Instructables
Picture of Digital multimeter shield for Arduino

This instructable describes a shield, which converts "Arduino" board in a digital multimeter (DMM).


The shield can be inserted on "Arduino" UNO und Duemilanove boards. It can work in three modes:

  • standalone - the measurement data can be seen at the character or graphical LCM
  • connected - the measurement data can be read on the PC screen using the "Arduino" IDE "Serial monitor"
  • combined - the data can be observed on both devices

The second mode does not require the presence of LCM, what makes the shield very cheap.

The "Arduino" based DMM has the following functions:

  • voltmeter with 3 ranges : 0-10V; 0-30V; 0-100V
  • amperemeter - it has a range 0-500mA
  • ohmmeter with 2 ranhes : 0-1KOhm, 0-250KOhm
  • diode, LED, connectivity checker
  • LED functionality tester
  • NPN BJT Beta meter
 
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Step 1: DMM "Arduino" v/s Standard digital multimeter

The following movie shows how the "Arduino" based DMM works in comparison with non-professional standard DMM

yamil1113 months ago

Hi Milen, im doing this project, and im wondering how i measure the current?, i mean, no how works the amperimeter or the VIC, im saying how i conect to the circuit that i want to measure? the way that connects a regular amperimeter (in series to the circuit) or the way that i connect a voltimeter (in paralel to the device measure)?

Milen (author)  yamil1113 months ago
Hi Yamil111,

I have tried to explain this on step 2.... never mind,
the idea is the following - if you use the DMM device connected to you PC, then you can measure only currents which are sourced by devices having the same ground - in this case you connect only one terminal of the device (the red one), the current loop is closed through the common ground. In the case you want to measure a current flowing in some network, with unknown (different form you PC ground potential), it is better to supply the whole assembling (arduino+the shield) by a battery pack or isolated DC/DC converter.
So you use the whole assembling as standard DMM. You have to be aware, that this device does not measure negative currents - it means - the red input connector shall be always connected to the more positive node of the measured branch (the node, from which the current flows out). The black one (the local ground of the assembling supplied by battery) shall be connected to the node with less voltage potential (the node where current flows in). In this way it is possible to measure currents flowing in branches, which potential could be kilo-volts. In all cases, you have to be aware of the possible high voltages and to work carefully.
yamil111 Milen3 months ago

Thank you Milen, i understand what you're saying. Sorry, i miss up what you explain in step 2. But now, you clarify my doubt. Thank you, and excellent project.

AvrDon7 months ago

Hi Milen

Hi can I convert your DMM project to read voltages say from 80v ac and 150v volts ac. I'm wanting to building ac voltage monitor for single or 3 phase that I can read with the w5100 ethernet module was thinking a web page or telnet.

Thanks

Donnie

Milen (author)  AvrDon7 months ago
Hi Donnie,

You can rectify the AC voltage to DC and read its value. You have to change the voltage divider resistors (make them higher) and to change their ratio - at 150-160 V AC, after rectification, at A0 you must have 5V. You have to be aware that the AC voltage is measured as RMS - the amplitude is higher and the generated DC voltage is different. You have to calibrate the voltmeter with the proper coefficient. May be you have to add some correction because of the voltage drop over the rectifier diodes. And must be very care full - the banana sockets are not insulated and these voltages are dangerous!
Mr. E7 months ago

A quality project and great presentation ! The design discussion provides great information useful well beyond the scope of this wonderful project !

Have you considered storing additional calibration data in EEPROM to help deal with your non linearity concerns?

Milen (author)  Mr. E7 months ago
I did not considered this, but under desire it can be done. This is nice idea for people who want to go deeper in the problem. I preferred to try to correct mainly the linear behavior errors. Practically with careful device choice you can have only such kind. This is most valid for the transistor choice - if the transistor has low Vsat voltage, for Vds/Vce voltages over Vsat it has more or less linear behavior. The used NTD2955 is not the best choice, but using the PNP improves the linearity. Also suitable PMOS can be chosen so that it works also in the linear regime, and than the non linearity problem disappear. But, for the pure science...your idea is very good. It require more than two measurements and making some kind of look-up table or trying to find the equation, which models the performance...
achand87 months ago

