Electronic – low power displays

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.

1. They are common-anode, so charlieplexing is out.

2. Multiplexing does not reduce power consumption, it only (may) simplify the drivers. You only have two displays, so you may as well not bother and drive them statically. The ratio of peak to maximum current is very low on these displays (only 1.5:1 for 10% duty cycle).

3. There are two parallel strings of 6 series LEDs for each segment, as shown the the datasheet. The decimal point has one fewer and apparently a resistor (not as shown). The voltage is for all LEDs in series. Since that voltage is around 12V you should use a higher voltage supply such as 15V or maybe a 19V laptop adapter.

4. You can use 2x ULN2003A drivers along with the 2x 74HC595s and 14 resistors to control the displays. The ULN2003A drivers will drop around 0.8V at 15mA, so you can find the resistors as around 150 ohms with a 15V supply or about 430 ohms with a 19V supply. If you use the decimal point be prepared to change its resistor value to match the brightness, and you'll also want to use the ULN2803A which has 8 drivers per chip rather than 7.

5. 20mA is the absolute maximum current at 25°C so don't attempt to run them at that current unless they are going to live in a refrigerator.