Author Topic: Open Source Multimeter  (Read 238450 times)

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Offline ivan747

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Re: Open Source Multimeter
« Reply #50 on: July 03, 2011, 04:37:46 pm »
This is about Dave's multimeter:

Add math features, so you can do calculations with ANYTHING, including calculations of calculations. Example:
Input power = Vi*Ii
Output power = Vo*Io
Power loss = Input Power - Output Power

Percentage of power lost = (I can't remember and I can't figure it out on my mind without some paper and a calculator)

It needs a good labeling system though.

For Bluetooth / Zigbee / whatever you can use a removable module to save some money on the initial cost.
 

Offline hans

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Re: Open Source Multimeter
« Reply #51 on: July 03, 2011, 04:52:12 pm »
If you power the thing from 4x 2500mAh AA batteries, in most ideal terms you can only draw 5mA @ 6V maximum to do a simple voltage measurement. So that's 30mW. It doesn't sound like a lot to me.
Of course this goes down FAST if you start to think about constant current sources, generators, quad channel, seperate temperature probes (probably requires seperate frontend), SD cards, dual screen (still draws more than a single screen!), etc.
I can imagine that isolation is quite hard to achieve that way. The hardware will get quite complex as in , analog frontend, display capability, automatic ranging, calibration, measurement stability (i.e. time/temperature drift or noise).

I think a quad channel accurate measurement device with all the protection circuitry is very hard to create, not even talking about batteries. Protection is a big issue because that kind of circuits use big components, which eats boardspace. Now, 4 fuses could be fitted into a device I'm sure, but you would need to have dual channel mob, power resistor and trace clearance taken into account. Powering the isolated circuit isn't an easy thing to do neither I can imagine.

I think it's better to have 2 devices with 1 V/I channel (that have seperated ground terminals) than 1 device with dual V/I channel. Possibly it would be nice to have a linking cable of some sort so you could link the multimeters together to have unique capabilities (you could transfer the data between devices to log multiple channels onto an SD card, or calculate effiency from figures taken on both meters). Isolating digital communication signals (RS485/RS232) is still a whole lot easier than designing an isolated DC/DC converter + digital isolation within 1 case.

Possibly , if you want a removable/external display module you could use RF to transport measurement data. This would be combined with the idea of having 2 meters linked together to do easy logging and efficiency calculations.

Another advantage with 2 meters is that you got 2 battery packs, like in you don't have to power all the features of quad channel from the same AA batteries.
« Last Edit: July 03, 2011, 04:54:55 pm by hans »
 

Offline Vertigo

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Re: Open Source Multimeter
« Reply #52 on: July 03, 2011, 06:28:48 pm »
This is about Dave's multimeter:

Add math features, so you can do calculations with ANYTHING, including calculations of calculations. Example:
Input power = Vi*Ii
Output power = Vo*Io
Power loss = Input Power - Output Power

Percentage of power lost = (I can't remember and I can't figure it out on my mind without some paper and a calculator)


now that sounds interesting :D
 

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Re: Open Source Multimeter
« Reply #53 on: July 03, 2011, 06:32:02 pm »
Make sure this doesn't have the fate of Segway.
 

Offline ivan747

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Re: Open Source Multimeter
« Reply #54 on: July 03, 2011, 07:13:10 pm »

1. A modern reflective or "transflective" graphic LCD would not consume much power with the backlight off, probably 10 times less than the isolation of a single input channel.

2. The range switch can be replaced with software controlled relays as in bench multimeters, but that would significantly increase the overall power consumption. A combination of opto-MOS and bistable electromagnetic relays would probably be acceptable.

3. When using batteries, it would be essential to prevent the unused channels from drawing any supply current.

1. Maybe a variable frequency DC-DC converter could be used to reduce quiescent current.
2. I would like a slider switch. It is unnecessary for most of the channels because there's only one multifunction channel and it doesn't share the thermocouple input and it is auto-ranging (at least that's what I expect). For current there must be range sliders.
3. The microcontroller will take care of this. The will be lots of isolation. The opto-isolators should be off most of the time. Also please get some DC-DC converters with control inputs, not that 5 pin SOT-23 junk that doesn't even have an output disable pin nor a undervoltage protection function for the input voltage.
 