Awesome !

granz7 months ago

To all potential builders: If you are powering this thing from your computer USB port you need to be very very careful what/where you are measuring. The USB port ground is usually earth grounded. This means that you can't go about measuring taking measurements with both leads on other mains-powered devices like you might with a commercial meter. Consider what happens if you try to measure a current in a circuit that is also mains powered, and you connect you ground lead somewhere in the circuit which isn't earth ground (Hint: you have created a short to ground via your PC). Commercial meters are either A) battery powered or B) isolated if they are mains powered. If you want to keep things safe, just power the Arduino from a battery pack.

dieferman7 months ago

Excellent , Thanks For Sharing !!!!

murfmv7 months ago

Milen, very nice job. Easy to follow good idea. How does one go about having a shield printed?

Milen (author)  murfmv7 months ago

Hi Murfmv. I have attached the design data. It is in "Eagle" format.

Download the ZIP file and un -zip it in some folder.

You can download the free lite version of the tool from here :

http://www.cadsoftusa.com/download-eagle/?language...

Install the program, start it and in the File menu>open one of the files (Schematic or Board) - the other will open automatically. Work with the board window. Press the "Ratnest" button - like "X" . Yo have to print separate the top and the bottom layers if you want to use the ink transfer method.

The top layer must be mirrored (in the print menu). Scale factor must be 1.

Solid and black checked. For the top layer - in the layer menu, first make all layer invisible, and then activate the following layers Top, Vias, Pads, Dimension.

For the bottom layer (not mirrored) activate the layers Bottom, Vias,Pads,Dimension.

The files are checked with the Iteadsudio checkset. They can be sent there, and you will have 10 boards for ~ 25 USD. If you want, I can attach also the gerber files.

Build_it_Bob7 months ago

Milen , this is a very impressive Instructable ! We all know that there are professional meters that can be purchased , but the impressive thing is this Instructable explains so much about what goes on inside a meter.

I feel ( and we are nearly the same age :) , that in our lifetime we have seen such an escalation in technology . I am not an expert in any aspect of electronics , but I am a lifetime student of this fascinating field. My father was enrolled in National Technical School correspondence learning and I was able to look through all his course materials in my early teens . Getting a basic understanding of Vacuum Tube theory , and then moving into transistor theory was so fascinating .

Integrated circuits started to simplify design , but we don't always take the time to appreciate what goes on inside and the engineering it took to create the special function chip.

Radio Shack had a great book out that I purchased on how to use your DMM , and that book helped me a lot . I hope that this Instructable gets a lot of attention as it covers in depth how a DMM works , and shows just how much design goes into a multimeter .

I think this would be a great project for College electronics students to build as it ties together electronics measurement theory and uses the Arduino controller which can be used for so many creative things.

I am going to download this Instructable as it is a keeper ! I will read through it more in depth as I am sure there is some interesting areas that will help me further my understanding.

Truly , I thank you for all the time and effort you put into this Instructable.

Build_it_Bob

Milen (author)  Build_it_Bob7 months ago
Hi Build_it_Bob,

Thank you for the nice words. I also went similar way. I started to solder some parts when I was 12. My first working detector receiver I did in 1977. Long history from this time... :-).
I want to mention - the design is not the perfect one, and no very universal.. - it has few week points : the low input resistance, the accuracy, the ranges comparing with standard industrial DMM. But his main purpose is to show a way of thinking - how using few devices a lot of functions can be implemented, how to have relatively accurate device, even you use standard and not trimmed building parts...This instructable is still not finished. Remain few additional things to be done - the ohmmeter trimming...and may be the conclusion.
aaguilar217 months ago
does anyone know some good books for easy to learn basic on electronics my nephew wants to learn n build
Milen (author)  aaguilar217 months ago

How old is he? Does he understand formulas?

