Author Topic: 7-Segment 4.5 Digit Autoranging Multimeter Project  (Read 3027 times)

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Offline sean0118

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7-Segment 4.5 Digit Autoranging Multimeter Project
« on: October 29, 2016, 02:28:42 am »
Hi everyone  ;)

I thought I would share a 7-Segment bench top multimeter project I'm working on. It's still very much a work in progress, I'm not even sure it will work yet.

It's based around the Intersil ICL7135 with a CD74HC194 4-bit shift register to add autoranging ability. I'll add a proper theory of operation (assuming it works) once it's built.  ;)

Any feedback is welcome. I'll post all the updated schematics and PCB files as the project progresses.
 

Offline ebclr

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #1 on: October 29, 2016, 03:07:42 pm »
This ic will make your project easy

http://www.cyrustek.com.tw/spec/ES5128.pdf
 

Offline rob040

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #2 on: November 29, 2019, 08:09:55 pm »
Hi Sean,

I came up with the same idea, building a 4,5 digit DMM. Found this thread while searching for info on the ICL7135.
I was wondering, did you ever finish the project successfully?

BR, Rob.
 

Offline sean0118

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #3 on: December 14, 2019, 07:08:40 am »
Hey Rob,

I sort of finished it, it's here somewhere...

It seemed to work but I never calibrated it. I was also going to build a lab power supply and use it as the display, but never did.

I'm going to build a voltage reference for calibrating multimeters soon, so I might get back into this project then.

Let me know if you do need any help.    ;)
 

Offline coromonadalix

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #4 on: December 14, 2019, 04:18:25 pm »
Dont want to break the fun,  but on ebay you have 4.5 digits  led panels at 20$ usd ??  icl7135 based ??  bought 2 of them to put in one of my psu.

https://www.ebay.com/itm/4-1-2-Digital-DC2V-Voltage-Panel-Meter-Voltmeter/391667118739

https://www.ebay.com/itm/4-1-2-Digital-DC2V-Voltage-Panel-Meter-Voltmeter/172397129492

Just tweak your input voltage divider ...
« Last Edit: December 14, 2019, 04:22:03 pm by coromonadalix »
 

Online Kleinstein

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #5 on: December 14, 2019, 04:28:27 pm »
I don't think modern panel-meters use the ICL7135 any more. It is more like a small cheap sigma delta ADC (e.g. MCP3421: 18 Bit ADC with reasonable accuracy reference) and a µC to control the digits.  A DIY DMM solution could use a similar setup. The concept is different in that scale factor adjustment is done in software and no longer with a pot for every range.
 

Offline not1xor1

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #6 on: December 14, 2019, 04:48:24 pm »
I don't think modern panel-meters use the ICL7135 any more. It is more like a small cheap sigma delta ADC (e.g. MCP3421: 18 Bit ADC with reasonable accuracy reference) and a µC to control the digits.  A DIY DMM solution could use a similar setup. The concept is different in that scale factor adjustment is done in software and no longer with a pot for every range.

The ones linked by coromonadalix do look like are based on icl7135 or something similar. The ones you're referring to are much cheaper, usually around 4-5 euros, and do not accept negative input voltages.
 

Offline imo

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #7 on: December 14, 2019, 06:19:08 pm »
An example of an analog front end for MCP/ADS/LTC 16-22bit ADCs..
Input 10Meg and -20..20V. You may add over-voltage protection with BAV199.
The first opamp - for example an OPA388 or ADA4528. The second opamp could be any 5V single supply one.
R17 simulates internal impedance of the ADC (not used).
« Last Edit: December 14, 2019, 06:39:47 pm by imo »
 

Offline wraper

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #8 on: December 14, 2019, 06:22:05 pm »
I don't think modern panel-meters use the ICL7135 any more. It is more like a small cheap sigma delta ADC (e.g. MCP3421: 18 Bit ADC with reasonable accuracy reference) and a µC to control the digits.  A DIY DMM solution could use a similar setup. The concept is different in that scale factor adjustment is done in software and no longer with a pot for every range.

The ones linked by coromonadalix do look like are based on icl7135 or something similar. The ones you're referring to are much cheaper, usually around 4-5 euros, and do not accept negative input voltages.
Those cheap panel meters are usually based on some cheap MCU.
 

Online Kleinstein

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #9 on: December 14, 2019, 06:59:36 pm »
Some of the cheap panel meters are known to use MCP3421 and an STM8 type CPU. Newer ones could use an even cheaper Chinese CPU.
 

Offline ormaaj

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #10 on: December 14, 2019, 07:08:33 pm »
What's the basic circuit for the auto-ranging anyhow? I'd suppose a window comparitor sets the range boundaries relative to a reference defined via PGA. Can't say as I've seen an example.
 

Online Kleinstein

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #11 on: December 14, 2019, 07:31:17 pm »
For the cheap DMMs the normal circuit is more like an inverting amplifier with switchable gain. So a 10 M resistor to the input and a switchable resistor like 1 K / 10 K / .../ 1M (maybe 10 M) as feedback. This way one can get away with all low voltage CMOS switches.

The control can be with comparators (one for range up and one for possible range down) or a small µC. Sometimes the comparator of some kind is part of the ADC. Often the whole function is integrated in a special DMM chip.

For more bench top DMMs the alternative circuit is with a divider and relays to select the divider, possibly combined with a PGA to select gain and maybe attenuation before the ADC. Some modern SD ADCs include a PGA like function that can be used. If the amplifier can work to a relatively high voltage like 10 V a single divider setting (e.g. 100:1) and thus 1 relay can be sufficient.
 