Offline DavidJRobertson

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Re: Open Source Multimeter
« Reply #55 on: July 03, 2011, 07:48:20 pm »
This is about Dave's multimeter:

...

For Bluetooth / Zigbee / whatever you can use a removable module to save some money on the initial cost.

I'd recommend the RFM12B by HopeRF (http://www.hoperf.com/rf_fsk/cob/RFM12B.htm). They're low cost compared to ZigBee, at £6.17 individually from Farnell (UK), are available in smaller (Yet still easy to solder by hand; XBees are pretty large), more convenient SMD modules and, if I recall correctly, they consume less power. There's quite a lot of info about this module spread around this blog http://jeelabs.org/ (I'd recommend you take a read of this anyway, it's quite good).
 

Offline House91320

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Re: Open Source Multimeter
« Reply #56 on: July 03, 2011, 08:17:13 pm »
Cool vid Dave do you mind if use an develop on some of your ideas. Also do you know of a good place to get cheap lcds like the one in you micro watch but bigger.
« Last Edit: July 03, 2011, 08:21:21 pm by House91320 »
 

Offline EEVblog

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Re: Open Source Multimeter
« Reply #57 on: July 03, 2011, 10:35:04 pm »
Thinking about it, you don't really need 4 isolated channels - for the majority of cases you only need 2 isolated channels, each doing V & I, as it will usually be possible to arrange a common point (even if it means readings are negative)

There are problems with such a common ground terminal, as used on the Gossen. Watch my AA battery capacity tutorial for a practical demonstration of the problem.

Dave.
 

Offline nitro2k01

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Re: Open Source Multimeter
« Reply #58 on: July 03, 2011, 10:35:47 pm »
Here my criticism of Dave's ideas. The few parts that are not just whining and complaining, but actual innovative ideas are bolded.
Quote
4 isolated channels (2xV, 2xI)
So my initial thought on Dave's idea is, four isolated channel seems a bit overkill. As Hans pointed out, what you really need most of the time is one or two V/I pairs for measuring power. And it also seems like you'd get a hard time getting the isolation done properly in that small space, even with two channels. How do you expect to power the second channel? Having a transformer for the power transfer seems like asking for interference problems, due to the AC needed to transfer the power. USB is easy in comparison since USB brings its own power. All you need is a simple USB controller and two optocouplers.
Quote
Output jack for programmable current, function generator or power supply
While this might be tempting to add to the current jack, is it really a good idea? As far as I understand, it's generally a good idea to keep the current circuitry as simple as possible to minimize the risk of failure. Like, only a low resistance shunt and a fuse. Switching the shunt seems like asking for long-term reliability issues.
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SD card negates the need for USB...
Seems like a good idea at the surface, but does it really solve a problem? The destination is still the computer. You can do USB isolation on a whim. It also seems like the complexity of dealing with a memory card and file system would require a faster and more power-hungry microcontroller than you could get away with otherwise, unless you can live with slow transfer times. Taking the card in out of the unit is mechanically stressful to the card. Also, in terms of storage, do you really need that much space? It seems like a traditional interface (USB/IR) + something like a 1 MByte serial flash would do the job cheaper and better. The only time I could see you need that kind of logging space is if you're testing a lot of repetitive stuff, maybe on an assembly line or testing power/telephone stations in the field, or if you want to leave the thing on overnight probing something.
Quote
Bluetooth/Zigbee for wireless secondary display or PC/Phone interface
Again, bluetooth is a short distance protocol, and I doubt you'd get enough bang for the buck (power consumption in this case) compared to a more power-conservative wired interface. Then there's always the issue of interference. I could see the point for certain niche applications, and perhaps also for some people who want to show off, but for day-to-day work, I'm not too sure.
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Two 5-digit displays for V/A/W/AC etc. Three would be nice, but adds complexity
Right on. I don't see how having three displays adds very much to the complexity compared to two, though. You'd still need to make a custom LCD panel, anyway. The number of segments should be the smallest issue. (Perhaps steal the Metrawatt layout, BTW, with one big and two small displays?)
Quote
Low power monographic display for whatever. Soft buttons, graph, etc...
Sure. However, I'm wondering if this is worth the complexity, unless you can fit onto the same glass as the digital display. (Novel idea, BTW: Ditch the LCD and go for an e-ink display...)
Quote
Does decent LCR/ESR
Power display/power factor
Sure.
Quote
Voice output
Eww! No! It would just sound tinny and you won't be able to hear what the bloody thing is saying. And besides, by the time the meter is done reading the numbers, the values will have changed. It's simply not an efficient way of conveying information. If we're talking novel ideas, one idea could be to use a tone to indicate the displayed value. Let's say it starts at 1 kHz. If you get a load spike, the pitch of the tone will go up. It won't tell you the exact value, but it will tell you in a very direct way that something is happening, and approximately what is happening.
Quote
Two/three temperature inputs.
Let me put it this way, if this was a commercial product, and this was not a feature that you wanted to use, you would complain about it. Also, putting any measuring points on the top of the device seems like a bad idea for signal integrity, since that's where you typically have all the digital stuff.
Quote
4xAA, aim for 500+ hours.
I think you were the one who complained that 4xAA multimeters typically couldn't measure the forward voltage of blue/white LEDs in diode mode...
« Last Edit: July 03, 2011, 10:40:22 pm by nitro2k01 »
Whoa! How the hell did Dave know that Bob is my uncle? Amazing!
 

