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Reading data to PC from a cheap multimeter – 3

The mighty DT830D multimeter.

DT830D Multimeter battery removed

After opened, the meter looks like this.

DT830D Meter Opened, not showing the back cover.

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.

LCD Pads on the PCB

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.

Probing with logic analyzer(believe me it was under the multemeter! Sorry saleae, it is an chinese one).

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.

One Comment

  1. Mario Overmann Mario Overmann

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