Adjusting a negative voltage with a PIC

[sanctuj]

I am using a graphic LCD that requires a negative voltage (approx. -10V) for the contrast control. I would like to be able to adjust it with a digital pot connected to a PIC, however, the digital pots that I have found will not work with the negative voltages. Anyone know of a way I can accomplish this?

Also, is there a way to monitor a negative voltage with the PICs ADC?

Thanks

 

[Dave#2]

Found this link the other day, might help:

https://www.electro-tech-online.com/custompdfs/2006/05/2001SEP03_AMD_MSD_AN2060PDF.pdf

Edit more info: I have not worked with digital pots much, so won't comment. But to shift a negative voltage up to where you can measure it, you could just use a resistor pullup between the ADC reference voltage and a zener diode's cathode. Connect the anode to the negative voltage to be measured. Whatever voltage you measure at the cathode is a zener voltage above the negative voltage.

 

[sanctuj]

Thanks for the reply. That pdf has what I am looking for.

Also, I will try the zener diode idea for measuring the negative voltage.

 

[justDIY]

Figure 1 worked very good for my smaller 2x20 lcd, which only needed around -3v ... but my large 2x40 "cold weather" lcd needed -8v, which the charge pump couldn't deliver.

I found the LM2682 to be a great solution:

**broken link removed**

It's a negative voltage doubler, taking your +5v and turning it into -10v ... nice and small, and works well with three inexpensive ceramic caps.