Author Topic: LED display for HP 3457A multimeter - I did it :-)  (Read 13172 times)

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Offline xi

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LED display for HP 3457A multimeter - I did it :-)
« on: November 10, 2017, 01:07:51 pm »
    Hi there,

    The HP 3457A is a well known really good DMM, I bought one for my hobbyist lab (got two for some reasons). Its main drawback is the LCD display! It's hard to read, especially when doing some critical measurements on a board, and trying to read the DMM at the same time :-[

    I know there are some mods to add a backlight - though this is a good idea, I am not a big fan of LCD displays at all: there is always some angles where you don't see anything

    So I though it could be a good idea to rebuild a display with some LEDs :) There were two major difficulties:
    - reverse-engineer the HP LCD protocol.
    - recreate a display identical to the LCD one, eg with two point (colon) and a comma between each digit.

    I succeeded and here is the result (before / after):


I am going to show the different steps of the design.

  • HP LCD display protocol spying

I used 2 floppy drive disk ribbon cables to get a straight extension in order to connect my Saleae logic analyser. Then I tried to understand the HP LCD protocol (this was far from trivial, because the data for one digit are scattered at various places in the transmission frame).


But I succeeded and was able to decode the digits with an ATMega32:


  • Mechanical considerations
Second biggest problem was to add the two points and the comma between each digit. I initially though about using some really small LED 14 digit display and use some discrete LED between each digit, with eg some 3D printed light pipes. But there were two new problems with this solution: the smallest display I found are 10mm width (there are 12 digits, and the DMM window is about 120mm with - meaning that there is absolutely NO room for some LED between each digit). The other problem was the price of the 3D printed light-pipe: more than 60€!
So I thought about another solution: machine the displays to remove about 0.5mm on each side and use an additional PCB with some miniature LED in order to display the punctuation :-/O And it worked :) Below is the image of the aluminium support to machine 4 display at a time:


  • Electronic board
Next steps were to design the electronic board. I choose to design the display in 3 floors: one for the CPU and power / one for the 14 seg. displays / one for the punctuation. The board are single side PCB, so that I can easily engrave them at home. But I designed them with two layers, thus no straps are needed if they are engraved with two layers ;)
I used an Atmel ATMega162 which has 35 IO in a TQFP44 & DIP40 package (most of the other Atmel in this packages only have 32 IO lines). I will publish the schematics and some detailed explanations if there are some interests in that!
Below are the boards just drilled and cut on my CNC machine.


And here is the board just before engraving:


  • Assembly
The 3 boards after mounting all the components:



The hardest part was to solder the miniature LEDs, especially the comma, because normally these LEDs are designed to be mounted horizontally, but I mounted them vertically ... (I wasn't able to find any LED with an elongated form factor, except these ones).


Once assembled, comparison with the original LCD display:


The original HP ribbon cable is reused and inserted into a DIP support:

[/list]

To be continued...
« Last Edit: November 10, 2017, 01:16:12 pm by xi »
 

Offline Vgkid

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #1 on: November 10, 2017, 01:32:45 pm »
That is really good looking , look forward to seeing more.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline xi

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #2 on: November 10, 2017, 02:01:49 pm »
follow-up

  • Tests
I used the SPI hardware module of the Atmel to retrieve the info from the HP 3457A. The frame are not SPI at all (10 bits vs 8 bits, ...), but adding a few discrete logic gates I was able to decode every frame of the HP DMM using the Atmel SPI.

First step was to display the decoded values in a serial terminal (Values in RED):


Then on the LED display - this is one of the first test I made:



All digit lit, once assembled:


  • Final result
I am really pleased with the final result: viewing angle are - as expected - really really good (see top and side views just below), and the display is really fast - data are displayed as soon as they are received ; I think it's even faster than the LCD version, because the CPU is faster. As for the refresh rate, I achieve 80Hz, so there is no visible flickering at all.

Power consumption is directly related to the brightness of the display. The yellow version of these Kingbright display would need about 10mA per segment to be really bright (meaning nearly 2A @ 5V for the 12x 14 segment display when everything is lit) ; this is way too high for the transformer of the DMM. I ended up in sending about 5mA per segment (mean value): when everything is lit, it adds about 4.5W of consumption on the transformer, and when some useful stuff is displayed, the consumption increase is about 2W, which is perfectly acceptable.
For the next module, I will choose red display, because there is a high luminosity version for this color.