Offline EEVblog

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Re: Open Source Multimeter
« Reply #59 on: July 03, 2011, 10:53:33 pm »
Ideas like the tones etc are a given. I deliberately did not go into detail on those types of features that are nothing more than simple software additions. It's a top level idea based around a floating multi channel concept.

Re. the number of common channels. As I said, go watch my AA battery capacity video where I demonstrate a real problem with the single ground System used on the Gossen Energy meter. Floating V and I inputs would fix that, or at least the jacks to allow kelvin measurement.

The AA's have nothing to do with diode test voltage. Most meters have the problem, yes, but only because the designers are lazy. You simply design in an appropriate converter to fix that.
The meter could be powered from a single AA and still have any test voltage you wanted.

Dave.
 

Offline House91320

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Re: Open Source Multimeter
« Reply #60 on: July 03, 2011, 10:56:09 pm »
Here is an early block diagram. also thinking of having little wireless bugs that you can measure voltage or current with an they would all transmit to the meter so you could petulantly read many different values at the same time wiresly.
« Last Edit: July 03, 2011, 10:59:40 pm by House91320 »
 

Offline nitro2k01

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Re: Open Source Multimeter
« Reply #61 on: July 03, 2011, 11:23:06 pm »
Re. the number of common channels. As I said, go watch my AA battery capacity video where I demonstrate a real problem with the single ground System used on the Gossen Energy meter. Floating V and I inputs would fix that, or at least the jacks to allow kelvin measurement.
I see your point, however you don't really need true galvanic isolation in that case. You just need current measurement plus a two-terminal differential input with neither of the inputs shorted to ground. Of course that's just bickering about the exact definition of isolated more than anything else.
The AA's have nothing to do with diode test voltage. Most meters have the problem, yes, but only because the designers are lazy. You simply design in an appropriate converter to fix that.
The meter could be powered from a single AA and still have any test voltage you wanted.
Of course you can boost the voltage all you want, but then you need to consider things like the power consumption of the converter itself, and the noise it introduces. That's just design decisions, but I suspect that it would be easier to use a 9V battery.
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Offline DavidJRobertson

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Re: Open Source Multimeter
« Reply #62 on: July 03, 2011, 11:27:27 pm »
Ugh. 9V PP3 batteries are awful...
 

Offline zunklimt

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Re: Open Source Multimeter
« Reply #63 on: July 04, 2011, 12:00:06 am »
Hello to all !

I'm happy to finally join this forum after a long time just watching all the very fine Dave's videos

I love the idea of an open source multimeter !

