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Reading data to PC from a cheap multimeter – 4

To determine the segment pins, i had to remember the theory of seven segment. Seven segments usually has 7 LED for showing a single digit and one led for a dot(i dont care for it now). 7 pins to drive the segment 1 pin to drive the dot and one common anode or cathode. The LCD has 29 pins to be analyzed. It has 3 full functional digit and a semi segment to show only number 1. So it should need 7×3=21 pins for 3 digit and 2 pins for the last 1.

I have to generate a truth table in excel to get all combination of the 6 pins of a single 7 segment. The number in right side column AC shows the decimal weighted equivalent of the combinations of the segments.

I have to compare those pin state with the COM pin showing the COM pin in channel 0 while other 7 pins connected to other channels to observe to determine each single digit.

Channel 0 is COM comparing the other pin state.
Channel 0 is COM comparing the other pin state.

Determining the segment pins is straightforward now. the steps are :

  1. Decide one segment to capture.
  2. Set the multimeter knob to a suitable ohm position and rotate the variable resistor to a position that the segment will show 1 and will be fixed to that position.
  3. Capture the data with respect to COM line.
  4. Note down the pulses in excel when the COM line LOGIC is LOW state or 0 because in this state, the lighted up segments will have opposite to COM that is HIGH state and the turned off segments will have LOW state.
  5. Then start again from point 2 for the digits 0, 2, 3, …. and so on.
  6. After that, start with next sgment.
  7. There will be inaccurate reading in the channel for the segment. If this happens, change the pin and search for the right pin.

For my case, the excel sheet looks like following image.

Left column is the name of channels. the bottom row shows each digit and the right most column shows the blob pin number.

The right numbers are the weighted value to recognize the individual segment from the previous truth table.

From this image, the pin numbers and their usage becomes crystal clear.

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