Scullcom Hobby Electronics #44 - Millivolt Meter MK2

[@fritzt0815]

That is what open source and open hardware is all about. Continually improving each other's designs and ideas. This is really nice.

[@scullcomhobbyelectronics1702]

Thanks. Well said.

[@mrfrog8502]

Your schematic drawing skills are exeptional. Straight lines, junctions clearly marked, I love it. Great work.

[@scullcomhobbyelectronics1702]

Thank you, i try my best !!!!! :)

[@DavidLeeMenefee]

This is by far some of the best electronic stuff on the internet that I have seen. You sure have put a lot of work into this, very inspirational. It's too bad you aren't asking for donations because you sure would do rather well I think. I hope you keep these videos coming....PEACE

[@scullcomhobbyelectronics1702]

Thanks for your kind comments, much appreciated. Regards, Louis

[@brunofonseca9686]

Louis, thank you so much for taking your time to make these videos. Thoroughly explained, very good indeed.

[@scullcomhobbyelectronics1702]

Thanks Bruno.

[@Kermal111]

Great update, I really like this version of the design. I have a few additional suggestions: - Currently the software calculates a moving average. This does indeed eliminate some noise, but you can reduce refresh rate and calculate regular say 8-sample average to get optimal noise performance. Another option is to have a dedicated button to switch between fast update/slow update/long-term average modes. - It would be nice to have this project as a null-meter. To fully exploit the hardware, add an option to short out upper divider resistor to get 1:1 ratio and put a front panel switch to do a quick offset adjustment. - The board could be optimized a little to divide the analog/digital parts better (better ground planes, dv/dt reduction). That's not an important issue unless one wants to aim for sub-millivolt performance.

[@scullcomhobbyelectronics1702]

Thanks for your comments. When I was testing this update I did test it using different update rates and different number of samples by simply changes to the software - as you will see in the code these variables can easily be changed. An additional button for this option could be added later as an additional feature. One of the reasons I left it off for the time being was that I wanted the response of the other feature buttons to respond quickly. One option would be to use an interrupt feature in the software but I am not to familiar with that method at the moment. As we work on the software I am sure improvements and additional functions can be added in future. I am already looking at a number of other features such as Minimum and Maximum Readings, logging a range of voltage readings over time in memory for review later, input filter, additional calibration voltages to improve setup accuracy, some maths functions, auto adjustment to cancel out linearity error of the ADC (LTC2400), improvements to the bar graph to cater for lower and higher voltages, switch input resistor divider to change ranges. There are many option and feature we could look at in future upgrades. The more you think about it the more you can do - it just takes time!!! :) Regards, Louis

[@javierpallalorden]

Hi Louis, this is an excellent follow-up video.

[@scullcomhobbyelectronics1702]

Thank you.

[@johanandersson3926]

Thanks for a excellent project. I also added the filter by Paul Versteeg. I've never worked with filters before, I think I add a button to test different filters and filter factors.

[@scullcomhobbyelectronics1702]

Thanks for your comment.

[@berniken6511]

Very nice upgrade Louis I will certainly be building it.. Thank you for the artwork downloads..................Berni

[@scullcomhobbyelectronics1702]

Thanks Berni.

[@Mtaalas]

Averaging function would be really great. It would smooth out those jumping numbers. it could be only 8 or 16 averages or something :)

[@Steven_Bennett_YT]

Look at the code, it has a configurable averaging feature.

[@DavidLeeMenefee]

It's great to see you back online.....thumbs up. Also thanks for the file downloads.......PEACE

[@scullcomhobbyelectronics1702]

Thanks.

[@piotrliszka8481]

your videos are very inspirational for me. because of you I recently start to doing my own lab gear. thank you for doing a good job. best channel ever. best regards.

[@TheFunkman]

As usual, another great project! Thanks Lewis!

[@MarcoFranceschini1971]

Hi Louis...great project from a great, great electronic engineer.

[@makerKID5]

Simply brilliant! I can't even find the words.

[@scullcomhobbyelectronics1702]

Thank you for your kind comment.

[@markj3851]

Great videos Louis. Can I ask if you can share the gerber files for the new v1.5 pcb? I can't find the link anywhere and oshparks site doesn't list ths project any longer. Thanks in advance from the US!

[@qzorn4440]

o my this is so exciting and practical 🤓 i just watched another video on a 10 digit analog meter with parts like the LTZ1000 voltage-ref for $75 alone 😳 thanks for a diy-project 🥳🤸

[@stoptheirlies]

Why did you remove the input protection?

[@isidoromaich7226]

Well done! excellent project, I will build this new version (I've already made the previous which I really enjoy and use a lot) (possible improve: min/max option)

[@scullcomhobbyelectronics1702]

Thanks for your comment. I am looking at a number of addition feature which we could add with new software versions which I will try and do over time. I will provide updates if I make any future changes or additions. Regards, Louis

[@tubeDude48]

When viewing the schematic, you didn't cover the 50Hz & 60Hz jumper. Newbies may not understand that.

[@tonybell1597]

Excellent, thank you, and thanks for sharing all the files to create it.... Winter project coming up me-thinks....

[@scullcomhobbyelectronics1702]

Thank you.

