Contrast Circuit (PC Engine LT)

The contrast circuit generates the colors on the screen. This is different to the backlight circuit which is a high voltage circuit with a single purpose to drive a fluorescent tube.

The contrast circuit is an adjustable negative DC voltage that changes as the user turns the BRIGHT wheel.

Most people think the BRIGHT wheel controls the backlight, but it doesn't, that is always solid. The wheel controls the twist in the pixels of the screen allowing more or less of the backlight through.

The contrast wheel has 4 pins. I believe one isn't connected/used, two are 5V (although not directly connected to the 5V rail), and one is the adjustable voltage between 4.5V and 4.8V.