LCD contrast control

Discussion in 'The Projects Forum' started by maxthestork, Nov 1, 2012.

  1. maxthestork

    Thread Starter New Member

    Oct 13, 2011
    Hi All,

    The basic thing I am doing is to create an electronically controlled negative voltage for the contrast of an LCD screen.

    The -5V comes from the screen and the screen contrast is set by a negative voltage that is usually around -3.5V. This is usually set by a voltage divider or pot from the -5V.

    I would like to vary the contrast under software control. The PWM signal is from a MCU. The PWM signal is 0 to 3.3V.

    As I understand it, using a P channel MOSFET with a gate to source threshold of around 4V will allow me to turn it on when the output of the MCU is off (0V).

    When the PWM signal is high (3.3V), the gate to source voltage is -1.7V and so the MOSFET output will be off and the V_contrast will be -5V.

    Can anyone see anything glaringly dumb and naive about this?

    I could increase the resistance connected to the -5V to allow the bleeding to -5V to be slower?

    Thanks in advance,