[@galileogalilei7844]

Very nice project. The only problem I see is the battery status hardware, you used a 100K ohms resistor to ground and a 1M ohms resistor to the positive of the battery to measure it's voltage at startup using the Arduino's ADC, but Atmel recommends less than 10K ohms source impedance at their ADC pins. So you could get bad readings doing this. The right way to do it (especially since you only read the voltage once at startup) would be to use something like say a 510 and a 5.1K ohms resistors and after doing an ADC conversion turn that pin into a digital input which is high impedance and will not draw much out of the battery.

[@scullcomhobbyelectronics1702]

Good idea, thanks.

[@edgarskarklins3572]

What is the maximum voltage it can currently measure? Would be nice to see if extended range would be added. And interesting to see how to switch between them. And I would like to see this built together with the current source from the miliohm meter instead of the eBay module.

[@supyrow]

wow! what an improvement. gonna have to build this one up! Thanks for the update and have a good one! -Bobby

[@scullcomhobbyelectronics1702]

Many thanks Bobby.

[@ZobiLaMoucheLeRetour]

Very very interesting project. Thank you so much to share it with us !

[@scullcomhobbyelectronics1702]

Thanks.

[@andreweastland9634]

Why do you have the resistor to ground on the buttons? This will create a voltage divisor with the input pullup resistor which are also 10k so you will only get halfway down to ground which may cause indeterminate read errors. Also why not some simple cap denounce on the buttons?

[@scullcomhobbyelectronics1702]

Thanks for your question. During my testing for this project upgrade I tried a number of options in order to overcome key bounce issues. Initially, I did originally have a small capacitor across each push button but after playing with adding a key bounce timing delay times in software I found I could remove them. Also I tested with having the buttons return direct to ground which also worked but found that adding a earth return resistor also worked well and also help to reduced the possibility of key bounce as long as the value was 10K or less. The internal pull-up resistor of the Arduino Pro Mini are between 20K to 50K (typically around 30K or more) and so using a earth return resistor of 10K did not effect cause any any issues with logic 0 level which for the Arduino is around 1.5V or less. If you do experience any issues the replace the 10K resistor with a 4.7K. The other benefit I found of using a resistor on my build is that it reduced any switching spikes which could radiate and possibly cause interference with the readings. Hope this explains my reasoning. Regards, Louis

[@Hugatry]

Great update video! Very well explained and demonstrated, like projects in your videos always are.

[@scullcomhobbyelectronics1702]

Thanks for your kind comment.

[@nathansavery3298]

Excellent video, I was wondering why you don't use a negative supply voltage, rather than ground for the buffer op-amp. wouldn't this allow a full range, rather than being limited by the offset voltage of the op-amp? Thank you

[@parragoriouse0138]

Hi is there a pcb file available for this new version anywhere? as the links bellow dont work for me

[@DoRC]

very cool. I think I'm going to try to make a single side board from the current schematic so I can try to make it at home. is there any reason that wouldn't work? Also I think it would be cool to have it take an average of the following 5 or 10 readings and display that when you hit the hold button. That way you can avoid accidentally hitting a reading outside of reality.

[@scullcomhobbyelectronics1702]

Thanks. Checkout the earlier parts to this project as the original design used a single sided PCB: kzbin.info/www/bejne/rammiGaml9uFn5o

[@argcargv]

Do you think it might be a good idea to add some low pass filter in the buffer stage. The noise on the lower digits reduces the utility and I wonder if a bit of low pass filtering would clean that up. The Keithley didn't show all that jitter and I wonder if it doesn't have some filtering in as well...

[@scullcomhobbyelectronics1702]

Thanks Ed. I have been looking at the possibility of adding some filtering on the input. Keithley have a filter button as a option and this is something I am also considering. I have been looking at a number of other features which I have already mentioned on an earlier reply to another followers comments. Thanks for your comment on this also. Regards, Louis

[@tubeDude48]

At 9:35 to 9:42 You said to use the CP2104, but then you say to use to CP2102 - was that a mistake in what you're saying?

[@audiotecnologia4084]

Great video soon I'll be building my milivoltmeter thanks for sharing your knowledge

[@scullcomhobbyelectronics1702]

Thanks. Enjoy your own build. Regards, Louis

[@boldford]

I've just come back to this to find some of the libraries are no longer available. Doh!

[@michaelhawthorne8696]

Nice project.... Maybe the update rate should be slowed down a bit, make the extra resolution more useful. I get to see plenty of the AD8628 OP Amps at work as the company we build and test for use them extensively, When they work they are great Rail to Rail OP Amps but boy do I get a lot producing noise which have to be replaced.

[@scullcomhobbyelectronics1702]

Michael, Thanks for your comments. I have been looking at a number of ways to improve the resolution. Changing the update rate is quite easy to do in software and I have considered this as an option in any event. Also we could set different update rates for different voltage ranges. Another way is to increase the number of sample readings taken, again this can be easily done in software - I think this would have the best result. I will be looking at this and testing in future software upgrades for this project. With regards the AD8628 I was surprised to hear you have had problems with some producing noise. I would be interested to hear in what type of applications you are using them and whether you have them wire on a single supply voltage or split supply voltage. I have not really had any problems with noise with them in this project as only DC levels are used. Also I have found this OP works best when powered from a single +5v supply. I chose the AD8628 as it has the lowest input offset voltage of any OP Amp on the market with virtually no drift due to temperature changes. Regards, Louis

[@michaelhawthorne8696]

Thanks for your reply Louis, The AD8628 is used with both Dual and single rails. There is one board I test and repair which uses 8 of them as dual rail (+/- 12V) analogue amplifiers fed from a DAC, (not to mention others dotted around the board too but on single supply rails.) This board sends out the analogue signals and the test rig returns this signal back and feeds it into 24 channels all using an AD8628 (+5V) in the signal path. This then goes into an ADC then on into a Xylinx FPGA. To be honest I've thought for awhile that the oven we use may be just too hot because when I replace the offending device, the fault clears, the iron not doing the damage the oven maybe doing. There's usually 1 or 2 to change on each board, rare to get one through without getting one bad one. When they go, they usually change the sine wave input to a poor looking square wave either that the o/p is rail to rail noise.

