howto switch off backlight for TFT Display?

Hello,

maybe someone can help me:

I have a small panel pc here which runs Linux 2.4.20. I use QT/E. I
want, that the backight of the display switches off after a certain time
while the touch screen is not touched.

I don't know, at which place this should happen:

- is it a kernel function
- has the window manager to do this

And:

- Do I have to program something new or is there a function for
switching off usually implemented somewhere, e.g. the framebuffer driver?

Thank you for your help

Ernst