Old panel meter replacement with an arduino based one

Projects General Guidance
1

Hello all,

This is my first post here and I hope you can guide me thru a "little" project that came into my hands recently.
First, i'll give you a few insights of my background in this area: I've been fiddling with electronics all my life just as a hobby, designing amplifiers and other small signal devices, mostly analog electronics.
My experience in arduino until now is the "blinkin led" sketch. I understand the language and if i open a sketch, depending of its purpose, i can make sense of 50% of what's going on.

Done!
Now with the project..

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I came across this unusual multimeter, Tekelec Airtronic Microdigit TE 924. its unusual beacause it can read uV, nA and GOhms, and also beacause there is no service manual that can be found!
The panel meter is a module inside the multimeter ref: TA 305, there is a manual on the web.

The panel meter receives an analog signal from the equipment and translates it to digital thru some sort of ADC, then outputs it to the LCD display. Along the way, it has a serial BCD output and also a clock signal on the rear connector that i was able to decode in my scope, but i can't make any sense of it. It has an option board installed that converts the serial data into parallel data.

Now, the original LCD is melting digits and malfunctioning, randomly displaying all the decimal points and polarity signals and i hope to create something modern with some LED digits i have.

I might be inclined to use multiplexing for the display, since i already got a chip for it (Maxim ICM7218A) and i've been experimenting with a ready made sketch, but i may not have all the pins available in the arduino for implementing some kind of interface to read BCD serial data from the multimeter, converting it, and displaying it on a multiplexed display.

At this point, i just need guidance to seek the optimal way to do this.
Should i take the serial output and clock from the original panel meter and program the arduino with SPI to interface it? Should i go parallel? Or maybe get the analog signal before it enters the panel meter?
The next step would be to convert BCD (binary) to something to be displayed. So feel free to throw me some pieces of sketches to experiment with.

Is this something i would be able to pull with my level of knowledge. Or is this something considered "expert"?

Thank you all in advance

2

Get a working DAC module for use with a µC using I2C, SPI...

Edit: replace DAC by ADC!

Or get a panel meter module with display and connect it to your equipment.

Displays for Arduino come with libraries for immediate use. Even if you want to build your own display there exist libraries for every (common) display driver or direct access to 7-segment digits.

3

This:
https://www.reichelt.nl/nl/nl/bcd-7seg-spei-dec-stu-74hc-4543-p3255.html?PROVID=2809&gclid=EAIaIQobChMIxrDG1ard-AIVyIjVCh3qdw7EEAQYAiABEgJbrvD_BwE&&r=1
Can convert bcd to 7 segment display.
Maybe these ic's are also available for 4 digit 7 segm display (starting from your serial bcd).
Or you could look for a 4 digit 7 segm display that does the conversion for you.
Maybe a bit old school, but might still be for sale.
Than you would have your panel in almost original state...

4

Thank you for your response.

I've already searched for a ready made panel meter, there is one that sounds promising, i has 8 led digits and a MAX7219 to drive it and it have a BCD decoder (i dont know if this decoder is useful or not for my project) . The thing is i need a configuration like:

|+-1. | 8. | 8. | 8. | (led digits on the meter)

or i can substitute de 1st digit ( |+-1| ) by two "normal ones" in order to display only the "minus" on the first one and the "1" on the second:

| 8. | 8. | 8. | 8. | 8. |
displaying:
| +- | 1. | 8. | 8. | 8. |
a total of 5 digits

I would happly try one of this 8 digit panel meters but the window on the equipment can only accomodate a maximum of five digits.

Can you point me in the right direction for the DAC module you purposed?

Thank you

5

ADC modules are available with up to 24 bits. How many of these bits are really useful (valid) for display purposes depends on the analog circuitry before the ADC.

6

Can this also be used for driving LED displays? I thought that for driving LCDs one would need some kind of AC signal on the segments as for LEDs it would require DC.
edit: Or maybe some kind of high frequency AC...i can make sense of this

I'm open to any kind of idea to add up to the list :grinning: Maybe we can all choose only one route in the end.

Thank you

7

I see now. I guess you meant an ADC in the first place. I was wondering why should i convert the digital signal again to analog :grinning:
I'll search again for one of these modules.
Thanks

8

What you have now is a 4 digit 7segm led display. Not a lcd.

9

Yes. But the datasheet for 74HC4543 states that it is meant to drive Liquid cristal displays, flourecent,... but not LEDs.
I wonder it works with LEDs without any addicional circuitry?

10

What i have now is this individual LED segment displays, a mutiplexer, and a Nano:

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edit:
I'm begining to like the external ADC option since i could bypass all the potencial problems on the original panel meter. This is from 1974 and i only replaced the old and leaky electrolitics on the main instrument.

edit2: there's a thing i didn't made clear about the original display:
actualy it is a liquid cristal display, the manufacturer calls it "Field-effect liquid cristal display" possibly some inovation from the 70s. I wish to re-use these LED digits to "replace" the original.

1 Like
11

I guess not then. But similar ic's should exist for led displays.

Found one:
TTL 74LS47
Here:

12

Never heard about field effect lcd's.
Maybe I am too young...
Might be difficult to find a new one if it is that old...

13

I think that effect applies to all LCDs. The field changes the crystal orientation and makes it transparent or opaque.

14

In that case, I know what they are, but I do not remember having seen this alias before...

15

From the original manual:

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Possibly a common lcd back in day

16

Yes! this could work, but, i would need at least four of them and wouldn't be able to drive a "minus" segment without another one of those chips (if they allow me to drive individual segments)
I have an ICM7218A multiplexer that allows a no-decode function to drive individual segments on a total maximum of 8 digits. (last picture i posted)
I know there is the MAX7219 that does all this in multiplex plus it has a BCD decode pin. i don't have this one but looks promissing.
I would prefer using mutiplexed signals from the arduino thru one of these chips since it uses 10 digital pins, leaving 2 digital pins free and pin 0 (TX) and pin 2(RX) if the arduino were to be fed the serial BCD from the panel meter.
Does this make any sense?

17

Then you really haven't decoded it. Can you take a picture of what you see or tell us more about the digital serial output?

Yes. It should not be impossible to decode the signal you have, and this would leave the cool old circuitry to do the other stuff.

Might not be exactly SPI, but you should be able to use the clock and data lines NF grab up enough to know what to put on the display.

It is possible the build in serial to parallel feature could be used, is there really no manual or other informations on the internet?

Post any and all numbers or other identifying data and nomenclature that you can see on and in the instrument.

a7

18

This if my first time ever using the decode function on the scope. I was able to trigger and got the decode results, but none would correlate to what i was seeing on the digital meter. Here's a pic:

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In the manual of the panel meter it says that the slew rate of the serial BCD signals and the clock were manipulated to minimize EMI. I can understand that the transient of having sharp rise times would create some noise on the circuitry. i don't feel confortable with those spikes on the serial line though.

Here's the link to the panel meter:
https://www.aef.se/Matteknik/Matteknikportal/Dokument/Manualer/TA305-Digital-Panel-Meters-9528.pdf

That's all i have. i?ve extensively reseached for a manual of the equipment with no luck.

On the faceplate: Tekelec Airtronic Micro Digit TE924
On the back the same reference and serial nº only

edit: I know that the DC offset of the signal probably would kill my arduino, but like i said...one issue at a time! :thinking:

Thank you!

19

From the panel meter manual:

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This connector comes from the main board of the equipment.

20

I realize you wish to learn / apply an Arduino for your project, however I suggest you look into something like this panel meter