[@scullcomhobbyelectronics1702]

Thanks Michael for the insight in to your dealing with these IC's, interesting. The only issue I have had with the AD8628 so far is when you try to run it off a split power supply (±2.5V), for some reason the input offset voltage rises to about 5 times the level of what it is when run from a single 5v supply. Running off the 5v single supply as in my project the input offset voltage is well within the 1uV spec. of the datasheet with virtually no drift with temperature. Regards, Louis

[@g1fsh]

Great video as always and some really nice upgrades to a great project. Thanks for the update. Can I ask what program u use to design the front panel templates for your projects?

[@scullcomhobbyelectronics1702]

Thanks Gary. For the front panel artwork and templates I simply use Paint Shop Pro from Corel and simply scan in the front panel plates for the correct size and then design the layout of the buttons and window and print a test sheet to check and then make any changes as required. It takes a bit of time to get it all lined up OK but work well for me. I sometimes cut out stiff cardboard templates and use them to test correct positions of buttons, sockets and LCD.

[@JoeSmith-gt3bf]

Hello, how would you implement the ability to read negative voltages as well? If you reversed the polarity would it not break the meter?

[@scullcomhobbyelectronics1702]

Hi Joe, You would probably need to change the ADC for an ADC with differential input (such as the LTC2412) and make some changes to the input circuit. If you applied a negative voltage to the meter it should survived as long as you do not apply a very large negative voltage. Regards, Louis

[@MrJetra]

It is a very nice demonstration of a precision instrument. I like the simplicity, but as a "cheap alternative" to on-the-shelf professional equipment, I miss the bipolar range (e.g. +/-10V). I have some ideas on how to do it, using the unipolar LTC2400, but I could like to see your solution.

[@scullcomhobbyelectronics1702]

Thanks for your comments. I may come back to this project again later. Regards, Louis

[@peterfielden-weston7560]

This was very interesting. I do have a question (or two.) Why did you choose the Arduino pro mini over, say, a nano? Wouldn't the earth return (current limiting) resistor be better suited placed on one (or both) of the matrix boards? Couldn't the 5 volt reference be included in the design? Does the meter have to be powered off to implement a new calibration setting? I like the idea of limiting the number of decimal places displayed as the readings on the last two digits were nearly random. Possible additional functions for consideration :- Maximum reading. Minimum reading. (Might need to go to a 2004 LCD display for these.) Averaging readings, taking the average of 2, 4, 8, 16 readings before displaying. (This could help with the accuracy of the device.) Possibly loop through the measurement cycle a few times, then do a battery check, then measurement cycles again. Only display the battery check for a low battery condition. A pre-warning that the battery voltage is getting low. Think I'll shut up now before you get sick of me. :)

[@scullcomhobbyelectronics1702]

Hi Peter, I chose the Arduino Pro Mini as it fitted the great PCB designed by Greg and is available from OSH Park. I did original fit the 10K earth return resistor on the front push button switch panel but later moved it to the edge connector as the rear buttons also used it. I did consider adding the calibration 5v reference on the PCB but later decided not to as one of my previous project had one of these. Also in future software upgrades I hope to improve calibration further by using a range of reference voltages to calibrate as must professional meters use that method. With regards the battery status I did initial simply show on the display when the battery requires replacing but later changed that to show status of voltage as the battery replacement level depends on whether the 5 volt regulator is a normal one or a Low Drop version. But we could simply added a replacement display in software if required. I have also been looking at more feature to this project but need to spend more time on the software. Will release software upgrades as I do them - hence the reason why I shoe software version on the intro screen. Regards, Louis

[@supersilve]

I liked this project very much and it looks nice and neat. However the display looks a bit dim with the red filter of the front panel. If for myself I would fabricate a piece of clear perspex for the front panel which will make the display much clearer. Well done and keep your good work. Thanks.

[@Brump1]

Hi Louis, this is an excellent project! Can you help me change the circuit to measure bidirectional voltages?

[@SirHackaL0t.]

Nice video. Have you considered using the usb connection to allow logging of the voltage data?

[@scullcomhobbyelectronics1702]

Yes this this could be a nice addition. Also you could use an SD memory card to record logged voltages, which could be controlled by the Arduino and the recalled and show on the LCD and toggled through them using another push button. I am sure there are lots of addition we could add to this project. Regards, Louis

[@Steven_Bennett_YT]

Louis are you still developing new projects?

[@douro20]

Looks like they're using NOS Philips I2C expanders.

[@MestreVintage]

congratulations for a amazing project. i kindely ask you for the new data as the link below isant working anymore, thank you very much

[@perrieiles493]

Was wondering why the zero cal was removed. at the very least it should use zero and full scale, as you suggest a multipoint cal would be even better. i favour at least 3 .

