DIY Benchtop DMM - Page 1
Forum section: electronics / testgear

author: mrKiwi1
author_rank: Newbie
timestamp: July 31, 2014, 08:25:15 am
content: Hello everyone,I had a accident 2 years ago and a eye operation has left me with poor eyesight, as a result I am finding it increasingly hard to read screen of lot of millimeters. I can read writing on components with the use of strong magnification but I find when working at my bench it takes some time to focus on my dmm to read its screen. I have tried v that has a smashed screen and it got me thinking. How hard would it be to turn a handheld DMM to a benchtop unit with a large LED display.please correct me if im wrong , but from what I can see the DMM LCD is made of 7 seg LCD digits. I was wondering if I could take the output pins to LCD and send them to a 7 segment LED driver with a 2 inch high 7 segment display. The meter is a chippie Digetech QM-1323 from Jaycar electronics here in australia. I am a electronics hobbyist and not to concerned about accuracy. The meter runs off a Semico CS7721CN chip. I havnt been able to find any datasheet for this IC but looking at similar dmm driver ic's they look to have segment outputs going to the LCD. Any advice is greatly appreciated.cheers

author: bktemp
author_rank: Super Contributor
timestamp: July 31, 2014, 08:42:40 am
content: It is basically possible, but is not easy, because lcd multiplexing is a complex thing using multiple voltage levels. And the segment/backplane arrangement is often optimized for easy manufacturing of the lcd glass: The multiplexing scheme does not always drive digit after digit, it can even be the top or bottom half of each digits or something like this.It is much easier to use a DMM with a RS232 interface an hook it up to a pc or microcontroller driving a larger display.

author: Wh1sper
author_rank: Regular Contributor
timestamp: July 31, 2014, 08:52:41 am
content: In your situation I would go for an old one. i.e. Keithley 179a or such alike.They are cheap and pretty good to readI

author: miguelvp
author_rank: Super Contributor
timestamp: July 31, 2014, 09:05:51 am
content: I agree, it will be cheaper to get a benchtop DMM than to hack one.I would recommendhttp://www.mortoncontrols.com/index.php/ut802.htmlShipping is almost double than the discounted price of $35 ($60 shipping to Sydney Australia) I'm sure you can find something similar on your whereabouts) regular MSRP is $70 so not worth it to you, but I'm sure you should be able to find similar pricing/products for that price in there.

author: sleemanj
author_rank: Super Contributor
timestamp: July 31, 2014, 10:19:25 am
content: As a cheap alternative, you could get one of the meters with a serial connection (or one that can be hacked in, like the VC99) connect it to your PC and run for example QTDmmhttp://www.mtoussaint.de/qtdmm.htmlHavn't used it so I don't know if it can change the size of the "display", but it's open source so you could change it I'm sure if suitably motivated.

author: Smith
author_rank: Frequent Contributor
timestamp: July 31, 2014, 10:24:50 am
content: I second the keithley. They are very good meters. I have a 199 witch has been used daily for about 10 to 20 years and the red 7 digit displays are still very bright and reasonably large. Even in bright light they are easy to read.

author: wiss
author_rank: Frequent Contributor
timestamp: July 31, 2014, 10:50:49 am
content: Keithley 192 have pretty large digits, same size as 199 I guess?Solartron rack-multimeters might have larger digits but then your bench is full

author: frenky
author_rank: Unknown
timestamp: July 31, 2014, 11:46:42 am
content: It's not too difficult to decode multiplexed signal that goes to LCD.I have done it with kitchen scale + HC4050 buffer IC + arduino.My setup:I was planning to write an article on this topic but never got to it. I'll try to be short.It's helpful understand how multiplexing works. I have found this document (and many others, just google lcd multiplexing):http://www.nxp.com/documents/data_sheet/PCF8576C.pdfFirst you need to examine pins to find out which are backplane pins and which are segments.Then use the oscilloscope to record the timings of the signals.I connected lcd pins to arduino like this:In arduino code I set rising interrupt on backplane 0.After that I waited, read signals, waited, read signals etc...I was reading 8 signals at 4 different times to get status of 32 segments.But to figure out which of the 32 bits is responsible for each part of segmented characters I made setup like this:Connect each backplane and each segment pin to 3.3V square wave and one segment will turn on.I hope this helps.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 31, 2014, 01:11:28 pm
content: I can second the old Keithley dmm. They have tall VERY easy to read displays. Plus because they aren't as aesthetically pleasing you can usually get them for cheaper than the hp/agilent or fluke. I have two Keithley 199 and they (plus the rest of the 192, 194, 195, 196 family) are by far the easiest bench meter to read.

author: mrKiwi1
author_rank: Newbie
timestamp: August 01, 2014, 02:52:21 am
content: Wow thanks for all your very helpful reply's. I took a look at the keithley's online and they look like they do have very good bright display. If I can find one close enough to make the postage worth it I will see if I can get one. The RS232 DMM and multiplexing idea is a very good suggestion as well. will need to do some research online as to price and see what I can come up with in way of $$$ thanks everyone for your help

author: Bored@Work
author_rank: Super Contributor
timestamp: August 01, 2014, 05:09:29 am
content: Quote from: mrKiwi1 on August 01, 2014, 02:52:21 amI think the cheapest (not safest or "best") meter we have ever seen here with a serial interface is the Digitek DT 4000ZC. It is also known under names like TekPower TP4000ZC.A forum memberhttps://www.eevblog.com/forum/profile/?u=26834sells themeBay auction: #200912705694. Some time ago he offered a discount for eevblog members. Just ask him.BTW, instead of connecting a meter with a data readout to a display you could also use a text-to-speech converter, or both. Some years ago I did this for shit & giggles with one of my bench DMMs and a Linux machine. Linux comes with several TTS engines, and all I had to write was a bit of software reading the bench meter data, converting it to text and piping the text into a TTS engine. Quite funny to switch the TTS language to another voice when the measurement range switches.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 01, 2014, 05:39:44 am
content: Quote from: Bored@Work on August 01, 2014, 05:09:29 amReview by Martin. Even though my "name" is mentioned, I have no $$$ affiliation with Martin or Franky.

author: mtdoc
author_rank: Super Contributor
timestamp: August 01, 2014, 06:42:59 am
content: Quote from: sleemanj on July 31, 2014, 10:19:25 amI've use used qtdmm with the TekPower (Digitek) meter mentioned above. It works well (Mac Os or Linux only I believe). The display is fairly large and easy to read. As I recall it cannot be resized independently but of course you could resize it by changing a monitor's screen resolution.I've used it with a Raspberry Pi. So an inexpensive option might be using it with a RPi running linux and a cheap used monitor. For the TekPower meter you'd also need to get a serial to usb adapter.

author: Berni
author_rank: Super Contributor
timestamp: August 01, 2014, 06:49:01 am
content: If you are going to go that route get a use a multimeter with a optical interface on the back for serial communication to get readings out of it that way.Proabobly easier to just buy a used bench meter off eBay cheep.I bought the Agilent DMM with a OLED display (the other model than what dave got for review that runs from AAA cells so it actually has a good battery life) the display on it is as readable as any other bench meter. I didn't really need it since my eyes are fine, I was just sick of leaning over to read the displays on multimeters. It was not cheep but good flukes go for even more and it came with a cal certificate showing all mesurments and it was almost dead on in DCV, so worth the money