Control a cheap multimeter via microcontroller GPIO Project or mod? - Page 1
Forum section: electronics / projects

author: NetDr
author_rank: Contributor
timestamp: February 13, 2023, 11:20:43 pm
content: A prototype I am working on needs a low cost, physically small, low power, embedded autoranging multimeter to measure, with self-protected inputs: AC/DC volts, amps up to 20A, ohms, capacitance, temperature, etc. on contacts to the outside world that I don’t control.I am considering modifying a Uni-T UT61D+ multimeter (about $65 new) by adding a microcontroller to control the UT61D+ to do the measurements.My plan is to program the low-cost microcontroller to:1. accept a SCPI command from the controlling PC via isolated serial/USB/LAN2. execute the command by:a) make, using microcontroller GPIO, the connections normally done using the UT61D+ function selector dial & buttonsb) read data from the UT61D+ serial/USB portc) process & format the datad) send the data to the controlling PC via the same isolated serial/USB/LAN.3. RepeatQuestions:A. Is this sane or is there a better plan?B. Is there a better supported, low cost, more programmable small multimeter that accepts SCPI or other control commands via RS232/USB/LAN with specs at least as good as the UT61D+?C. Does anyone know of an existing mod/project for controlling via SCPI a typical cheap multimeter via microcontroller GPIO?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 14, 2023, 12:54:39 am
content: I would probably not modify a DMM for this, but use a DMM with serial output, and then just add a stepper motor to the dial on the front, and maybe a few extra solenoids if some buttons have to be pushed. Disadvantage of that approach is that it is not very "physically small". Or is it acceptable to turn the dial manually? Even then, I would still start with a DMM that has a serial output, and then maybe connect a uC internally to it.Digilent used to have a DMM in "arduino footprint". I'm not sure, but it may have been discontinued, maybe there is old stock left somewhere.Another possibility is to use the Owon SDM1041 as a donor meter. It's a bit more expensive (around EUR130). The advantage of this is that it has One PCB with the DMM inside, and another with a uC that controls the display, buttons and I/O. This meter would need very little modification, just swap some connectors and only use the DMM print of it.If you want "small", you could start with a Mooshimeter. It's got an ADC with too much bits and it does "auto ranging" in software.------------------------------------------------------------------------------------Edit: Update.XDM1041 costs EUR110, and already has SCPI. It seems unlikely you will find a "less expensive" meter with SCPI.https://www.eleshop.nl/owon-xdm1041-multimeter.htmlI was tempted to buy one myself once, but I did not as it's auto ranging is quite slow and it has some other limitations that are simple unacceptable for a benchtop meter for me. So be sure to read some reviews to decide whether it's limitations are acceptable for you.

author: NetDr
author_rank: Contributor
timestamp: February 14, 2023, 01:55:11 am
content: Thanks for your input Doctorandus_PFrom your detailed reply it looks like have not yet made a large, obvious omission.Physical constraints cause the small, thin UT61D+ circuit board to be a close fit. There simply isn't room for an external stepper & solenoids. A small mezzanine pcb for a supervisor microcontroller does fit. I am quite comfortable adopting a microcontroller & its GPIOs to the DMM supervisor function.Before settling on the UT61D+ I did take a hard look at the Owon SDM1041/SDM1241 (battery powered). For this projectPros:It does have an implementation of SCPIMuch higher countsCons:Current measurement limited to 10ALarger PCB sizeThe separate power supply board is integrated to the DMM board. Not clear the DMM board will work without the power supply board connectedTriple the price (for the battery powered version) for the version that is most likely to work with only one board.I did not know about the Mooshimeter, Very interesting !Unfortunately, it too is limited to 10A measurement.Not sure it can be configured for wired operation.

author: james_s
author_rank: Super Contributor
timestamp: February 14, 2023, 02:05:37 am
content: If you want to modify a multimeter, I'd look for one that has buttons only instead of a dial. How much accuracy do you need? It may be easier to just measure directly with your microcontroller. If it's built into something then in theory you don't need all the fancy input protection and safety features offered by a real multimeter.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 14, 2023, 02:21:55 am
content: Why do you need to change V/A/ohm/capacity/etc measurements from SCPI? Those measurements need wiring changes. SCI is not enough, unless you have a controlled switch upfront.If you settle with wiring + selecting the measurement type manually, then auto ranging and reading out with the host then you’ll find several non-SCPI but USB or Bluetooth connected ones with simple opto cable.

author: NetDr
author_rank: Contributor
timestamp: February 14, 2023, 02:44:53 pm
content: You are correct dobsonr741, there are PC controlled relays between the DMM inputs and the various contacts to the outside world. I don’t control what will be connected to those contacts so self-protected inputs are required.These measurements will be required up to several times a minute every hour, 24/7, under PC control.The “per unit cost “of the final prototype is IMPORTANT.

