The mighty DT830D multimeter.
After opened, the meter looks like this.
The LCD pads has 31 pins. Among those, 3 pins are shorted so making 29 pins. If i have to determine signal from 29 pins, an arduino uno will not be capable because it has only 20 useful pins and among those, 2 pins is reserved for serial txd and rxd.
An arduino mega is large enough that it will not fit into the meter case so, by analyzing, I have decided to use a STM32 Blue Pill board. Because it has a lot of pins available and a lot of memory, onboard USB, runs on 3.3V (has a 5V to 3.3V regulator onboard) so it can run on a li-ion battery, and size small enough to fit into the free space of the multimeter case.
I had to determine which blob pads has routed to the LCD pads with another analog multimeter. In my case the blob IC had a pin marked with white dot and connected with +V. And after that a series of 25 pins routed to the LCD and 3 pins are directly routed to the main knob of the multimeter(why? i dont know but it seems funny).
I am going to use a cheap logic analyzer to determine the COM pins and each segment pins. Before that, to connect the logic analyzer with the blob IC, I had to solder 29 thin wires with a vero board in which i have soldered a IC rail before.
Determining the COM line is simple and straightforward but tedious. It is a single/multiple pin which has no phase change according to the manual and the instructable(i provided the link in previous post). To determine this in the right way, I have to put a variable resistor between the two probes of the multimeter, then set the knob to a suitable ohm measurement position and capture the data by logic analyzer while trying to generate all possible combination of the LCD. This will take several attempts because i have figured out there is 29 pins to capture while my cheap logic analyzer has only 8 pins to capture data.
The captured data looked like the following image to the naked eye i was afraid how i am going to change this into a recognizable number, but i knew that i was looking only for phase change. A pin that the phase will not change during rotating the variable resistor ad generating all combination of the LCD.
After a brainstormed afternoon, i found the COM port on the blob pin 20. Sorry for the unconventional pincount, i have counted anticlockwise from the white dot while considering the white dot as 0th pin.
My next post will be on determining the seven segment and other LCD pads.