[@philspargo9543]

Hi Louis! I just completed this version and calibrated it with the 5 volt reference. At 10 volts it is 2mv high and at 1 volt it is about 1mv low. How can I calibrate this error voltage ?

[@adrianschneider4441]

I am just wondering whether you might have considered grounding the metal plates on the base and on the front as they may act as antennas and coupling in noise. Cheers.

[@scullcomhobbyelectronics1702]

Thanks for the comment. Yes I did ground the copper side of the base. As you say the front panel metal back-plate could also be connected to ground.

[@christianmock1815]

Very nice upgrade. How about leaving the USB interface on, and adding an RTC, then you could log time and voltage on a PC.

[@scullcomhobbyelectronics1702]

Thanks Christian. I did think about leaving the TTL to USB adapter wire in the box permanently but for different reasons. But your idea of also adding a Real Time Clock to log voltage reading over time is a very interesting option. I need to look further in to that idea. Regards, Louis

[@tommyallehamn]

+ Scullcom Hobby Electronics As always, brilliant! But I have a few questions. I'll start with, why you didn't power the op amp with dual supply since the input value is so close to zero (say tens of micro or nano volts). Second, was it so so hard to get exactly zero on the output when shorting the two inputs? (You got around 40uV in best cases). I mean, getting 0V without modifying the software (or calibrating), because I'm working on a similar project, but an ammeter, and it was so shame that I couldn't get zero when there was no current through shunt resistor (zero volt deference between the two op amp inputs). I tried OPA277 (because it can be trimmed) and AD8629 (because it has input offset voltage less than OPA277) with single and dual power supply. I tried also different supply, 5, 3.3 and +/-1.65 v, and noticed that better results was with lowest voltage. Any ideas or help would be so appreciated! Regards, Tommy

[@scullcomhobbyelectronics1702]

Thanks Tommy. The project was designed to run off a single supply and to achieve that I used a AD8628 which is designed to run off a single supply and is an auto-zeroing OP Amp with a very low input offset voltage of only 1μV. There may still be some noise picked up at the input and so you may still get around 40uV or so at times. Also the two protection diodes on the input can cause input noise. The AD8628 is hard to beat for very low input offset voltage and offset voltage drift of only 0.002μV/°C. You could try shielding the input circuit and ensure you have guard tracks around the IC input pins. Regards, Louis

[@tommyallehamn]

Thank you for your reply! Actually it's a good idea (guarding and shielding), I wanted to try it, but after I saw Barbouri's results, I skipped it. In best cases (after guarding and shielding) he got +/- 12 uV. (And by skipping, I don't mean it doesn't worth the effort, but, I mean I need something more in addition to guarding and shielding.) But in the other hand, I came over this: www.analog.com/media/en/training-seminars/tutorials/MT-037.pdf It's simply how to trim input offset voltage externally. (Please check figure 6 page 6). Because of limitation of my multi-meter (wich is min 10uV), I couldn't test this method as well. So can you kindly think of this maybe in your free time and give a shot? Regards, Tommy

[@him481]

Lovely project

[@scullcomhobbyelectronics1702]

Thank you.

[@dkng32]

that is a great work you are doing, by the way is it possible to add a battery charging circuit to the project? that will be a nice improvement for the millivolt meter and a nice feature to have.

[@scullcomhobbyelectronics1702]

Yes it would be easy to add battery charger option. Will look at that. Regards, Louis

[@Moonbrony]

Great improvement, I've been wanting to make one of these for a while. Even more reason to now. Do you have a link for the box?

[@scullcomhobbyelectronics1702]

Thanks. There is a link for the box is on the parts list which is in the download zip file. But I also put another link below: www.ebay.co.uk/itm/131573248544?_trksid=p2057872.m2749.l2649&var=430944212817&ssPageName=STRK%3AMEBIDX%3AIT

[@zaprodk]

I'm confused. In the picture you made and printed out, you are talking about RX/TX on the display - wasn't that I2C (SDA/SCL)?

[@scullcomhobbyelectronics1702]

Thanks. Yes you are correct for the I2C it is DDA and SCL which simply go to A4 and A5. Regards, Louis

[@karlmuhlberghuber5918]

hello and thanks for this amazing video, the quesiton is what other ic can be use as AD8628? it seems it is Zero Drift and rail2rail

[@scullcomhobbyelectronics1702]

Hi Karl, Two alternative could be OPA2187ID or TLV333IDCKR. Try doing a search for Zero Drift OP Amps. Regards, Louis

[@simonbaxter8001]

Yet another great project! The displayed accuracy could be improved by having an option to average a number of ADC readings (say 4, 8,16 or 32 ... perhaps selectable like the decimal place modes), that would reduce the noise and home in a little on the real mean of the input voltage.

[@Steven_Bennett_YT]

Look at the code, it has a configurable averaging feature.

[@simonbaxter8001]

@@Steven_Bennett_YT The code uses a 'resource hungry' rolling average which needs an array of readings to implement. This has a number of issues when scaling up. It's limited to the size of the RAM you have, which on a typical Arduino micro is not much. A simpler, more scalable averaging can be achieved by taking say 1024 samples consecutively, adding them to a total, then dividing by the number of samples (a bit like the getADCAverage function does). That would use a handful of bytes and would be infinitely scalable. Only downside would be that the display update rate would reduce as sample count increased, but that is how precision bench DVM's achieve their last digit accuracy anyway, so a simple trade-off to get much, much higher accuracy. Saying that, it all depends on the spread of the ADC noise, and ADC ref voltage is, to how accurate the reading is anyway!

