Dot Matrix LCD contrast adjustment

old_cow_yellow

Some of the external Dot Matrix LCD Modules need a contrast adjustment negative voltage. I think we can use one of the Timers to generate PWM and convert that to a negative DC voltage. The contrast can be adjusted by varying the frequency and/or the duty cycle. Could any body with such a LCD try this scheme?

over 12 years ago

0 Ilmars over 12 years ago

Guru 46710 points

Consider following circuit too (figure 1):

http://e2e.ti.com/blogs_/b/powerhouse/archive/2013/10/01/powerlab-notes-don-t-be-negative-on-negative-supplies.aspx

0 old_cow_yellow over 12 years ago in reply to Ilmars

Guru 58965 points

Thanks. That circuit can supply more current and higher (more negative) voltage too.

For LCD contrast, we need a few uA at about -1V. Either way will work.

The LCD contrast is temperature dependent. MSP430 could sense the temperature and automatically adjust the contrast voltage

0 Ilmars over 12 years ago in reply to old_cow_yellow

Guru 46710 points

old_cow_yellow said:
For LCD contrast, we need a few uA at about -1V. Either way will work.

Your circuit for just LCD contrast indeed seems like best choice. And feasible. Thou careful selection of components & PWM freq is key. I guess left cap 10nF or even smaller, right one 100nF, small signal low drop diodes like BAT42. It would be necessary to add some load resistor (100..330kOhm?) for circuit - if you say that LCD contrast load is so small. Anyway only tests will show

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Dot Matrix LCD contrast adjustment