So I've made a little drawing with how I see the think

It's still just a first draft, there are lots of details to be adjusted/corrected but here it is by now:

- ONE SINGLE BIG SCREEN (E-INK or OLED)
- USB or WIFI or BLUETHOTH to configure the meter from a computer and for realtime readings on a PC
- NO VOICE (numbers are universal, a voice solution would require to think about what languages to use)
- USER CONFIGURABLE SCREEN DESIGN AND FUNCTIONALITIES

i've included two versions of the idea: one with the connection at the bottom like standard, another with the connection over the screen to keep all the cables away when using on a flat surface (though if used vertically, the classic disposition is better)

i hope you like it

ZundKlimt


« Last Edit: July 04, 2011, 12:05:54 am by zunklimt »
 

Offline EEVblog

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Re: Open Source Multimeter
« Reply #64 on: July 04, 2011, 12:07:15 am »
Of course you can boost the voltage all you want, but then you need to consider things like the power consumption of the converter itself, and the noise it introduces. That's just design decisions, but I suspect that it would be easier to use a 9V battery.

The converter would only switch on when required, so power consumption is not an issue.
Noise, well, you design it so it doesn't interfere  ;D

A 9V battery would be suicide for a meter of this complexity. Not nearly enough capacity. The AA's will struggle, power management would have to be aggressive.

Dave.
 

Offline zunklimt

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Re: Open Source Multimeter
« Reply #65 on: July 04, 2011, 01:46:09 am »
here is a second version of my drawing, this time i separated the connections from the controls and screen into two separated type of boxes

All the necessary electronics and protection circuits are located inside the small CONNECTION BOXES which should be as flat as possible.


Each connector can be freely configured as an iput or as an output and several readings can be made simultaneously at each connection.
(of course, here i don't take in consideration how possible or not this would be in terms of electronic circuitry, this is just an idealized wish...

INPUTS: V, mA, Watt, Capacitance, Freq, Temp, Etc...
OUTPUTS; dcV+mA or WAVEFORM+FREQ+AMPLITUDE or TRIGGER or Etc...

All the data is transmited via WIFI or USB between the CONNECTION BOXES and a dedicated CONTROL UNIT (including a big E-INK/OLED screen and soft buttons) or to a SMARTPHONE or to a COMPUTER

several CONNECTION BOXES could be used with one single computer to get more channels of I/Os

with this equipment there would not be much diference between a multimeter, an osciloscope, a function generator or a power supply... this would be all in one ...

well, i'll keep dreaming...



« Last Edit: July 04, 2011, 01:47:42 am by zunklimt »
 

Offline s_lannan

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Re: Open Source Multimeter
« Reply #66 on: July 04, 2011, 02:22:26 am »
put a capacitor ESR meter in it...
 

Offline Tony R

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Re: Open Source Multimeter
« Reply #67 on: July 04, 2011, 02:34:19 am »
Ok - The first couple pages of this thread makes me a little upset to see people are thinking an open source project must be cost effective. To cite the most well known and well used open source project out there, Linus Torvalds did not want to make his operating system profitable or economical, he just wanted an OS that worked the way HE wanted it to. Who says it needs to be a competitor, that's one thing I don't like about the engineering type (Which, at times, includes myself.) Linus Torvalds had no idea his OS would run on many many servers and computers, nor did he know it would be the OS of 459 of the top 500 super computers in late 2010 (which is over 90%.)

My point is do not end this now, because who knows what it will become! Engineers tend to think "Is it worth doing? can I get the same thing somewhere else?" Yes, there are 1000s of different kinds of multimeter for all different markets. I can not argue with the fact that yes, it will cost more to make one then it will be to buy one. But Linus could have just got an OS and ran it instead of creating his own OS UNIX would have worked fine for him.

With all that said, here is what i think the multimeter should be sure to have...
*Rechargeable battery is nice
*Data Logging (sd card)
*A alarm - say voltage gets over or under a center value have it make a noise
*Screw USB, Bluetooth would be nice, you can also avoid isolation this way.
*I like the idea of having the standard thermocouple input, maybe have 2 (like dave came up with)
*Of course a diode tester, one that can go beyond 3V
*LSR meter
*High Voltage sound, make a noise when the voltage goes above 90V (or whatever is though necessary)
*I would like computer software that makes it easy to interface with MatLab
*I would also like to see a DMM with no rotary switch, so if it could be all push buttons i think that would be a unique design.

