Interpretation of LCD Signals

A first look at the LCD traces shows us a number of interesting features: the signals are multilevel, they repeat and there are lots of them!

LCDPin13

Just picking one of the random images shows us that there are four levels: let’s call them A, B,C and D.  A=0v, B=1v12, C=2v24 and D=3v36.

The first observation is that at 3v36 the voltage is boosted over cell battery voltage so there must be a voltage regulator (charge pump) somewhere; secondly the four levels suggest a multiplexing arrangement called 1/3 bias. We have 0v, 1/3, 2/3 and 3/3 voltage levels where 1/3=1.12volts.

Secondly the underlying waveform has a repeat after 8 units.  The whole 8 transition waveform takes around 16.6mS; i.e. it repeats with a frequency of 60Hz.

Now the display has quite a few segments.  It has at least 4 7-segment digits and also 2 digits for the fractional part (it measures 1/8ths of pounds) and there are also a few decimal points, colons and the kg,lb and st symbols.

My current theory is that there are four back-planes Pins 15,14, 13 and 12; with Pins 0-11 being segment pins.  The scheme employed is 1/3 bias, 1/4 duty at a refresh rate of 60Hz. If 4 pins are back-planes and 12 pins are segment lines that would give a potential 4×12=48 segments which seems feasible to drive six digits and a few extra segments.

In order to establish which segment lines and which back-planes drive which LCD segments will take further experimentation for which I will have to build a suitable AC voltage supply so as to not damage the LCD.

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About nivagswerdna

Professional Geek
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