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Offline coromonadalix

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #12 on: December 15, 2019, 05:08:46 am »
The 2x  panels i have  are indeed  7135 based,  i could see the main ic behind the panel .... not sure the 2 Ebay links are the same,  but i think so.

For the range selection  you had to change a resistor value  for the input voltage divider,  they were setup for 2 vdc operation,  changed one for 199.99 volts  and the other stayed at 2 volts

My Psu  was outputting an voltage in ratio of the output current, i did not needed an power shunt ... just  "trimmed" the displayed value to fit.

And for theses 2 panels they have independent supply rails /  floating supply ...

Other links
Current meter
https://www.chinahao.com/product/39066261938/

Voltage meter
https://www.chinahao.com/product/549555539676/





Oh  and an dual icl7135 display rs232  project
http://www.kswichit.com/DVM7135/DVM7135.htm
« Last Edit: December 15, 2019, 07:41:06 am by coromonadalix »
 

Offline floobydust

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #13 on: December 15, 2019, 08:39:16 am »
I wish there was an LED multimeter, I bought a used 4-1/2 digit bench meter with ICL7135 and it's easy to read. No glare, no squinting.
But it's old, I hit it with 10kV by accident and realized it has no input protection aside from the arc taking out a digit driver transistor  :palm:

I wanted to take a HY3131 and have an MCU drive a LED display and get laughed at by the interweb  ;)
 

Offline tooki

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #14 on: December 15, 2019, 10:38:15 am »
I hear you. LED displays are superb for indoor use. One of my forever-unfinished projects is to replace the LCD in my Keithley 197 with a new display, either 7-segment LED, dot matrix LED, or a graphical backlit LCD or OLED (there just aren’t that many graphical displays with the right aspect ratio and size). There’s another thread on the forum where someone wrote the arduino code to convert the display signals back into actual number strings, which then can be displayed on whatever.
 

Offline imo

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #15 on: December 15, 2019, 12:50:18 pm »
I would not mess with those DMM chips today, as Kleinstein wrote. A sigma delta 16-24bit ADC (MCP/LTC/ADS), a simple front end, a small cheapo MCU, and an 8digits 7segment LED module (for a $1-2 off ebay) and you get a decent voltmeter.
« Last Edit: December 15, 2019, 12:54:07 pm by imo »
 

Offline coromonadalix

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #16 on: December 15, 2019, 02:02:09 pm »
You have on Ebay theses meters, Instek GDM-8245

Not perfect in every specs, can be bought for cheap ??  Haven't seen a teardown / insides pictures :(  nor seen a schematic ?

 

Offline floobydust

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #17 on: December 15, 2019, 08:40:40 pm »
But it's a multimeter thread, 4.5-5 digit voltmeters have been conquered for less than the price of an ICL7135 alone:
https://www.eevblog.com/forum/projects/$5-voltmeter-with-5-digit-(0-1mv)-resolution/
https://www.eevblog.com/forum/testgear/are-sub-$10-5-digit-ammeters-worth-buying/msg1623889/#msg1623889/


The old LED bench multimeters have no input protection at all  :palm:  like GW Instek GDM-8045, GDM-8145, GDM-8135, Wavetek, Amprobe too. It seems to be one manufacturer in Taiwan that did the boards and the design is 1980's.
They die fairly easy and a new ADC or AC RMS IC almost costs more than the meter is worth.

I can't find Hycon HY3131 for sale anywhere, didn't want to buy a tray of them, so I never moved on the LED multimeter project.
 

Offline schmitt trigger

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #18 on: December 15, 2019, 09:27:32 pm »
I have also toyed with the idea of designing and building such a meter.

I understand that the cost will be higher and the performance perhaps lower than commercial units.

But it is a learning challenge, it may prove to be a lot of fun, and another circuit challenge conquered.
« Last Edit: December 15, 2019, 09:29:09 pm by schmitt trigger »
 

Online Kleinstein

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #19 on: December 15, 2019, 09:46:35 pm »
The critical parts for performance and also the costs today is not so much the ADC (the ICL7135 and 18-24 bit SD ADCs are cheap  today).
Expensive parts can be current shunts and a good input divider. At the 4.5 digit level the reference can also start to cost some money and require some thoughts. If it is just for learning, one could start with lower grade parts (e.g. higher TC resistors) and do the good parts only when everything is complete. I would even consider to top out at some 100 V, so one is not temped to use the DIY meter with mains.

For the current ranges auto ranging may not be that practical. It is doable but could also be confusing as a changing shunt value can effect the circuit.

Even the old meters usually had some protection in the form of a series resistor at the input. However the protection for the Ohms modes were often not that good and still is with some meters.
 

Offline rob040

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Re: 7-Segment 4.5 Digit Autoranging Multimeter Project
« Reply #20 on: October 25, 2020, 12:36:50 pm »
Hey Rob,

I sort of finished it, it's here somewhere...

It seemed to work but I never calibrated it. I was also going to build a lab power supply and use it as the display, but never did.

I'm going to build a voltage reference for calibrating multimeters soon, so I might get back into this project then.

Let me know if you do need any help.    ;)

Hi Sean,

I am very sorry for the late reply. Didn't get a notification and forgot totally about it.
For the same reason as mentioned by others, I stopped it. Looking for parts I found out that it's much cheaper to buy a secondhand DMM and I have enough projects in mind to realize and finish.
I considered to search for a defective one, re-use some components (like the divider resistors) and build one with the ICL7135, but also that idea didn't reach the finish. 8)
Thanks for your reply!  :-+

Rob.
 


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