Tony R.
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Offline EEVblog

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Re: Open Source Multimeter
« Reply #68 on: July 04, 2011, 03:06:12 am »
Ok - The first couple pages of this thread makes me a little upset to see people are thinking an open source project must be cost effective. To cite the most well known and well used open source project out there, Linus Torvalds did not want to make his operating system profitable or economical, he just wanted an OS that worked the way HE wanted it to. Who says it needs to be a competitor, that's one thing I don't like about the engineering type (Which, at times, includes myself.) Linus Torvalds had no idea his OS would run on many many servers and computers, nor did he know it would be the OS of 459 of the top 500 super computers in late 2010 (which is over 90%.)

Linus Torvalds never made hardware. He did not have to care about how much something cost, or how expensive or difficult it was to manufacture, or if you could get the parts etc.

Cost must be considered in OSHW projects or else they the concept will fail because no one will ever build anything.
The goal of OSHW projects is usually to encourage other people to build one too, and that often means custom parts like PCB's, cases etc must be made readily available. That won't happen if it's not cost effective.
If your goal is to produce one unit, and not have anyone else ever build one, and just share the design info, then fine, but that's usually not the reason people do major OSHW projects like this.

Dave.
 

Offline Zad

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Re: Open Source Multimeter
« Reply #69 on: July 04, 2011, 03:54:15 am »
Okay. Well, my first reaction is: Nope, not gonna happen. Seen it a thousand times before, mostly in my own notebooks. A product with lots of enthusiasm, but some huge flaws.

Sanity check!

Try and get a ballpark quote for the custom enclosure. Chances are that it is going to cost more than the price of the house you live in, and certainly more than the cost of the electronics. I seem to remember that tooling for the TV-B-Gone was well over $10,000, and that is a pretty simple and small case. While you are at it, try and get a quote for custom  LCDs. Now realise how attractive those dot matrix greyscale trans/flective LCDs look. E-paper is currently way too hard to get hold of and has an awfully slow update rate. OLED looks sexy, but power consumption is going to be a killer in this application.

So, having got the grumpy realities out of the way, what next? Well, the engineers most familiar enemy is among us: Creeping Featurage. That temptation to add more and more and more stuff, just because you can. Still, nobody is going to buy a standard A-V-O meter for $200 when they can buy one which is better for $50, so it needs extra features.

Personally, I would forget about it being a hand held multimeter and go for a bench meter. Appropriate enclosures are much easier to buy (and fabricate) for bench equipment. If the problem you are dealing with is *that* difficult that it requires such a fancy meter, then you are going to be at your bench, not in the field.  To get even half of the features mentioned into a hand portable case is going to be impossible, even for commercial production density.

Having 4 channels alone is going to need considerable space, and to retain isolation it will need quite meaty relays for some range selection. 4 properly isolated channels with good resolution in a small space isn't easy. If you are still determined to go hand-held then consider slim high-density mobile phone batteries as an alternative to 4xAA cells. Mini-USB connection would be ideal for charging, as this would allow the use of any modern mobile phone charger. Galvanic isolation is quite easy.

Why not use the standard channels in voltage configuration for temperature sensing?

Using an Amps/Volts input socket for power supply / signal generation is going to run the risk of connecting it to a live mains potential (if inadvertently selected). I would feel nervous with 4 potentially mains level (or even low voltage 10A) connections physically so close to each other on a hand held device.  Maybe have a secondary 2 channel V/A measurement remote headless slave unit?

I would have 3 additional (or 4 if we are going for Kelvin sensing $$$) terminals for component testing, which also function as a signal generator (easy to do with a cheap DDS+buffer) and power supply. This physically separates it from the V/A, primarily for safety reasons, but it also disconnects a lot of hardware from the LCR sensing system. You want as little capacitance and inductance on the LCR measurement terminal as possible if you are going to get reasonable resolution and range. Yes, this would remove resistance sensing from the standard measurement terminals but increases precision considerably. Maybe add a separate low resolution resistance measurement? In my experience you rarely need to do precision component characterisation when you are doing mostly V/A work.