[@Steven_Bennett_YT]

@@simonbaxter8001 Thanks very helpful

[@cccyberfamilydk]

Thanks for the nice video. As always a great video. Can you provide a link to the PCB?

[@Barbouri]

Hi Christian, The current version that is available from OSH Park is 1.51 oshpark.com/shared_projects/qgv0fpKN Barbouri (Greg C)

[@scullcomhobbyelectronics1702]

Thanks Greg for answering Christian. I did also put the link on the parts list download. Regards, Louis

[@springrollwang4441]

Love your videos. a silly question. does this only measure DC voltage?

[@scullcomhobbyelectronics1702]

Yes for DC volts only.

[@Stereosandotherfun]

Why is there no shielding? Noise looked a little high...

[@HendrikLipka]

Is there a reason to change the input resistor from 10Mohm to 1MOhm?

[@scullcomhobbyelectronics1702]

No reason. It was just the way Greg designed the PCB. This could be changed to what I did originally by raising the Caddock resistor network off the PCB and adding some wire links. That will then give you the 10M input that we had before. Thanks for asking this question - I should have mentioned it in the video but forgot. Regards, Louis

[@BeetleJuice1980]

can you provide a replacement for the caddock its 25$ and hard to find in Europe.. can we fix the pcb chem for the extra two resistors you soldered?

[@BeetleJuice1980]

if I use 100k and 1meg 1% resistors is not the same as with the caddock pack?

[@scullcomhobbyelectronics1702]

You would need to use a 100K and a 900K to give the divide by 10 ratio required. Also I would suggest using 0.1% tolerance. The Caddock are much better when it comes to stability due to age and temperature but you could use cheaper resistors if you wish.

[@hankus253]

Why the choice for the Pro Mini? The Nano has USB and is no wider and just a couple of mm's longer. Is detachable USB the reason rather than a fixed in place?

[@Barbouri]

Hi hankus253, The only reason for the Pro Mini over the Nano, was that I had zero Nano's and a dozen 5V Pro Mini's in my parts stock and a FTDI Basic Breakout board already. Otherwise I would have stayed with the Nano that Louis used initially. Barbouri (Greg C)

[@scullcomhobbyelectronics1702]

Hi Greg, Thanks for your answer. I do like the small form factor of the Arduino Pro Mini so I though it worth using that to benefit from your great PCB. I am sure others would also welcome the fact that this PCB is available for this project. Regards, Louis

[@YunusCanKurban]

Hi Louis, is there a bi-polar input version? or dou you have any suggenstions for making your design bi-polar?

[@scullcomhobbyelectronics1702]

Hi Yunus, Sorry for the delay only just seen your message. As it stand the circuit is not bi-polar input. To do that you would need to make some changes to the input circuit of the LTC2400. As a start you might want to have a look at Linear Technology design solution document which you can download from the link below: cds.linear.com/docs/en/design-note/dsol05.pdf Regards, Louis

[@YunusCanKurban]

Thank you for answer. Do you think adding -2,5 v ref. to negative input pin of adr4540 is a nice solution? that way we can add +2,5v bias in to adc input and chance software to work bi-polar.

[@satishkumar-gg5fm]

Sir can you make short circuit finde???

[@timothysvec8568]

Amazing project. Well done Sir! Well done! Approximately how much would the component's cost in U.S. Dollar's?

[@scullcomhobbyelectronics1702]

Thanks. The cost depends on where you source your components from but an estimate would be around 90US$.

[@irgski]

What happened to the blue display?

[@scullcomhobbyelectronics1702]

It still uses the Blue LCD display its just the front panel on the case is clear red perspex.

[@philspargo9543]

Louis, What is the input impedance of the Milliohm meter ? (900K ?)

[@scullcomhobbyelectronics1702]

Hi Phil, The input impedance is 1Mohm in this version as Greg Christenson designed this small PCB to use the 1K, 9K and 90K (total 100K) for the bottom leg of the potential divider and the 900K for the top leg (of the Caddock resistor network 1776-C681). In my original design I used the 1K, 9K, 90K and 900K (1M) for the bottom leg of the potential divider and the 9M for the top leg. So in the original design it was 10M input impedance. You could stand-off the Caddock resistor network from the PCB and then hard wire it as my original circuit design, in which case it would then be 10M input impedance. Regards, Louis

[@philspargo9543]

Hi Louis! Do you prefer the 10 Megohm impedance ? Certainly it would have less loading. What are the pros and cons?

[@scullcomhobbyelectronics1702]

10 Megohm would probably be the best. My original design was that. But they both should work well. Regards, Louis

[@stephenbeeson1521]

2 Things i'm not sure about on this. 1. Your using a 4.096V reference but your calibrating for 5V. 2. Your calibrating at 5V when your opamp and ADC are both powered by a 5V regulator, what happens if it outputs less than 5V?

