LCD c ontrast pin

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Hi guys/gals,

I have a Mega128 controlling a Hantronix 2X8 LCD. I was wondering how to control the LCD pin. When it it cold or hot, the LCD is unreadable. The problem is that I cannot use a trimpot because the user would have to open the case each time. Is there a rogue cheap way you guys do this? Perhaps a thermistor somehow in line with the divider gthat sets up he contrast voltage? I was thinking, if the Mega had a D/A, that would work, but it does not know the temp and I have no temp sensor on the board. Any idea's?

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Do you have the contrast pin connected somehow? How HOT or COLD does the lcd get? Is it within it's working range? I haven't heard of anyone having to adjust the contrast, it is usually set and forget.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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For normal temperature ranges, the LCD data sheet generally allows you to have a fixed voltage (viewing angle dependent).

For extended voltage ranges, I seem to remember a circuit that used a thermistor and a negative supply for a larger temperature range.

Harvey

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If an LCD display is used in an automotive environment say, you can see wide variations in temperature, e.g sunlight through the windscreen can produce ambient temps of over 50 degrees C.

I've seen designs which use extended temperature range lcd displays - these typically have a negative contrast voltage setting.

How about DAC control of the contrast voltage for a nice touch, then your users can adjust the contrast easily to taste.

Cheers
Robin

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If the contrast voltage is in the range of 0V to VCC then use AVR´s PWM and a lowpass filter. (simple RC)

Klaus
********************************
Look at: www.megausb.de (German)
********************************

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Right now I have a trimpot for testing, which will eventually have to get tossed. I tested at 0 - 50 degrees. At 0, scrolling was totally unreadable, which is fine since I will do static displays from now on to fix that. When it was hot at about 50 degrees, the background looked very dark making it hard to see the illuminated blocks of the LCD. I only use 5V supply. I just may have to use a DAC. Hmmm... Thanks for the replies.

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You might try a different brand of LCD that is "extended temperature".

[Hmmm--we have the Hantronix 2x8 in several products and I haven't had reports of those problems, but most are "indoor" apps and would not have that wide a temperature swing.]

Try Gary at http://www.newhavendisplay.com/

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Lee,

So you just have a trimmer that you set and forget? And that seems to be fine from 0-50? Hmmm.... Maybe I have another issue that I am delaing with.

Thanks for the info.

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Is your 5v supply still stable at 50 degrees ? If not it will cause a contrast change, though I'd expect brownout or 'magic smoke' loss problems too.

Slightly O/T - I've recently been playing with a 2x20 OLED display which I have to say looks lovely. Can't see me ever wanting to use a bog standard LCD again. Nice bright display and no backlight required.

Has anyone had any negative experiences with OLED displays ? (other than price of course !)

It would be interesting to try cooking this display to 50 degrees and see what happens... :twisted:

Cheers
Robin

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Quote:

So you just have a trimmer that you set and forget?

Yes.
Quote:

And that seems to be fine from 0-50?

Not exactly. They are indoor apps, so probably do not normally get either that cold or that hot.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.