SD-Card. I think having an SD card socket in the case is a BAD idea (so bad that I put it in capitals). Even when populated, that slot is going to be an entry point for dust, moisture, bits of solder, you name it. If it were a commercial design, I would make it accessible under the battery cover. As an Open Source design intended for 1-off and small runs, I would mount it on the main PCB.

I don't think I would trust touch-buttons to have sufficient isolation for mains measurement. Even if the ADC modules are correctly isolated (and let us not for get this is open source, people may cut corners!) then something in my caveman survival instincts tells me not to touch even insulated metal pads. Conventional tactile pushbuttons are much more reassuring but still need ingress prevention via a shaped neoprene (or similar) button mat. A bench meter would have panel space for a rotary input device and 2 separate graphical displays.

Sound generation is easy on a controller like a Cortex M3 with 512MB of onboard flash. If speech isn't used, audible cues are still very handy. With regard to remote access, I would prefer WiFi to Bluetooth / ZigBee. That way, it is possible to use your mobile phone or ipod touch etc to view web pages generated on the fly, without and application software. With an appropriate router you could (e.g.) monitor experiments at work from home, on holiday etc.

So, my advice would be that if you want it all-singing all-dancing, go with a bench meter, with more room for isolated high-res low-noise ADCs in multiple channels. Power isn't a problem then, and you can use colour LCD panels. Measurement of mains level voltages and high currents can be done with remote head units.

If you go with hand-held, then stick to 2 (isolated) channels with what is effectively a separate LCR meter and component analyser (2-terminal diode, 3-terminal transistor and regulator). Design an external display-less 2-channel unit for measuring 2nd and subsequent ports.

One more thing. How about proper low-current measurement with a transimpedance amplifier? I think you could make a very average meter, but with  zero-burden microcurrent measurement you would have a product with a distinct Unique Selling Point, which is what marketers love!


Offline EEVblog

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Re: Open Source Multimeter
« Reply #70 on: July 04, 2011, 04:33:52 am »
Okay. Well, my first reaction is: Nope, not gonna happen. Seen it a thousand times before, mostly in my own notebooks. A product with lots of enthusiasm, but some huge flaws.

I agree. Doesn't mean it's not fun to toss around ideas  ;D

Quote
Try and get a ballpark quote for the custom enclosure. Chances are that it is going to cost more than the price of the house you live in, and certainly more than the cost of the electronics. I seem to remember that tooling for the TV-B-Gone was well over $10,000

IIRC he paid $30K up front before the first unit rolled off the production line.
And that's a project with an 8 pin PIC, a battery, one switch, and an IR LED.
He went nuts though, could have done it for MUCH less than that.

Quote
Personally, I would forget about it being a hand held multimeter and go for a bench meter. Appropriate enclosures are much easier to buy (and fabricate) for bench equipment.

I agree. A bench meter with an off-the-shelf enclosure stand much better chance of success. There is a thread on that for the open hardware bench meter some time back.

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Why not use the standard channels in voltage configuration for temperature sensing?

Simple. Then you lose the voltage channels for logging. The whole idea is being able to log voltages, currents, and temp at the same time.

Quote
SD-Card. I think having an SD card socket in the case is a BAD idea (so bad that I put it in capitals). Even when populated, that slot is going to be an entry point for dust, moisture, bits of solder, you name it. If it were a commercial design, I would make it accessible under the battery cover. As an Open Source design intended for 1-off and small runs, I would mount it on the main PCB.

Yes, it needs finer details sorted. There are several ways to make it work. So if you want an SD card in the meter, you can make it happen.

Quote
One more thing. How about proper low-current measurement with a transimpedance amplifier? I think you could make a very average meter, but with  zero-burden microcurrent measurement you would have a product with a distinct Unique Selling Point, which is what marketers love!