[@scullcomhobbyelectronics1702]

Hi Stephen, Thanks for your questions. 1. The 4.096V reference IC is used by the 24bit ADC and ensures each step of the ADC is an accurate value. The resolution of a 24-bit ADC is a total of 16,777,216 steps. Therefore if the ADC uses a 4.096V reference source then each step is equal to 4.096 divide by 16,777,216 which means each step is equal to 0.0000002 volt (0.2 micro volt). 2. Calibration is a separate issue. The Millivolt meter has a divider network at its input where the input voltage to be tested is applied. This voltage is divided down by 10 before been applied to the ADC to be read. So if you apply an accurate 5 volt reference source at the input of the meter then in theory (assuming no losses or error) 0.5 volt is feed to the ADC. The ADC then generates the correct number of steps to represent 0.5volt. The meter software then calculates actual voltage reading back up to 5 volts by multiplying the output reading of the ADC by 10 before being displayed on the LCD. In this case, if the ADC reading does not equal 0.5volt then a difference voltage is calculated which is then used in software to calibrate out this error so that the reading on the display is correct and shows 5 volt. The error is a result of tolerances in the input circuit which results in the ADC not receiving the correct voltage that reflects the input reading. Hope this helps the understanding. Regards, Louis

[@stephenbeeson1521]

Hi Louis, Ah, I looked at the unity gain amp and forgot about the Caddock Network. Another question, what would be a good way to calibrate if we don't have any other calibration reference equipment?

[@scullcomhobbyelectronics1702]

Hi Stephen, You will need some form of accurate 5 volt reference source. Checkout my earlier videos where I show you how to make one at low cost. kzbin.info/www/bejne/qXyphWyCpNqsirs kzbin.info/www/bejne/ZmKsc5eemKqghdk Alternative search on eBay for 5 voltage reference and you will find some cheap units from China which will do the job for you. Regards, Louis

[@robertomartin8731]

I wish you have a playlist organized for each of your project so its easier to follow. Thanks for all the videos.

[@scullcomhobbyelectronics1702]

Thanks. Good idea about a play list. I am still trying to get to grips with all the features of youtube. I will try and look in to this option. Regards, Louis

[@questioneverything6860]

Very impressive!

[@scullcomhobbyelectronics1702]

Thank you.

[@SatyajitRoy2048]

Nice stable reading from a measuring tool is a pure joy. average out the readings and show only the valid stable value. Also any precision voltage measuring tool should be able to read voltages as near to the zero as possible. When there is no voltage applied, then also your meter shows many millivolts which is annoying. May be you need to power the opamp with dual supply because any rail to rail opamp are really not exactly rail to rail. and they tend to be noisy when working at those edges. you could simply supply it with ±2.5V easily. You could use any arduino pin to generate negative voltage and with double LC filter you could easily manage low noise supply for the opamp (instead of 9V, ±4.5 arrangement could even be better). another point is, you need to use multipoint calibration so that it gives linear measurement reading to greater extent. I havent gone through the LTC2400 yet, but it seems it will be better to use differential mode ADC rather than this single ended one. Say, you want to measure milli ohm of a resistor and you pump it with a known current from a source, then you just cant measure the drop across the resistor precisely. You are not always going to measure if its 5.0002V or 5.0003V, sometimes you just need to check whether its 0.00002V or 0.00003V, thats where this tool will just fail. Anyway thumbs up for the good work and I am pretty sure that this will come up with better design someday.

[@scullcomhobbyelectronics1702]

Thanks for your comments. With regards improving the stability of the resolution shown on the display I am looking at a number of possibilities. Either by slowing down the ADC update rate or simply taking more samples for the averaging (or both). I think the second of the two options should work well as more averaging should stabilize the display reading. With regards using a split ±2.5V supply for the AD8628 I have found that when I did that on testing this chip it caused the input offset voltage to increase. I found this IC works best off a single 5v supply. Off a single 5v supply the input offset voltage was only about 1uV as per the datasheet, or even less. But when I used a split supply it increased by 5 times or so. with regards calibration as I mentioned on my video I am looking at multi-point calibration using several reference voltage across the range of the meter, say 0V, 100mV, 1V, 5V and 10V. I also want to look at ways of some auto adjustment to cancel out linearity error of the ADC (LTC2400). These are some of the upgrades for the future as this is an on going project of mine. All the best, regards. Louis

[@IanScottJohnston]

Scullcom Hobby Electronics - Dont forget the averaging/tracking formula I sent you a while back, very, very useful in noisy ADC type scenarios.

[@SatyajitRoy2048]

I dont know why the offset voltage is increasing when you used ±2.5V split supply, any opamp should have guaranteed offset range in its full supply range. But the important part is, the output of this opamp doesnt reach rail voltage and I just saw that its quite a large amount. Somewhere like few millivolts and may be thats causing the zero reading so unusable. You cant calibrate that out properly, because this rail-to-rail behaviour of the opamp is not properly characterised and hence will drift over time, temperature and other parameters. So the catch is, you need to have split power supply in order to get 0V properly matched with the ground. Similarly the ADC will have some issues at the bottom line as well. My concern is only to the proper reading near 0V level and that's the most important part of any measurement. I havent gone through PCB design yet, but thats another story and I am sure the person designed the PCB has taken care of proper grounding topology right for this precision work.

[@scullcomhobbyelectronics1702]

Thanks for the reminder Ian. I am still trying to get my head around what that code is doing! Also how it could be used in my existing code.