May be the best solution should be to buy him some design kit with possibility to experiment with plenty of circuits and to learn the electronics practicing it... this will keep his interest, instead reading some book with theory.

aaguilar21 Milen7 months ago
he 14 n i have bought him some kits before n he really good but now he wants to take things apart but i rather he made stuff on his own from parts from old radios n stuff like that ... but i would like him to learn the basics
Milen (author)  aaguilar217 months ago

I did a small research in the internet...it seems that a lot of people recommend more or less the same books:

http://www.circuitstoday.com/4-great-books-to-stud...

and a classical example:

http://frank.harvard.edu/aoe/

There are also some very useful sites like:

http://www.allaboutcircuits.com/...

I hope that this would give you him a nice start.

and thank yu for yur replys :)
thegoodhen7 months ago

That's nice! I am just thinking about designing and building my own universal multimeter-but mine will be benchtop and higher end. I will use dedicated adc chips, voltage references and relays. But... I have to point out that your design has EXTREMELY low input impedance. I understand that you don't wanna go all the way into megaohms because of noise, but maybe you could add a low noise input buffer?

Milen (author)  thegoodhen7 months ago
You are absolutely right. The input resistance is low. I decided to make a compromise, also with the ranges... only to keep the schematics simpler and cheaper. But I think that for applications, for which this DMM could be used ( Arduino projects, digital projects... etc), the low resistance would not be such a problem. I wish you good luck with you project, and I hope that we all will see after a nice Instructable describing it.

Regards
Milen
thegoodhen Milen7 months ago

I see. I didn't understand the reasoning behind the low resistance, but now I do-it's to keep the complexity down. I wasn't sure if you are fully aware of this being a flaw, so I wanted to point it out! I would also like to mention that I am not trying to nitpick the flaws of your (I think brilliant) design, but with this in mind... Is the spacing between the rails sufficient? It seems like the 100 volts might easily spark over... Maybe that's just me though.

Milen (author)  thegoodhen7 months ago

As I wrote, before - I did compromise also with the ranges. May be 100V is not the suitable range, but here is another problem - I do not have any insulation of the banana socket - if there is really high voltage, it can be dangerous for the users. I did not want to have any dangerous high voltages on the board.The Zener diode is able to pass over 100 mA. This current multiplied by the input resistance of 10 KOhm, gives a voltage drop over 1000V at the input resistor. It is 1W and for some time will survive. But I do not suggest, that anybody tries that ! :-).

I want also to mention something additional about why I put so small input resistance - I did not want to use a buffer. I do not know how exactly the AD conversion is done in the Atmega chip. There is Sample & Hold circuit, but I do not know is there any buffer inside the chip before the S&H circuit. If no buffer exist, it is possible that the time constant of the S&H circuit capacitor and the big input resistance is enough high comparing with the sample time to introduce huge error. (I had some problems with ADC's without buffers before the S&H circuit, which were sampling false value because of the high output resistance of the previous circuit). OK, here the filtering capacitor of 10 nF helps, but with higher input resistance this capacitor also will create time constant and if the measurement was short in time, it would be also not correct.

msabhi637 months ago

ommg/////.........nice......one more possibility......

rpotts27 months ago
oh! realistically, how would this compare to a harborfreight model? same? better? WAY better?
Milen (author)  rpotts27 months ago

The described DMM has some limitations - The ranges are not the most appropriate.

The reason for that - the ADC has reference voltage 5v - that defines an input range for Vmeas 5V. This DMM can be done relatively precise inside this region, but comparing with even harborfreight model, it has the following disadvantages:

1) the ranges...

2)the accuracy - for the industry DMM special dedicated chips are used, which are trimmed internally, have the required resolution. For the "Arduino" based DMM, the resolution is limited to 10 bit...

3)the input resistance is relatively low...

In other words - the proposed solution is cheap, relatively precise, but with some limitations

rpotts27 months ago
Nice writeup! So you used a combination of opamps and voltage dividers for the ranges?
Milen (author)  rpotts27 months ago

Hi,

The voltage dividers are used for the voltage measurements...this approach use also the professional voltmeters. For the current measurement using voltage gain amplifies, may be is not the best solution, because the error is also amplifies. There a different set of very precise resistors is used. Their connection is done by mechanical switch, which contacts have practically 0 resistance and do not influence the accuracy. I wanted to use this approach for the current measurements (also for the voltage), I had to use some kind of relays (reed) , what would make the circuit more expensive and heavy. That is the reason I have only one range for the current measurements.