author: NetDr
author_rank: Contributor
timestamp: February 14, 2023, 02:58:21 pm
content: I to like to see buttons when trying to automate electronics, james_sIn this case the "dial" on the DMM is really just a series of single buttons arranged in a circle. The "dial" control ensures 'break-before-make' and that only one button at a time can be pressed by the user.The accuracy of "6000 counts" is good enough for this application.The DMM guts are being "built into something" but the contacts are computer controlled relay connected to the outside world to contacts controlled by users. You know what can happen. :-)

author: james_s
author_rank: Super Contributor
timestamp: February 14, 2023, 06:09:18 pm
content: Quote from: NetDr on February 14, 2023, 02:58:21 pmAre you sure about that? I haven't taken apart a modern DMM but older ones had quite a complex array of contacts on the knob.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 14, 2023, 10:07:44 pm
content: If you go in the direction of adding relays, then you can add a custom shunt resistor to get any current range you want.But at some point it won't make much sense anymore to use an existing DMM as a donor, and it makes more sense to use an uC with external ADC or a chip designed for a DMM.

author: TimFox
author_rank: Super Contributor
timestamp: February 14, 2023, 11:34:20 pm
content: If you need volts and amps, would it be easier to use two cheap meters?

author: thm_w
author_rank: Super Contributor
timestamp: February 15, 2023, 12:34:39 am
content: Really odd requirements.Makes me think you either aren't giving us enough details, or you are misinterpreting a spec that was given to you.Or if this is your own idea, why not tell us what the idea is?

author: NetDr
author_rank: Contributor
timestamp: February 15, 2023, 03:00:17 am
content: I am not fully understanding your description of the DMM's dial contacts, jame_sLet's look at the DMM together.The outside view of the UT61E+ function dial is the 'dial' picture below.The inside of the switching dial is the white circular piece on the left side of the picture 'inside' showing the DMM insides.A close-up of the white dial copper switching contacts is shown in the 'white' picture. The springy plastic arms give the clicky feel when switching ranges.The copper contacts shown short the appropriate circular segment traces in the 'green' picture.I suspect for many DMM functions the appropriate traces are grounded or connected to Vcc by the shorting contacts of the dial.For each DMM dial position function I will determine which traces are grounded or connected to Vcc. Those same traces will be grounded or connected to Vcc by the added supervisory microcontroller GPIOs for each SCPI commanded function.There may be traces that are not simply connected to ground or Vcc. These traces may instead pass a signal from one trace to the next. I will have to examine those more closely for the correct supervisory microcontroller response.More details of the DMM's circuit board can be found in the very similar UT61E+ model DMM which is in a nicely done review video on YouTube. A “skip ahead” link to the “teardown” part of the review is here:https://youtu.be/m23jIF8ve6I?t=2071

author: NetDr
author_rank: Contributor
timestamp: February 15, 2023, 03:29:57 am
content: QuoteI agree, thm_wMy customer's intent is to add the ability to do remote circuit diagnostics on the internal guts of a large piece of outdoor farm equipment. The farms are huge, far away and a pain to get to - think an entire day or more of travel to get on site to do warranty repair work.If the failed or failing part can be identified remotely, a lot of time and money can be saved if I can make the idea work.

author: NetDr
author_rank: Contributor
timestamp: February 15, 2023, 03:39:43 am
content: Quote from: TimFox on February 14, 2023, 11:34:20 pmThe meter expense is from the meter's data, counts, and 20A capabilities. The one meter can do volts, amps and a lot more.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 15, 2023, 04:05:17 am
content: I think your complicate things a lot, just find an higher "specced meter" witch you know it will work with scpi commandsyou are venturing way too far playing with the dial etc ... put the dial on a stepped motor with position detectionyou want to do serious stuff sorry to say act seriously ? because a customer will depend of your project ?And all the time you will invest to make things work should be paid by your said client, in the end it would be better to buy another meter typeAnd for big currents, you should use an shunt resistor not the internal resistor of the meter for obvious safety reasonsExamplehttps://github.com/TheHWcave/OWON-XDM1041

author: inse
author_rank: Super Contributor
timestamp: February 15, 2023, 12:02:21 pm
content: If you go for a used Keithley 199 multimeter?It has GPIB interface and a scanner unit to connect eight different measurements.But it only measures V,A and Ohms.I recently bought one for €130.-Would that also be an option?

author: NetDr
author_rank: Contributor
timestamp: February 15, 2023, 12:43:59 pm
content: Thank you for your reply coromonadalixRelays, solenoids, and the Owon XDM1041 you referenced were suggested by Doctorandus_P in his reply above.In replies above I discuss in detail why each is inappropriate for this project.While these would certainly be possible solutions in another situation, they don't meet the requirements of this project.

author: NetDr
author_rank: Contributor
timestamp: February 15, 2023, 01:01:12 pm
content: Thank you for your suggestion, inseI had not thought of the Keithley 199 multimeter. Many interesting capabilities.Looking on eBay, used 199s have sold for over $150. this over double the cost of a new UT61D+ ($65)I did a quick search for a GPIB to USB adapters an found prices around $50 on eBay. GPIB to serial adapters were MUCH more expensive.The physical size of the 199 + GPIB-USB is too big to fit in the allocated space for the project.While the 199 would certainly be a possible solution in another situation, it doesn't meet the requirements of this project.

