's are based on this chip, BTW.

[Punisher69]

just a idea, but you could buy a cheep $15 digital multimeter and tear it apart and use it.

[jivetrky]

I had thought of that. And if I can't come up with anything else, that may be what I do. If that happens I'll just have to change the scope of the project a little bit and make the case a tiny bit bigger.

I still hope that I can find the original idea though, maybe if anything just for the sake of actually doing it instead of conceding to the DMM :)

[MarkS]

Multimeters are really nothing more than voltmeters with a microcontroller that converts the input voltage into the correct Ohm, Amp, capacitance, etc., reading. Try doing a search for voltmeter projects and go from there. Ohms = Voltage / Amperage.