7-Segment 4.5 Digit Autoranging Multimeter Project - Page 1
Forum section: electronics / projects

author: sean0118
author_rank: Frequent Contributor
timestamp: October 29, 2016, 02:28:42 am
content: Hi everyoneI 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.

author: ebclr
author_rank: Super Contributor
timestamp: October 29, 2016, 03:07:42 pm
content: This ic will make your project easyhttp://www.cyrustek.com.tw/spec/ES5128.pdf

author: rob040
author_rank: Contributor
timestamp: November 29, 2019, 08:09:55 pm
content: 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.

author: sean0118
author_rank: Frequent Contributor
timestamp: December 14, 2019, 07:08:40 am
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 14, 2019, 04:18:25 pm
content: 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/391667118739https://www.ebay.com/itm/4-1-2-Digital-DC2V-Voltage-Panel-Meter-Voltmeter/172397129492Just tweak your input voltage divider ...

author: Kleinstein
author_rank: Super Contributor
timestamp: December 14, 2019, 04:28:27 pm
content: 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.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2019, 04:48:24 pm
content: Quote from: Kleinstein on December 14, 2019, 04:28:27 pmThe 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.

author: iMo
author_rank: Super Contributor
timestamp: December 14, 2019, 06:19:08 pm
content: 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).

author: wraper
author_rank: Unknown
timestamp: December 14, 2019, 06:22:05 pm
content: Quote from: not1xor1 on December 14, 2019, 04:48:24 pmThose cheap panel meters are usually based on some cheap MCU.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 14, 2019, 06:59:36 pm
content: 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.

author: ormaaj
author_rank: Contributor
timestamp: December 14, 2019, 07:08:33 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 14, 2019, 07:31:17 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 15, 2019, 05:08:46 am
content: 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 voltsMy 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 linksCurrent meterhttps://www.chinahao.com/product/39066261938/Voltage meterhttps://www.chinahao.com/product/549555539676/Oh and an dual icl7135 display rs232 projecthttp://www.kswichit.com/DVM7135/DVM7135.htm

author: floobydust
author_rank: Super Contributor
timestamp: December 15, 2019, 08:39:16 am
content: 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 transistorI wanted to take a HY3131 and have an MCU drive a LED display and get laughed at by the interweb

author: tooki
author_rank: Super Contributor
timestamp: December 15, 2019, 10:38:15 am
content: 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.

author: iMo
author_rank: Super Contributor
timestamp: December 15, 2019, 12:50:18 pm
content: 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 7segmentLED module(for a $1-2 off ebay) and you get a decent voltmeter.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 15, 2019, 02:02:09 pm
content: You have on Ebay theses meters, Instek GDM-8245Not perfect in every specs, can be bought for cheap ?? Haven't seen a teardown / insides picturesnor seen a schematic ?

author: floobydust
author_rank: Super Contributor
timestamp: December 15, 2019, 08:40:40 pm
content: 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 haveno input protection at alllike 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 15, 2019, 09:27:32 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2019, 09:46:35 pm
content: 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.

author: rob040
author_rank: Contributor
timestamp: October 25, 2020, 12:36:50 pm
content: Quote from: sean0118 on December 14, 2019, 07:08:40 amHi 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.Thanks for your reply!Rob.