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):
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I am going to show the different steps of the design.- HP LCD display protocol spying
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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).
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But I succeeded and was able to decode the digits with an ATMega32:
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- 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
And it worked
Below is the image of the aluminium support to machine 4 display at a time:
/?action=dlattach;attach=368664)
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.
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And here is the board just before engraving:
/?action=dlattach;attach=368668)
The 3 boards after mounting all the components:
/?action=dlattach;attach=368678)
/?action=dlattach;attach=368670)
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).
/?action=dlattach;attach=368672)
Once assembled, comparison with the original LCD display:
/?action=dlattach;attach=368674)
The original HP ribbon cable is reused and inserted into a DIP support:
/?action=dlattach;attach=368676)
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To be continued...
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