Seeking replacement for obsolete LCD

[hgmjr]

We have an existing product that uses an LCD module part number PC2002-LRU-AWB-K-Q made by Powertip. This display is no longer available so I am trying to find a replacement.

The contrast voltage supplied to this Powertip LCD is under the control of an embedded microcontroller. The actual contrast voltage is nominally negative at around -1V. This voltage is adjusted by the microcontroller based on an input from a temperature sensing circuit. The problem that I am faced with is that I am not familiar enough with the general specification of LCD modules to be able to recognize one that accepts a negative input to its contrast control pin. So far the couple of modules I have selected turn out to expect a contrast voltage of 0 to 5 volts. When connected to our interface board, all of the pixels are on or dark. I experimented and found that by supplementing the contrast control from the interface board with a potentiometer connected between ground and +5 volts, I can then adjust the contrast to a voltage slightly positive to ground and all is well. The display reads normally.

I notice that Crystalfontz has a 2x20 display that looks mechanically compatible. However, I am uncertain that it would be able to display normally with a negative 1 volt on the contrast control input.

I should mention that this LCD module needs to be of the wide temperature variety with LED back-light, dark pixels on a yellow-green background, and 116mm by 37mm by 10mm.

Please feel free to request any additional information that I may have overlooked.

Thanks,
hgmjr

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[CF Tech]

On the character LCDs, (the CFAH2002A-YYH-JT for example) the Vlcd is 4.2 to 4.8 volts. so that ends up with 0.2v to 0.8v needed on the Vo pin for a Vcc of 5v (Vo = Vcc - Vlcd). So like the others, our display will be too dark if you connect it to -1v.

It must be that the old LCD needed a Vlcd of about 6v to justify the -1v your circuit feeds it.

Can you change your circuit to be adjustable between -1v and +2v? That would accommodate a lot of displays.

A possible "quick fix": The Vo pin pulls up. So if you can put a series resistor in line between your -1v output and the Vo pin input of the LCD you might be able to get it to work without other modifications. You would have to experiment to find a good value for the resistor.

Does that help at all?

 

[hgmjr]

On the character LCDs, (the CFAH2002A-YYH-JT for example) the Vlcd is 4.2 to 4.8 volts. so that ends up with 0.2v to 0.8v needed on the Vo pin for a Vcc of 5v (Vo = Vcc - Vlcd). So like the others, our display will be too dark if you connect it to -1v.

It must be that the old LCD needed a Vlcd of about 6v to justify the -1v your circuit feeds it.

Can you change your circuit to be adjustable between -1v and +2v? That would accommodate a lot of displays.

A possible "quick fix": The Vo pin pulls up. So if you can put a series resistor in line between your -1v output and the Vo pin input of the LCD you might be able to get it to work without other modifications. You would have to experiment to find a good value for the resistor.

Does that help at all?

Thanks for the suggestion. I had not considered the possibility of introducing a series resistor.

You are correct in your assessment that this contrast circuit was designed when there were many more LCD modules available that required a negative voltage on there contrast inputs.

I had considered a common-base BJT stage as a possible level translator but I was hoping to avoid that if I could. The series resistor is certainly a simple method that would be worth further thought.

Thanks for taking the time to ponder my question and provide some possible workarounds.

hgmjr

 

[CF Tech]

When you said "level translator" it reminded me of an even better solution than the resistor, just calculate what the difference in voltage should be and put in the right size of zener.

 

[hgmjr]

When you said "level translator" it reminded me of an even better solution than the resistor, just calculate what the difference in voltage should be and put in the right size of zener.

Or possibly due to the very small amount of voltage offset, perhaps a couple of silicon diodes will do the trick.

This would doubtless add a temperature variable into the mix but who knows maybe it will be in my favor.

hgmjr