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Use an LCD IC to drive led matrix.

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Gigillo74

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led matrix driver

Hi guys,
i need to interface the attached ST7066Uv22 with a led matrix instead of a common LCD segment.
Can I drive a led matrix with this IC with a trasistor that interface the low power of the IC with the led?
Le led matrix is organized in row and coloumn. Can i drive them as page 40 of the datasheet using the transistor? How i connect the transistor to the led with the necessary 5V supply?
Hoping i'm clear.
Thank You.
Luigi.
 

betwixt

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led matrix driver ic

Unfortunately, no it will not work.

LCDs are driven with AC to prevent the electrodes migrating. LEDs have to be driven with DC. If you look at the drive waveforms in the data sheet you will see that between the backplane drive and segment drive, the voltage constantly reverses.

Even if it would work, you would have to add additional driver circuits as the chip can only deliver a few microamps of drive current.

Brian.
 

Gigillo74

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how is the lcd driven

betwixt said:
Unfortunately, no it will not work.
.....Even if it would work, you would have to add additional driver circuits as the chip can only deliver a few microamps of drive current.
Brian.
It is what i need, a driver circuit.
How can i build it with transistor?
Thank You.
 

Gigillo74

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led matrix high side driver ics

I don't think it will be suitable for my application.... i need to build a 16x02 module like LCD with the LCD controller but driving LED instead LCD segments.. ;-) The final result is a big LCD module with LEDS!!
So i need to interface the out of any row and column of the IC with the column and the row of my led matrix.
 

Gigillo74

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lcd matrix controller ic

Hi blueroomelectronics,
the solution with max6952 is an optimal solution, and new for me.
Maybe, for me is not suitable because it is not compatible with HD44780 based display and need a custom library to buid to use with a micocontroller.
I've no problem to use microcontroller.

A 5x7 16x2 matrix will need 1,120 LEDs or 32 5x7 displays.
I know!! there are more leds, but i've just them to test one display unit.

If you can tell me only how are the state of the two pin of the IC to put visible an LCD segment, i arrange a level interface for the leds.
Thank you.
 

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how to drive led matrix

The LCD drive signal is not simple.

Basically, one of the signals, usually the backplane for the character, is a square wave.

When a pixel is 'off' the signal to it is also a square wave and is in phase. The two signals, although going from 0V to the full drive voltage, are identical so across them nothing can be measured and nothing activates the LCD.

When a pixel is turned 'on' the signal to it is inverted so it is out of phase with the backplane. Now the voltage measured between the backplane and the pixel is reversing on each cycle of the square wave, it is AC. As the backplane goes high, the pixel goes low and vice versa.

You can run LCDs on DC, which is what you need for your LEDs but the electrodes quickly electro-plate to one side, hence using AC.

Although I wouldn't recommend it, you can recover the signals you want by using XOR logic gates. If you put the backplane signal into one input and the pixel drive into the other, the output will be '1' if the pixel is on and '0' if it is off. You need an awful lot of XOR gates though!

A better solution is to emulate the LCD controller in software and use multiplexed drives as suggested earlier.

Brian.
 

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ic in drives

Hi Brian,
you are clear in your reply.
Sorry, but what appened if i use a diode on AC voltage? it turn on only in 1/2 wave, isn't it?
And in this case, the led will turn ON for a time that is 1/2 of the original time. Is true?
 

betwixt

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lcd interfacing using ics

It would be very difficult using a diode for three reasons:

1. The voltage is AC between the backplane output and the pixel output. If you rectify either one alone they will always have the same voltage on them.

2. You would have to add a capacitor to filter the DC pulses and the IC would not be capable of providing enough current to charge it.

3. Because the pixel scanning is so fast, (KHz) there would be no time to charge the capacitor.

The LCD controller typically only draws 150uA and it would have to supply all the hundreds of transistors form that.

I really think you are going nowhere fast following this idea. The problem is not difficult but the LCD controller is not the right starting place to drive LEDs.

Download the data sheet for the ST7066U LCD controller which has the LCD waveforms on it, you will see it is not easy.

Brian.
 

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how to run a led array

I've seen the datasheet and i can operate with two value for the different voltage that LCD uses. All i need is to connect the pin V1,V2,V3 and V4 to VCC and V5 to GND throught a variable resistor.
 

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