Electronic – Maxim 7219 – how do you adjust the current going into each segment of a seven segment display

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.

You need to do two separate things:

1. Measure the light level. This should be done with a light dependent resistor and an analog input of the PIC.

2. Adjust the brightness of the display. The simplest way I can think of is to reduce the duty cycle, the fraction of the time that the LEDs are on.
   You are already be switching quickly between the separate digits, so it should be a simple matter to turn them each on for an even shorter period of time. This will make them appear less bright.

Then you can link the two with a formula or algorithm, which you can develop by experiment.

A note on multiplexing:
You can generally get away with flickering an LED at perhaps 50 or 100 Hz, if it's for a normal range of brightness. But for very short duty cycles (and you will need to go very short to get a dim display, perhaps 1/256), even 100 Hz starts to look strange, the lights leave dotted streaks as you look around the room. So aim for something more like 1 kHz, to be sure it looks smooth even at low brightness.