Electronic – 7-segment output: 4-6V, LCD input: 3-5V. How to convert

A simple and not very energy-efficient approach for driving two or three backplanes using 3 I/O pins is to wire each backplane to a different pair of I/O pins using two matched resistors (so one plane would connect to 0 and 1, one would connect to 1 and 2, and the third would connect to 0 and 2). Driving two pins one way and the remaining pin the other will cause one backplane to be driven to VDD or VSS while the others are driven to VDD/2 volts. Driving the segment wires high or low will activate or deactivate the corresponding segments on the backplane which is at VDD or VSS, but will not affect the segments on the other backplanes (which will receive half voltage regardless). Cycle through the six interesting combinations of backplane I/O voltages and you can put whatever you want on all three displays. Or, if you just have two backplanes, you may skip the parts of the cycle that would drive the third.

Note that you may discover that even the half-energized parts of the display are too dark. If that is a problem, you can spend a portion of each cycle with all backplane and segment wires driven low. If you make that portion variable, it will be possible to implement display contrast control entirely in software.

It is way too late for this I know, but there might be someone trying to convert AC signals for LCD into TTL signals to drive 7-segment LED. I have come across this issue when my boiler controller LCD lost so many segments that it was difficult to read the temperature and settings. I have studied the protocols and signals levels driving LCD display and I think I have come up with a way to convert signals.

First of all, you need to know how many segments are driven by one signal line and how many 'common' signals you have. Also, you need to know what combination of segment signal and common signal turns on/off segment on the LCD. This can be determined with multi-meter or even better with scope. Your original LCD driver must be functional and you should be able to drive the display (too much writing how to do it, but it is simple process).

Now, the circuit to convert LCD signals to LED (TTL) is not going to be simple as it was suggested above. The segment is ON when the differential voltage across common signal and segment signal is more than 2/3 of supply voltage of the driver. This could be only for 2ms or less. The segment is OFF if this differential voltage is less than 1/3 of supply voltage. This is simple - right?

Now, you need to capture this pulse, hold it long enough (latch it) and output this to the respective LED segment. You need to remember, that you need to detect differential signal between common line and segment line. I think this is it.

I am not expecting that anyone will post here any more and I am not expecting anyone to try building this. It is not worth it unless you are desperate or have too much time on your hands. I certainly have not built it.