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At this time I have an lcd where I hook up a 1k resistor between contrast and ground. As the battery voltage drops, the text on the screen also fades. I could make a manual switch and switch between resistor values to compensate but I want something more automatic. My lcd is a stanndard 5v hd44780 display. Could I get away with a zener in which cathode connects to 5v and anode to contrast pin with resistor in parallel? If so, what values should I use for the resistors and diode connected to lcd contrast pin for optimal display output?

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  • \$\begingroup\$ A datasheet and schematic would improve your chances of getting good answers. \$\endgroup\$ – Transistor Jan 15 '18 at 20:05
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To straight up answer your question we would need a schematic of your existing setup. As an initial guess I would say that a zener in parallel with a resistor is not a suitable method. Zener diodes require a minimum amount of current to operate correctly and this needs to be factored in with the amount of current that the contrast pin can source, etc... It gets way too complicated for getting such a simple LCD display to function correctly.

The HD44780 LCD display can accept a voltage between 0V and 5V (inclusive) at the Vo pin to control the contrast of the display. The optimum voltage value is somewhere around 0.5V. There are a few methods of making this work:

1) Simply connecting the contrast pin to GND gives a workable display on the HD44780.

2) One resistor between Vo and GND as you have described. This does not allow you to adjust the contrast without swapping it out for a different value resistor however.

3) 10k potentiometer between the 5V rail and the Vo pin is a very popular method as it allows you to fine tune the contrast of the display. I suggest this method.

4) A PWM signal from a microcontroller (Arduino is the easiest method) or 555 timer connected to the contrast pin through a RC low pass filter to smooth out the changing voltage and give you a DC voltage between 0 and 5V.

Keep in mind that if you are using an Arduino (very common with the HD44780) then the 5V rail will not be affected by the battery level you are using to power it. The 5V an arduino produces is internally regulated until the battery level drops below ~4V.

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  • \$\begingroup\$ My circuit does run on a regulated 5V source (LM2940 IC) but I'm looking for a solution for low battery. So if the battery is ready to die, I want the display to tell the user such a message without dimming. I'm aware of method 3 which is what I want to avoid because I'm making a product for someone and I don't expect them to turn a potentiometer each time the battery is almost dead. \$\endgroup\$ – user152879 Jan 16 '18 at 0:22
  • \$\begingroup\$ If you are worried about the contrast setting at very low battery you might as well worry about the temperature variations as well and how they will affect your display. The classic LM334 constant current source would be a good solution to your problem as it operates from 1-40V and counters any variations in power supply/temperature. It is also ~25% more efficient than a resistor. Check this out: edn.com/Pdf/ViewPdf?contentItemId=4427215 \$\endgroup\$ – user174867 Jan 16 '18 at 0:42
  • \$\begingroup\$ "if the battery is ready to die, I want the display to tell the user such a message without dimming." - what battery are you using, and what is its minimum operating voltage? Note: if its supply voltage drops below ~4.5V the LCD display may need a negative 'contrast' voltage. \$\endgroup\$ – Bruce Abbott Jan 16 '18 at 3:55
  • \$\begingroup\$ Well, until recently, my only option for a battery is one you can pick up anywhere (example: standard alkaline AA's). but I might be able to mitigate my issue already because I did receive a 7.2V NIMH RC car battery rated for 6800mah. Hopefully I can get lucky. \$\endgroup\$ – user152879 Jan 17 '18 at 20:45

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