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Reading data to PC from a cheap multimeter – 2

To read the data from a multimeter, at first i have to determine the individual pins of LCD, which is connected to which segment, the microcontroller voltage to drive the LCD and the nature of the pulse. I have opened the pcb from the multimeter and marked the pins that are going to the LCD.

Then I have attached a 40 pin header to the multimeter case because i have to experiment it multiple times. I thought a breadboarded version (the arduino people will call it a breakout board) will be better for me. I thought an atmega8 would be sufficient for this project but he meter for my test has a total of 31 pins. Among them 3 pins are shorted and 2 pins ar, so it become 27 pins for driving the LCD and the voltage to drive the LCD is very little, 2V or less. So i think i will use a stm32 blue pill for this project because that has enough pins, processing power, built in USB, runs on 3.3V and has small size to fit in.

Then i need to understand how an LCD works in the first place. I have found a friendly explanation on this instructable post. As he said :

Most people aren’t as interested in reading the LCD screen as they are in driving it, so the best way to learn how to read the screen is to learn how it is driven. One of the best explanations I have found is available in this Atmel Application Note – AVR340, but I will quickly go over the key points.

An LCD screen is a collection of segments that react to AC signals present on the control pins. The segments might make up an alphanumeric digit similar to a seven segment display, or they could be for a single icon on the screen such as the AM/PM text on a clock. One screen can have many segments driven by only a few pins – this is done through multiplexing.

A single pin is actually used to drive multiple segments, depending upon which of a set of COM signals is active at any given time. As mentioned above, the LCD is driven using AC signals, typically in the form of binary square waves (OFF or ON). However, the COM signals will have three states: OFF, 1/2 ON, and ON, where ON is full the source voltage. The linked application note gives the following example of this control scheme.

Notice, only one COM signal is active at any given time, and the LCD segment line will control different segments, depending upon which of the three COM lines is active. To enable a segment, the segment line needs to be out of phase with the COM line. In other words, if the COM line is HI, the segment line should be LO, and vice-versa.

The next step for us is to determine which of those LCD pins are COM lines, and which drive a segment!

To me, the LCD screen is a capacitor. It has a transparent thin layer of metal coating on glass and inside of two coated glass, there is a dielectric oily material which they called the liquid crystal. The liquid crystal changes its polarization when current flows through it. This is made visible by polarization filter on the glass. Because generally it has two metal pads and di electric material, DC can’t flow through it only AC can flow through it.

Now it becomes complex, because the signal is now AC, and i have to make the timing accurately to determine the signal correctly and it should be interrupt based on the com line.

On the next posts, I will discuss how i have determined the COM lines.

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