LCD Drive Voltages

Now we have established that of the 16 lines going to the LCD panel there are four back-planes and twelve segment lines we can therefore imply that this combination can drive a panel with up to 4×12 i.e. 48 segments.

Each LCD Segment is electrically connected to one back plane and one segment line (although in reality these segments and back planes also interconnect, but we can ignore this for now as the clever multiplexing logic hides this from us), to derive if the segment will be ON (dark) or OFF (clear) we must determine the AC voltage applied to the segment.  The easiest way to do this is to numerically difference the waveforms and to calculate the RMS (root mean square) voltage; If the voltage is above the LCD threshold the segment will be dark.


I arbitrarily assign a segment number to the combination of segment line and back plane and calculate the RMS for that combination.  The RMS voltages seem to be either around 1v0 or 1v7 so we can imply the LCD threshold is somewhere in between (let’s assume 1v4).  We then calculate if the segment should be ON or OFF.

When the scale is powered on but idle it displays 0.00kg which is roughly 6×3+2 i.e. 20 segments dark…. and we get 20 1’s… looks like we are on the right track.


About nivagswerdna

Professional Geek
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