How do I exchange LCD display with 7-segment LED's?

Question

I have bought a cheap radio controlled clock, and I want to make a hack that replaces the existing LCD display with 7-segment LED's. With my voltmeter in AC mode, I can measure voltages(with respect to battery minus) on the connectors to the LCD of 0.4V(presumably off) 1.4-1.6V(presumably on). I do not know the waveform or frequency of this voltage, but I suppose I can measure it, if I find an oscilloscope somewhere.

How do I convert this AC to a logic signal, that can be used to drive the 7-segment? Is there any standard regarding LCD drive voltages? Does there exist a single drive chip for this purpose? Do I need transistors, op-amps or a chip to make enough current?

UPDATE: There is absolutely no logic in the display itself - it is just a glass plate(with liquid crystal). I can even make the display turn on shortly with dc. All the connector seen on the picture drives the display directly.

Update (Wikipedia LCD) says:

Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field). Displays for a small number of individual digits and/or fixed symbols (as in digital watches and pocket calculators) can be implemented with independent electrodes for each segment.

Answer 1

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.

Answer 2

This may be more trouble than it's worth. The driver will probably put a ~30 Hz square wave on the backplane, and switch segments on by driving them with the inverse of that, and maybe even the in-phase signal when the segment is off.

In that case the segment will always get a signal and then Oli's solution doesn't work.

Even worse would be a multi-level drive; then the signal would look like this:

Not pretty. It may be easier to just use the receiver and replace the controller with your own. The WWVB/DCF77 receiver will be a separate IC, and most likely connected to the controller via three wires: ground, data (from receiver to microcontroller) and enable (from microcontroller to receiver). The enable is required because the receiver consumes a few mA, and would drain the battery too quickly if permanently on. On one radio controlled clock, powered by a single AA battery I measured 3 mA with the receiver on, and 7 µA with the receiver off.

The TI MSP430 family is known for its low power consumption. You'll have to write your own software, but the coding of both WWVB and DCF77 is kept very easy: the first decoders used common TTL logic rather than microcontrollers, which weren't available at the time. Microcontrollers can decode complex protocols, but with TTL you would need too many ICs, hence the simple coding and modulation format.

Answer 3

You could do with an oscilloscope to check the drive signal, but assuming it is just an LCD with no onboard drive IC, then it's likely to be driven with an AC voltage of between something like 50-500Hz.

To convert this for driving your 7-segment LED display, you could use a diode (may need a schottky if drive voltage is low) feeding an RC low pass filter (cut off < 30Hz. Something like 1kΩ and 10uF would do) to derive a DC voltage, then use this to drive the base of a transistor that controls the segment of your display (assuming common anode display with NPN on cathodes)

Answer 4

Just a guess, but it's not likely an AC signal, but either a parallel address / data bus or serial comms going to the display. Many LCDs use a 4bit or 8bit bus or SPI/I2C. I think you'll end up having to figure out what the LCD make is so that you can get the protocol. If it's a standard graphical controller, then you'll probably be able to get the protocol on the net. But then you have to write a command parser to figure out what's getting sent to the display, so that you can figure out how to light up the segments on your 7 segment display.

Answer 5

I suppose you could always try removing the reflector from the back surface of the LCD, and arrange photodetectors to scan the segments and copy them to an LED display. Or just do what Oil Glaser suggested, but with a trimpot. This way, you can enter a digit, and adjust the trimpot until the LED display correctly displays the digit you entered. Ima go ahead and try the latter first.

Answer 6

I have found that there are multiple backplanes, and that the voltages involved is stepping between several different levels. It is therefor extremely difficult to extract a 7-segment signal. Not only is amplification needed, but also a lot of logic testing the different voltages levels.

My best bet is to find a radio-controlled clock that already has a LED display. I have untill now been unsuccessful.