[@shadowland_86]

How to buy this item, i am very interested, give me buy link

[@edgarskarklins3572]

You use schematics, lookup the parts list and build it yourself silly 😅 That's the whole point

[@ufohunter3688]

Hi Louis, I like to build this one too :) Can you provide a bill of material? Specifically the switches, the ebay project box (love that red acrylic look and your mounting setup) and a link to the PCB. I have the rest and won't have to buy them :) You make the best projects and videos, but I guess you know that by now. Thanks.

[@ufohunter3688]

Just found the PCB on OSH park and they want $25 for 3. I only need one! I'll be using a perf board and wiring it myself. I was hoping to save time and not do that, but I can't see myself paying $25 +SH/H for it.

[@scullcomhobbyelectronics1702]

Hi, 1 x ABS Project Case (185x135x60mm) with Removable Plates KE112 (Light Grey with Red end panels) cost £6.55 plus (£3.70 delivery) from eBay seller “tippikat”. Direct link below: www.ebay.co.uk/itm/185x135x60MM-ABS-Project-Enclosure-Box-Case-Housing-with-Removable-Plates- KE112-/131573248544?var=430944212817&hash=item1ea2607220:g:nLkAAOSwu4BVokKL 1 x SCI R13-512B1 RED Latching Push Switch Button - cost £0.90 from Rapid Electronics Ltd. (order code 78-0233). Direct link: www.rapidonline.com/sci-r13-512b1-red-latching-push-switch-sm-button-red-78-0233 5 x Diptronics DTS-644K Square Button Through Hole 6 x 6mm Tactile Switch (Size 6 x 6 x 7.3mm) From Rapid Electronics Ltd. (order code 78-1150). Direct link below: www.rapidonline.com/diptronics-dts-644k-square-button-through-hole-6-x-6mm-tactile-switch-100gf-78- 1150 1 x Diptronics KTSC-62R Red Round Button for Tactile Switches (Order Code: 78-1162) 1 x Diptronics KTSC-62G Green Round Button for Tactile Switches (Order Code: 78-1178) 1 x Diptronics KTSC-62B Blue Round Button for Tactile Switches (Order Code: 78-1176) 1 x Diptronics KTSC-62Y Yellow Round Button for Tactile Switches (Order Code: 78-1175) 1 x Diptronics KTSC-62K Black Round Button for Tactile Switches (Order Code: 78-1160) www.rapidonline.com/Catalogue/Searches?query=Diptronics%20KTSC-62&filterSearchScope=1 You will find a full bill of materials in the download I gave which if you unzip you will find a full parts list. Direct link is below: www.scullcom.com/Millivoltmeter_MK2.zip Regards, Louis

[@scullcomhobbyelectronics1702]

Hi, Send me a message with your contact details in Private Messages and I will try to help you out with regards the PCB. Regards, Louis

[@ufohunter3688]

Oh thanks Louis, I kinda jumped the gun and ordered this case instead, before hearing back from you. A lot cheaper too as it turns out. 1/3 the price as it turns out. goo.gl/hnhhrv I ordered everything and now have to wait a YEAR for the delivery donkey to arrive from China :) That is, if the old donkey doesn't die in transit... I sent you my email address. Never used the private messaging on KZbin before and had to Google it like everything else. Google is my Chief Information Officer.

[@scullcomhobbyelectronics1702]

The case you have ordered is very good for this project I ordered and received the same case as you have ordered a few months back which I plan to use on another project. I will answer you private message.

[@dalwindersingh9455]

excellent video ,also plan to do Meggar (insulation resistance meter) video

[@scullcomhobbyelectronics1702]

Thanks for you comment. I may have a look at a Insulation Meter in the future. I am currently working on a new project.

[@boldford]

The second link seems to be broken.

[@scullcomhobbyelectronics1702]

Hi Brian, I just check it myself and it seems to work OK. You should get a box pot up after a few seconds with the option to download a zip file with all the other files in. Give it another try. Regards, Louis

[@boldford]

It's working fine now. Thank you.

[@mevlutcagdas8810]

grand master 12v mppt battery charger pls

[@radudum1]

Please make a video about speed control of 3 phase induction motor.Thx!

[@scullcomhobbyelectronics1702]

May be something for the future. I am busy at the moment working on another project.

[@seyedmarashi]

hello, how much is completed project? do you have one pcs for me to sell?

[@scullcomhobbyelectronics1702]

Hi Seyed, The cost of the project is about 60US$ if you have to purchase everything. The cost also varies depending on where you buy your components from. I do not have any PCB's but they are some available from OSH Park - one of my viewers Barbouri designed one for my project and had some PCB's made and has made them available on OSH Park: oshpark.com/shared_projects/qgv0fpKN Regards, Louis

[@seyedmarashi]

thanks alot, am i able to pay more and buy completed project with box and everything? it seems barbouri is only pcb printing Co. i need assembled and full working one and for payment no problem. thanks

[@DeeegerD]

If I was designing the circuit board for this project I most certainly would not have designed it using an Arduino module. Rather I would have just designed it using a Atmel ATMega328 chip - then you would avoid the pin out problems. Either SMD or THT would work fine (both are easy enough to solder).

[@scullcomhobbyelectronics1702]

Thanks for your comment. That is also an option, which ever you prefer to do. The cost of an Arduino Pro Mini these days is so cheap that they make a simple and easy solution for the hobbyist. Buying the Atmega328P chip, 16MHz crystal and capacitors can be the same cost as buying the completed Arduino Pro Mini board. Also you would need to install the bootloader.