author: MadScientist
author_rank: Frequent Contributor
timestamp: February 15, 2023, 01:10:08 pm
content: I have the Owen. That has good ascii based serial comms to easy configure and acquire data via serial

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 15, 2023, 05:22:47 pm
content: Can you tell us more about what environment it’s expected to work? Temperature, open air/closed room, etc?

author: NetDr
author_rank: Contributor
timestamp: February 15, 2023, 05:56:58 pm
content: QuoteThanks for your interest obsonr741The DMM circuit will be an addition in an existing IP67 electronics box that already contains the necessary computer and electronic controls for a large piece of farm machinery. The box is exposed to environmental temperatures but shielded from direct sun, rain & snow. There is no air-conditioning but a fan assisted heat exchanger keeps temps down. A heating element keeps temps above 0 Centigrade.

author: SeanB
author_rank: Super Contributor
timestamp: February 15, 2023, 07:01:20 pm
content: Not possible to simply grab one of the DMM application notes, like the reverse engineered ones on here, and simply use the application notes to build your board. A few relays to provide the needed switching, and appropriate input protection, and you will have a small enough meter to fit onto a single card in the enclosure.

author: thm_w
author_rank: Super Contributor
timestamp: February 16, 2023, 12:01:24 am
content: Quote from: NetDr on February 15, 2023, 03:29:57 amSo someone is at the location, operating the equipment? Wouldn't they be capable of turning a meter to "Volts" and attaching it to various points on the machine?If they are not capable, then you are better off attaching various monitoring devices permanently, with no relays or configurable outputs.Current monitor, say battery current, with a clamp or shunt.This is not uncommon on high end industrial equipment. You can even log data over time to anticipate failures.Either with custom hardware, or off the shelf DAQs (ethernet, wifi, etc.).Having a black box that you remotely choose "Volts" and then tell a person to connect it somewhere is going to be a huge pain in the ass.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 17, 2023, 12:00:10 pm
content: not giving enough details doesn't help, and i think you want to protect your ideathinking cheap in this case will not help .... you are not taking the right pathwireless monitoring exist, long range wi-fi etc .... but it's not cheap, if it's an important equipment piece, you have to properly analyze the requirements / needs etc... to me it sound like partially done or not done well at allthat's why this thread may or will die and you'll have to ask elsewheremy 2 cents

author: NetDr
author_rank: Contributor
timestamp: February 17, 2023, 05:04:32 pm
content: QuoteThe "idea" is literally in the title & repeated in the original post."Control a cheap multimeter via microcontroller GPIO"Anywhere a multimeter might be used is a possible use case for a remotely controlled multimeter. Each and every specific case of a how a multimeter might be used is irrelevant.My intent when posting here on EEVblog/forum/project was to find out: Does anyone know of an existing Open Source project or mod to DIY an inexpensive remotely controlled multimeter?Unfortunately, almost every comment here ignored the "low cost" requirement in their suggestion. OF course this problem becomes trivial to solve if money and requirements are of no concern.It is clear from the many comments that none of the commenters knows of any such project or mod.That is the answer to my question.Now that I have some assurance no one has already published a DIY remotely controlled multimeter, the fun can begin. For me, the fun is the CHALLENGE of a useful "low cost" DIY remotely controlled multimeter.Controlling a cheap multimeter via microcontroller GPIO does not look complicated to me but I am sure I will encounter complications. One always does in DIY world. Since no such project already exists, my project can enlarge DIY knowledge when I publish here what it took to reach success.Thank you for your interest, coromonadalix

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 17, 2023, 05:16:26 pm
content: I thought you wanted to modify an of the shelf meter.If you want to build something yourself thenhttps://hackaday.io/project/165857-stm32-open-source-multimetercould be a starting point.

author: inse
author_rank: Super Contributor
timestamp: February 17, 2023, 05:55:14 pm
content: You can have it fast, reliable, cheap - pick two!

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 17, 2023, 06:31:23 pm
content: How many do you want to build? The “cheap” will be relative to that.Here is a design example - with schematics and Arduino library links. It’s no longer available, but the design is about what you need.https://digilent.com/reference/add-ons/dmm-shield/start

author: NetDr
author_rank: Contributor
timestamp: February 17, 2023, 10:29:53 pm
content: QuoteYes Doctorandus_P - you are 100% correct. I only want to modify an off the shelf meter.Many commenters here think it is easier to design, layout, manufacture, test, debug [repeat several times] -> a custom meter. I am certain this is an extremely problematic direction. The DIY multimeter projects I have casually looked at have taken months to get working. I will avoid all that development by simply repurposing an already designed, laid out, manufactured, tested, debugged, working, mass-produced inexpensive commercially available multimeter.

author: thm_w
author_rank: Super Contributor
timestamp: February 17, 2023, 11:28:24 pm
content: I don't think anyone is claiming it will be easier.The easiest option is to buy off the shelf industrial monitoring equipment (eg a USB DAQ) which already accomplish the task.But you keep on driving back about low cost. So... that is the response you'll get.How much does this farm equipment cost, $50k? 100k?