That's all for now! If there are some interest, I will post the schematics and more detailed explanation ;)
« Last Edit: November 10, 2017, 02:06:29 pm by xi »
 
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Offline IanB

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #3 on: November 10, 2017, 02:12:19 pm »
Wow! That's some serious hacking. You must have been really upset with that LCD display  :)
I'm not an EE--what am I doing here?
 

Offline retiredcaps

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #4 on: November 10, 2017, 02:18:41 pm »
Awesome job!  :-+

I have one bench meter, but I never use it.  It is a Keithley 175 and it has the same hard to read display at certain angles.  I got it for $30 CDN out of curiosity more than anything.

I can certainly say that I would never even attempt this LED modification given the time required to do so.  It is easier to reach for another handheld multimeter with a bigger display and backlight.
 

Offline ludzinc

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #5 on: November 10, 2017, 02:26:52 pm »
WOW

A thing of beauty, and a Joy forever!
 

Offline drussell

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #6 on: November 10, 2017, 03:14:51 pm »
Slick!   Nicely done!   :)
 

Offline rsjsouza

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #7 on: November 10, 2017, 03:20:58 pm »
Congratulations on the modifications! The display looks stunning. I have two 3478As that have the same type of display... Perhaps some day I take the plunge to replace them...
Vbe - vídeo blog eletrônico http://videos.vbeletronico.com

Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline Tomorokoshi

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #8 on: November 10, 2017, 03:24:39 pm »
Very impressive. That would make a good upgrade kit.
 

Offline TheSteve

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #9 on: November 10, 2017, 04:21:52 pm »
Hardcore - I love it!
VE7FM
 

Offline razberik

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #10 on: November 10, 2017, 08:24:20 pm »
Amazing !
Shall it fit to 3478A ?
 

Online kripton2035

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #11 on: November 10, 2017, 08:40:08 pm »
you can start a kickstarter for this now ... ;)

Offline brabus

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #12 on: November 10, 2017, 09:08:43 pm »
Very, very well made. Congratulations!
 

Offline tooki

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #13 on: November 10, 2017, 10:56:00 pm »
Wow, that's beautiful!!!

I've never used that model or any relative. What does it use the colons between digits for?
 

Offline tooki

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #14 on: November 10, 2017, 11:01:32 pm »
Awesome job!  :-+

I have one bench meter, but I never use it.  It is a Keithley 175 and it has the same hard to read display at certain angles.  I got it for $30 CDN out of curiosity more than anything.
I'm working with Technogeeky to come up with a replacement display board for the Keithley 197/197A, and I wouldn't be surprised if it used a very similar display board, if not the same one. Keep your eyes peeled. :)
 

Offline Mr. Scram

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #15 on: November 10, 2017, 11:04:58 pm »
Nice job, and certainly something I would have been interested in if I didn't lose the auction the other day.
 

Offline capt bullshot

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #16 on: November 10, 2017, 11:06:25 pm »
Wow, that's stunning
Great job!
Safety devices hinder evolution
 

Offline ElektroQuark

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #17 on: November 10, 2017, 11:30:03 pm »
 :clap:

Impressive work.
As someone has noted, you could sell some kits.

Online Dr. Frank

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #18 on: November 11, 2017, 12:12:50 am »
Great job, very well done and documented!
Thanks for sharing.
 

Offline xrunner

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #19 on: November 11, 2017, 12:17:11 am »
Impressive - most impressive!  :clap:
I am a Test Equipment Addict (TEA) - by virtue of this forum signature, I have now faced my addiction
 

Offline Back2Volts

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #20 on: November 11, 2017, 02:20:12 am »
Wow, awesome, impressive, most impressive...   what else can I say !!!
 

Offline sokoloff

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #21 on: November 11, 2017, 02:25:53 am »
Hacks like this are simply amazing to me.
Such dedication and effort for such an elegant outcome; I love it.

I'd love to see the schematics and more detailed information about how did the decoding and reverse engineering. (It's not like I even own that equipment, but I love reading the nitty-gritty details.)
 

Offline bookaboo

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #22 on: November 11, 2017, 02:58:06 am »
Incredible job  :-+ :-+
 

Offline ealex

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #23 on: November 11, 2017, 03:16:30 am »
that's awesome  :-+

i'm interested in the protocol .
maybe a small CPLD can be used to capture the data an drive the display ?
 

Online cvanc

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Re: LED display for HP 3457A multimeter - I did it :-)
« Reply #24 on: November 11, 2017, 03:44:15 am »
Beautiful, elegant work.  Well done.
 


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