Yes, that's a given I would have thought.

Dave.
 

Offline Bored@Work

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Re: Open Source Multimeter
« Reply #71 on: July 04, 2011, 06:19:09 am »
Ok - The first couple pages of this thread makes me a little upset to see people are thinking an open source project must be cost effective.

You conveniently forgot to mention that the one asking the original question mentioned this is for a kickstarter project. If you don't know kickstarter, it is a web site where people are collecting funding for their projects. This project is right from the start business, not hobby. He just chose to make the project more interesting by labeling it open source.
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Offline ejeffrey

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Re: Open Source Multimeter
« Reply #72 on: July 04, 2011, 07:46:41 am »
Hmm.... it just occurred to me that there is a potential issue with having the amps channel isolated from the voltage channel.  It makes AC power considerably more complicated.  To measure AC power you need to multiply the instantaneous voltage and current.  In a non-isolated power meter you can use an analog multiplier followed by a standard sigma-delta or dual-slope ADC.  The alternative is to digitize the I and V waveforms at high speed and  multiply in software.  This costs a bit more power both for the ADC and the CPU, but is still easy to do by using a dual-channel ADC so the I and V are automatically synchronized.  The advantage of this is that you only need 2 ADCs and you can get everything in software: DC+AC volts  / DC+AC amps / power / apparent power / crest factor / phase lag.  With the analog multiplier, you need 3 ADCs and you don't get crest factor, phase angle, or separate DC+AC components without extra hardware.

With true isolation you either need to transfer the analog signal over the isolation barrier or have a common (opto-isoated) clock and transmit the 'high bandwidth' digital signal over the link.  I would probably go the first route -- the current input module would have only the range switching logic and a linear opto-coupler.  The voltage measurement module would have ADCs for both channels.  I would still go with the high speed digitizing approach and compute RMS and power in software because it is more flexible and powerful and you can display the waveforms if you want.
 

Offline FreeThinker

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Re: Open Source Multimeter
« Reply #73 on: July 04, 2011, 08:44:40 am »
Ok - The first couple pages of this thread makes me a little upset to see people are thinking an open source project must be cost effective. To cite the most well known and well used open source project out there, Linus Torvalds did not want to make his operating system profitable or economical, he just wanted an OS that worked the way HE wanted it to. Who says it needs to be a competitor, that's one thing I don't like about the engineering type (Which, at times, includes myself.) Linus Torvalds had no idea his OS would run on many many servers and computers, nor did he know it would be the OS of 459 of the top 500 super computers in late 2010 (which is over 90%.)

My point is do not end this now, because who knows what it will become! Engineers tend to think "Is it worth doing? can I get the same thing somewhere else?" Yes, there are 1000s of different kinds of multimeter for all different markets. I can not argue with the fact that yes, it will cost more to make one then it will be to buy one. But Linus could have just got an OS and ran it instead of creating his own OS UNIX would have worked fine for him.

With all that said, here is what i think the multimeter should be sure to have...
*Rechargeable battery is nice
*Data Logging (sd card)
*A alarm - say voltage gets over or under a center value have it make a noise
*Screw USB, Bluetooth would be nice, you can also avoid isolation this way.
*I like the idea of having the standard thermocouple input, maybe have 2 (like dave came up with)
*Of course a diode tester, one that can go beyond 3V
*LSR meter
*High Voltage sound, make a noise when the voltage goes above 90V (or whatever is though necessary)
*I would like computer software that makes it easy to interface with MatLab
*I would also like to see a DMM with no rotary switch, so if it could be all push buttons i think that would be a unique design.
Didn't the OP say it was a Kickstarter project? By definition this means it MUST be cost effective, otherwise who is going to pony up for it?
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Offline mikeselectricstuff

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Re: Open Source Multimeter
« Reply #74 on: July 04, 2011, 10:40:26 am »
Didn't the OP say it was a Kickstarter project? By definition this means it MUST be cost effective, otherwise who is going to pony up for it?
That assumes the project creators have a realistic idea of costs.
I always wondered what happens with a kickstarter when the money raised ends up not covering costs due to unforseen issues...
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