Electronics > Projects, Designs, and Technical Stuff
Replacement display board for Keithley 197A
alx2009:
The Keithley 197 / 197A is an old 5.5 digits bench multimeter. You can read about the multimeter here: https://www.eevblog.com/forum/testgear/keithley-197a-owners_-corner/
The meter is quite good but often the LCD display starts to fade, due to the zebra strip contacts failing, as documented in the previous thread. My own meter started to suffer from this problem. It can be mitigated temporarily, but eventually it reappears. I like this meter so I am looking to design & build a replacement display board.
Luckily rastro reversed engineered the display protocol (see https://www.eevblog.com/forum/projects/keithley-197-led-display-hack/msg493365/#msg493365), which makes the job a lot easier. Another forum user, Technogeeky, implemeted a decoder running on an Arduino https://github.com/technogeeky/keithley-197
this work inspuired me to build my own replacement board and I intend to document the design here. So lets start with my main requirements:
- Must fit inside the case of the instrument without too invasive/ugly alterations
- Given the effort, I want to add some new functionality. At a minimum data logging via external interface, serial or usb and a continuity test mode
- I want to be able to update the SW without opening the instrument. This means it must have an external interface for programming
- Another common problem for this meter is that the cluster of buttons at the left of the display require quite a lot of pressure, and it is getting worse. I wan tto improve that as well.
-It should not cost more than a new instrument of the same class/capabilities... so obviously no fancy custom displays :)
- And last but not least, it must be safe! among other things, this means galvanic isolation of the programming/datalogging interface and due consideration for isolating the new display.
Ok, so what is the first step? Well, my experience with other projects is that developing on the instrument itself is not ideal. A mistake can damage the multimeter. Not to mention the hazard posed by the exposed mains and the fact the ground of the board under development is shared with the device under test.
I usually prefer to use a simulator, and verify on target occasionally until the design is mature (and hopefully safer to work on). In adition, a simulator can improve the stress testing (e.g. I can change the timing to make sure I have a good margin).
The good work done by Rastro, Technogeeky and the information on the forum allowed me to design a simulator pretty quickly. I have used an Arduino nano on a breadboard. The original display board connect via a ribbon cable with a 16 pin IDC connector with 3x row spacing, like they have made it to fit on a breadboard - or more likely on the DIL 16 socket used in the multimeter ;D
The picture below shows the simulator connected to the original display board:
Via serial monitor you can turn on and off any segment in the display. The image shows all segments turned on (which is unlikely to happen in practice, but I may want to test nonetheless).
I also connected another Arduino running the Technogeeky code (with some modifications suggested in the forum posts above) and it works (forgot to take any image, sorry)
I have published the complete simulator sketch, schematic and fritzing breadboard view on my gitHub: https://github.com/alx2009/displayBoardTester
Questions, comments and wishes for next posts are welcomed :-)
Otherwise in the next posts I intend to discuss the choice of the key components (display, microcontroller) and the overall design of the replacement board (actually boards as we will find out).
free_electron:
The button problem is well know. take the pcb off, clean the pcb with alcohol and let it dry. most likely it will be solved. you can rough up the carbon contacts on the rubber membrane with a pencil eraser.
alx2009:
Thanks free_electron for the comments! They are good tips, and they work - for a while. I have also use graphite spray as in my case the problem was more on the key itself than the PCB. That worked for more than a year, but now a couple of keys are acting again...
If this was the only problem I would probably clean the PCB/apply more graphite and continue. But I am going to replace the PCB anyway so instead I am going to use MJTP1140ATR SMD tactile switches from Apen, which I got from Digikey: https://www.digikey.se/short/rqn9h5mm
They are small enough that they fit perfectly under the original key. I have done a preliminary test and they seem to work quite well, adding a clear and satisfying feedback when the button clicks.
As it happens, the terminals have exactly the right size to be soldered on top of the original PCB contacts. So if someone is not satisfied with the other workarounds, this could be an option. Most likely there is a need to add spacers to account for the extra height, but not much: the switch itself is 1.5 mm thick but there is some play between the key and the PCB. Adding a spacer on the order of 0.5 mm should suffice, maybe even less. I have bought an extra button, so I can do a test if someone is interested.
Brian of Romsey:
Hi,
What a great project. I have a 197A and would like to be considered as a beta testing site (I am in the UK now but may later turn up in Australia). Happy to pay for things.
Have you considered using the ESP32-S3 which would allow for adding Bluetooth connectivity at a later date?
Thanks, Brian.
tooki:
Great project, by the way. I’ve been meaning to do the same thing, once I have the time…
Wouldn’t tactile domes, or even thinner switches, be a better option than these switches and spacers? You can get ones under half a mm thick!
I don’t see how you could add continuity mode via the display. The update rate is what, 3-5 times per second? That’s useless for a continuity tester.
For data logging, it certainly seems to me like the GPIB option might be a better place to start. But it would work all the same. I’d skip a galvanically isolated electrical connection and use wireless, like Brian of Romsey said, if testing shows it doesn’t cause interference in the meter. (With the ESP32 on the display board, it might be too close for comfort. But if divided into two boards — an MCU board installed in the rear, where the original display cable plugs in, and a new display board — it’d probably be safe!)
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