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Topic: Get the LCD signal (Read 120 times) previous topic - next topic

WarrenHC

Dec 15, 2018, 06:53 am Last Edit: Dec 15, 2018, 10:39 am by WarrenHC
Hello everyone,

I have a pH meter like this one:
https://www.allaboutcircuits.com/news/teardown-tuesday-digital-ph-meter/

and I was trying to do the same thing like this, trying to catch signal from LCD pins:
https://quo.vadis.stojkovic.ch/hacking-a-weighing-scale/
 
But the problem is, when I connect the wire to the arduino, my pH meter will going crazy, the value is pretty unstable, seems like some noise effect it.

I was wondering, is anyone could give me some advise? Thanks a lot.

P.S. Sorry for my English.

6v6gt

From the picture, it appears that the 4 digit LCD has 12 pins which is to be expected for a multiplexed 4 digit display with seven segments for the digits and the decimal point being segment 8. That is, non of the pins are directly connected to a power rail.

Your main problems
1. work out if it is a common cathode or common anode type.
2. which 4 pins switch the digits
3. which 8 pins control the digit segments and the decimal point.

I have not done it myself but, without an oscilloscope, I would probably start with a 10k resistor with one end connected to a power rail and probe each of the 12 pins to see if the effect is on an individual digit or a specific segment on all four digits simultaneously. Then move the resistor to the other power rail and do the same. If you see nothing, maybe try with a smaller value resistor but you increase the risk of damage if its value  is too small.

Once you have understood the pin mappings of the lcd, you can then start working out how to connect it to an Arduino. You may need special value pullup/pulldown resistors on any lcd pins which may float when not active. It looks like it is a 3 volt device so it would be best to use a 3 volt arduino or some sort of level shifting.

This is not a trivial exercise and, depending on the multiplexing frequency, you may need to need to use direct port reads to get the status of the pins quickly enough.

WarrenHC

#2
Dec 18, 2018, 02:19 pm Last Edit: Dec 18, 2018, 02:21 pm by WarrenHC
Hi 6v6gt,

Thanks for your reply, very appreciate, but I think I might let you misunderstand.

I'm already can capture the signal from those pins, and already find out the function of each pin.
Normally, the pH meter will gave me an exactly number, at most 0.2~0.5 floating.
But when I connect the wire between LCD pins to arduino, the pH meter will going crazy, the value will floating between 1~2.

I'm already try to put diode in it, but the voltage drop makes my arduino can't capture any signal.

6v6gt

OK. You have got further than I thought.
You must also connect the Arduino ground to the negative rail of the PH meter. You haven't shown such a connection on your diagram.
Are you using pullup resistors on the 5volt Arduino Uno ? This may present a problem for the 3 volt device you are connecting to if the pins are not 5volt tolerant.

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