[@boldford]

Pro Mini clones can be got for about £2.00 each.

[@donaldfilbert4832]

Excellent !!

[@pwversteeg]

I have made an addition to the firmware for my meter, by adding a logging capability. Have a look at my blog for the details: www.paulvdiyblogs.net/2016/09/building-6-digit-digital-milli-voltmeter.html

[@Aga-Bubamara]

Why don't you try make a video using a ICL7135C or TLC7135C integreted ADC and LCD driver IC ,using a precision Voltage ref... ICL7135C has a 5ppm TC

[@scullcomhobbyelectronics1702]

Thanks for your comment. I have looked at the ICL7135 in the past. However this IC only allows up to a 4½ digit display. I may use this IC in a future project on my channel though. Regards, Louis

[@Aga-Bubamara]

excellent .. looking forward to that video.. true 4.5 digit only but if TC kept low even 4.5 digit is excellent as long as it's stable at wide range of temp.. seen a guy used Crystal heater for radio oscillator.. it keeps temp at 40.8 degrees constant if put over the voltage ref IC even if you use a cheap 10ppm or worse Vol Ref it sis till irrelevant that 10ppm as the temp is kept at a constant temp without oscillation or drifts , so it keeps the reference voltage stable regardless of room temperature... maybe you may consider using it while making a video. it is a excellent stabiliser for voltage ref-s... and you no longer need to be concerned about higher ppm.. Voltage reference is kept at a constant stable temp.. no need to replay. I understand you may get a lot of comments.. and that may be pain sometime.. Great work, keep doing it...

[@pwversteeg]

Dear Louis and others, I have made some significant changes to the software that makes the meter more usable and accurate. Look here for details: www.paulvdiyblogs.net/2016/09/building-6-digit-digital-milli-voltmeter.html I am very impressed with the meter, thanks for sharing this design! Paul

[@scullcomhobbyelectronics1702]

Thanks Paul.

[@Edmorbus]

Thanks for sharing

[@scullcomhobbyelectronics1702]

Thanks.

[@tardate]

Thanks for this great project. It's been a fun diversion for a while as I breadboarded my own variations and finally got around to making a PCB and putting it an enclosure. And it works great! FWIW I've shared my build notes, code and board designs in my GitHub repo: github.com/tardate/LittleArduinoProjects/blob/master/Equipment/MilliVoltmeterDIY/CustomBoardAndEnclosure/README.md Now I'm looking forward to the next Scullcom project I'll build ... do keep them coming, they are greatly appreciated!

[@scullcomhobbyelectronics1702]

Thanks for your comment and the link to your build of this project. Looks like you have done a great job. I am currently working on a new function generator project. Regards, Louis

[@JonnyDeRico]

Very cool

[@Stinktierchen]

Nice project BUT... a huge BUT. Almost all of the parts there used. The Opamps the reference and so on. Are VERY hard to get to. Especially this CADDOCK 1778-C681 is almost impossible to get and it is super expensive. A great project and well designed. But it is almost impossible to build it for a normal price as the parts are too difficult to get.

[@lis0x90d]

What will be interesting to see is to how to protect/extract measurements from noise. Using 24bit ADC with such level of noise in scheme is ridiculous. It is sad to see how steady keithley measurements on back according to your device.

[@CT1JRZ]

Nice

[@scullcomhobbyelectronics1702]

Thanks.

[@Brian_Of_Melbourne]

Hi Louis, There are some interesting variations on the bargraph here www.carnetdumaker.net/articles/faire-une-barre-de-progression-avec-arduino-et-liquidcrystal/ particularly this one kzbin.info/www/bejne/gIKxcqKbg8ihl7c Food for thought. I would also suggest, for consideration, that rather then having a separate buttons for Cal and Cal-Reset, perhaps Cal could be a short press and Cal-Reset a long, at least four seconds, press. This prevents accidentally resetting the calibration and saves a button and I/O pin. Cheers, Brian.

[@scullcomhobbyelectronics1702]

Thanks Brian for your comment and links. I had a look at the bar graph variation. Although it gives a larger range I think it would be difficult to read as a representation in voltage at a glance. It may be suitable for some other applications though. I feel the best way to improve the range of the bar graph is to alter the bar graph range to fit the input voltage range - this is something that Fluke also do on their multimeters - doing it this way ensures it can be interpreted easily and quickly at a glance. Good point about the calibration buttons. I am sure this could be modified. Regards, Louis

[@philspargo9543]

Hi Louis! I just completed this version and calibrated it with the 5 volt reference. At 10 volts it is 2mv high and at 1 volt it is about 1mv low. How can I calibrate this error voltage ?

[@scullcomhobbyelectronics1702]

Hi Phil, One issue with using ADC's is their Integral Nonlinearity (INL) Error. This error is the deviation of the values on the actual transfer function from a straight line. In an ideal ADC the transfer line should be straight but due to Integral Nonlinearity Errors it is curved slightly. This would most likely account for your small errors above and below the 5 volt calibration reference. I think it may be possible to reduce this error in software which corrects for the Nonlinearity Errors. When I have more time I will try to have a look at how this can be done in software. If I find a good solution I will post it on my website for this project (www.scullcom.uk/category/projects/millivolt-meter/) Regards, Louis

[@eboy536]

Awesome 👌