HP3548 / HP3457 OLED display

[Xyphro]

Hi Guys (and Girls),

I have since quite some time a HP3457 multimeter and hate the display. Had to do something about it. The typical LED Backlight mod was not resonating - it looked still very bad - low contrast.

I had to come up with something else. I loved this idea here from Xi: https://www.eevblog.com/forum/projects/led-display-for-hp-3457a-multimeter-i-did-it-)/50/

But mechanical wise it is very hard to reproduce for others (including me).

After many weeks of hunting for OLED screens in the right format I had a brilliant Idea: Why not use the typical small OLEDs that you can get for extremly low price on Aliexpress and use a single display per digit?! Immediately routed a PCB and ordered it and after some months I had time to take this up and program the SW (thanks again Xi for your groundwork in decoding the protocol!).

The current consumption is extremly low, while the display is very bright: only 20mA are consumed.
Instead of isolating the power supply, I isolated the datapath, so it can still be powered from an AC supply in an galvanically isolated way.

The PCB is from JLCPCB. No milling required and a second PCB (which is not shown) acts as a mechanical holder which has mounting holes and can be mounted without any modification of the multimeter. Also the original flatband cable connector can be used.

Work is nearly done. I think I will put everything together tomorrow and share the final pictures :-)

Best regards,

Kai

Edit: Here a very bad quality video of the current state: https://youtu.be/hoUbkyWuI_g

[EHT]

Looking great! I'd seen the backlight mods but didn't fancy the risk of cracking the display open, and then ending up with poor contrast anyway. A new module mechanically compatible with the LCD is perfect!

[free_electron]

coooooooool !

[Xyphro]

Have built it in and I am very pleased with the result.
Now I have to finish the annuciators (small triangles on the bottom of the screen).

[Xyphro]

Some more Photos...

[Hexley]

Have built it in and I am very pleased with the result.
Now I have to finish the annuciators (small triangles on the bottom of the screen).

That looks really great, Xyphro.
Question: is there enough resolution to display text annunciators below the digits, such as is done in the 3478A? Those vary from three letters, like "TLK" or "RMT", up to five or six, like "SHIFT" or "AZ OFF". If that could be done, your approach could lead to an excellent replacement for the 3478A LCD.

[Xyphro]

Hmm, Funny. My hp3547 lcd just had arrows pointing down and no Text. Underneath the display there is a Text printed on the frontpanel of the hp3547.

The witdh of the Oled is 32 pixels.

Challenge accepted :-) I have 3KB Flash space left of the total 8KB.

[Kleinstein]

12 digits time 32 byte for the graphics to display would only 384 bytes of the memory. So in princuple it could work with a little text / symbols.
It is stiff the question if just the same arrow / block for all positions could work as well.

[free_electron]

Hmm, Funny. My hp3547 lcd just had arrows pointing down and no Text. Underneath the display there is a Text printed on the frontpanel of the hp3547.

The witdh of the Oled is 32 pixels.

Challenge accepted :-) I have 3KB Flash space left of the total 8KB.

depends on the vintage of the machine.

i like how you even made the pressfit dip socket. so this is a real replacement for the hp 5 bit drivers.

[Xyphro]

I'd make it configurable (triangles or different Text variants) . If 32 pixels results in unreadable Text, I can go for 2 line Text.

[Xyphro]

I think it should work. I designed below icons (Mix of icons for different multiple multimeter types) and converted them to .c / .h. Will try this out tomorrow and post some pictures. The selection between booring triangles and device specific variants can be made during compile time (select 1 icon per digit position).

[Xyphro]

...and I think I am done!

Now it is possible to select between 3 different flavors:

HP3457_ORIGINAL => Uses triangles as annuciator
HP3457_ICONS => Uses Icons matching HP3457
HP3478_ICONS => Uses Icons matching the HP3478

The Icons I made are well visible, but very hard to read. Allthough it looks nice on the pictures, I would make the Icons a bit bigger if I'd seriously use them. But as the source is a .GIF file it is easy to do that. Maybe in the next days I'll try this out.

Here an updated Video (unfortunately it started looking blury once I uploaded it to youtube): https://youtu.be/EYaLf55-z-o

BTW: On the pictures it looks as if the digits are not perfectly centered towards the labels underneath. Actually it is perfectly aligned. It looks shifted because the camera was very close to the display.

[Xyphro]

I remeasured the current consumption with the triangular annuciators. When supplied with 12V DC it is 30mA.

There is a 5V Regulator on the Digital Board which is supplied with 12V on its input side where I hooked it up to. So no additional supply is needed.

[Dr. Frank]

Hello Kai,
great work so far!

Your headline implies that you have another version available, maybe for the HP3458A, or is it just a typo?

Would be great, but you'd need a 16 digit, 12 annunciator version.
Frank

[TERRA Operative]

If the 3478 version icons can be made to be nicely readable, I'm interested.

Will you sell PCB's? Or provide gerbers etc? I have two, maybe three 3478's that can do with an upgrade...

[Xyphro]

Good remark Dr. Frank. I though they both have 12 digits :-) the scheme can be scaled, but obviously not with the same PCB. The parallel i2c control would be ready to control 16 displays at 100kHz.

[Xyphro]

I plan to put it on Github soon. Hp3478 is not tested, but I guess the protocol is not different. A confirmation if it works would be great.

[dietert1]

Great work! One could invert those icons though to increase font size and readability, maybe using only three letters if four don't fit.

How does the durability of those OLED displays compare to a VFD?
The best OLEDs nowadays are good for 100 000 hours, that is 11 years. But there are differences. As far as i remember the first OLEDs were good for 2 years only. And as far as i remember they degraded even when not in use (oxygen leaks to organic material).

Regards, Dieter

[EHT]

Such a professional job! I love that the blue OLEDs even match the colour on the front panel. I'm in for one for a 3457A, if there is likely to be any bulk order of PCBs etc.

[Xyphro]

About burnin: Not sure, but I had that thought also already. Time will show.

[rsjsouza]

This is a fantastic mod, Xyphro. I look forward for your design!

[Xyphro]

About the annuciators: I have tried to increase the textsize a bit and inverted them. Attached a photo for the HP3547 version of the textual icons.
Might still be improved a bit, but I am happy with it.

Edit: A PM reminded me, that I also have A HP6651A power supply with a very similar kind of display. Unfortunately that supply is lying like a ZOMBI (half dead half alive [1]) in a shelf, but still I can try the display out and maybe that stimulates me also to repair the supply...

[1] (one of the very expensive DACs is dead and I lost interest because it is such a heavy beast.)

[TERRA Operative]

That latest design is much nicer IMO.

I have a few HP power supplies, but the LCD setup is quite different to the meters, so I'm not sure how cross compatible it will be, I can open them up and take a look when the time comes.

[EHT]

I have a few HP power supplies, but the LCD setup is quite different to the meters

+1: also have 6644A and 6622A PSUs that could benefit from readable displays!

FWIW, the new design with non-inverted annunciators is great!

[Xyphro]

The Hp66xx supplies have mechanical same displays, also with 12 digits. Of course supplies like E3632 and similar ones are very different.

Btw. My hp3547 shows when the input is left open in Volt Mode a very huge drifting voltage until I connect it to a source or I change the input impedance to the lower setting. Is this normal or an indication of something dying?