Beware LCD's with ext temp range need more volts

I just got caught out by a 20x4 LCD display with "extended temperature range".

I wired it up and switched on confident that it would work as I'd done it before. However all I got was a backlit screen with nothing on the display regardless of what I did with the "contrast" pot.

After studying the spec a bit more I found that, although "normal" displays work with 0-5V on the contrast pot, this extended temperature range one needed -2.9V to 5V on the pot to work under all circumstances.

I had to run up a quick voltage inverter (1 resistor, 2 caps, 2 diodes) to generate -4.2V (a bit of drop due to the diode) and wire that in place of the 0V on the contrast pot. Then all was well.

I ran the voltage inverter from a PWM output - maybe I could have used a control pin on the LCD if it was running enough, but I haven't run out of pins yet so went for the easy route.