Switching LCD Backlight on and off

[gregmcc]

I'm busy fiddeling around with a microcontroller, keypad and lcd and have question about the lcd backlight.

everytime I press a key on the keypad I would like the lcd backlight to come on for say 2 seconds. if another key is pressed the light stays on. if no key is pressed within 2 secs the light goes off.

the problem is that i've run out of pins on the microcontroller so am looking for another way to do it - otherwise I could have used some coding to do it. When a keypad is pressed a buzzer makes a short beep.

Now my question - can I link the buzzer to some kind of timer (555?) to do the switching of the light on and off.

Can anyone offer any help - am I on the right track, or is there an easier way?

 

[phalanx]

You can use a 555 timer wired as a one-shot (monostable multivibrator). Set your pulse width to 2 seconds and fan the output pin from your controller to both the buzzer and the one-shot.

Every time your buzzer goes off, you will get a single pulse 2 seconds long.

 

[gregmcc]

If I press the keypad within the 2 seconds, will the output be extended by a further 2 secs.

 

[justDIY]

If I press the keypad within the 2 seconds, will the output be extended by a further 2 secs.

no, not with a basic monostable setup

can we see what the rest of your io lines are doing? maybe there's a way to optimise one of them, to free up a dedicated line for the backlight ... then you have the ability to control the backlight, without sounding the beeper, like turning the backlight on when it's dark out and stuff

 

[phalanx]

If I press the keypad within the 2 seconds, will the output be extended by a further 2 secs.

Oops, I overlooked the fact that you needed a retriggerable one-shot.

It just so happens that you can get dedicated retriggerable monostable multibrator (retriggerable one-shot) chips and they cost less than $1. If you go do Digikey, search for "monostable", click the multivibrators link, and under the features column select one of the "retriggerable" descriptions and then narrow your search. A number of possibilities will pop up.

 

[Oznog]

But basically putting in a 555 when you've got a uController is just heresy. Suggest getting a controller with more pins or try to find a way to save a pin elsewhere.

 

[gregmcc]

thanks for all the replies. I found a chip on digikey which might do the trick. Will try it out tonight and let you know.

At a later stage I might have to look at using another microcontroller more pins - a light that comes on after its dark is a good idea!