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Topic: How can I use this for a display(s) with an arduino? (Read 889 times) previous topic - next topic

geofmureithi

A pal of mine gave me this display from a Chinese scale and I would like to use it with an arduino. A google search didn't yield much and am not sure its even possible.



Any assistance or direction would be really appreciated! :)


Paul_KD7HB

The reality is there is not much you could do with the display.

But the good part is there are only 8 connector pins to worry about.

Look at the back of the board and you can determine the pin connecting to ground. That leaves only 7 pins.

The little disk ceramic capacitor is most likely connected between ground and the + power pin. So, now you know the power and the ground pins.

Now, what are the common values of all the resistors nested in bunches? They will be the current limiting resistors for the LED displays. Once you know the resistance, you can use Ohms law to compute the LED display voltage, assuming 20ma or even 30 ma per segment. So, now you know the ground pin and the +power pin and have a good idea of the voltage used by the device.

Now you have to figure out the rest of the pins and how they are used. Good luck.

Paul

MarshaJ847

The problem is the ChipOnBoard under the black blob on the left of the PC board...
was it just taking power and measurements from strain gauges, or was the a secondary board.

xArt

I'd cut all traces to the COB, and from there, you couldn't ask for an easier PCB.
It's already arranged as a POV display. You send the value you want to display to ALL LEDs,
and select the common pin for the single display you want to light, typically with a shift register or decade counter.

dougp

Looks like only three signal wires run to the COB.  It could be something as simple as a shift register input feeding a built in muxer. 
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geofmureithi

The problem is the ChipOnBoard under the black blob on the left of the PC board...
was it just taking power and measurements from strain gauges, or was the a secondary board.
It was from a weighing scale.

Johan_Ha

I see only four pins. This whole unit is connected with only four pins to something else, right? Those four pins go to battery +, battery -, something else? If this "something else" is nothing but a pressure gauge, you can try feed the lines with a voltage created with a potentiometer. But my guess is there's another chip somewhere taking the info from the pressure gauge and maybe from a switch, where you select lbs/kg.
You can always figure out the traces and recreate the logic how to light up each of the 128 leds. Or compare with this chip, which seems to connect with 4 wires.
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Hiddenvision

#7
Jun 15, 2019, 07:28 pm Last Edit: Jun 16, 2019, 03:37 pm by Hiddenvision
Edited to make more sense.!

I see there is 8 pin header
Six separate pins in use as 3 of them are connected together

Not sure what order.

1: Positive, should be easy to find
2: Negative, should be easy to find
3: Red AC Led.

Then perhaps....
4: Clock,
5: Data,
6: Select,


Could be a Max 7219 or two under the black dot.
Otherwise carefully remove the COB (black dot) and replace with 2 x MAXIM 7219 or similar

Each 7219 controller can control 64 leds or 8x7segments plus dots.
As I said, quite possible that is what is already used.

Find the power pos and neg, +5v Maximum.
Making the red AC led light up in combination with the cap being across pos and neg should give you the correct layout for those 3 pins.

Then run some crazy TTL signals into the other 3 remaining pins.
Just don't blow it up too much or you will have to use two new max devices.

You could actually just run some example 7219 driver code in your Arduino and move the 3 control wires around till you hit lucky 3 x 3 only 8 or 9 possible combinations.
As long as you don't jam >5v into the pins you should be fine as they are all inputs anyway, but you could use 1k - 22k resistors inline to limit the current on those pins for testing.

The Maxim 7219 devices can be daisy chained to work on 3 control wires.




Hope that help a little

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