LCD Backlight Proper Wiring

[DavyJones]

What would the proper way to wire the LCD backlight to a port and setting that port so that it comes on. Currently I wired the backlight to +5v on the plus side and the minus side of the LCD backlight to portd.0. The LCD backlight is an LED. So to turn the backlight on I set LOW PORTD.0 which I understand and to turn it off I set it HIGH. This method puts +5vdc on the LED at all times. Of course it does not come on until I set portd.0 low which is understandable. I'm just trying to figure if this is standard way to hook something like this up. Can a variable be set to HIGH to end this confusion?
I'm sure this is a dumb simple question but I'll ask anyway.
Example:
_OFF = High
_ON = LOW
portd.0=_on
Thanks
David

[Archangel]

What would the proper way to wire the LCD backlight to a port and setting that port so that it comes on. Currently I wired the backlight to +5v on the plus side and the minus side of the LCD backlight to portd.0. The LCD backlight is an LED. So to turn the backlight on I set LOW PORTD.0 which I understand and to turn it off I set it HIGH. This method puts +5vdc on the LED at all times. Of course it does not come on until I set portd.0 low which is understandable. I'm just trying to figure if this is standard way to hook something like this up. Can a variable be set to HIGH to end this confusion?
I'm sure this is a dumb simple question but I'll ask anyway.
Example:
_OFF = High
_ON = LOW
portd.0=_on
Thanks
David

Hi David,
It is not uncommon for devices to be "hot" and the controller provides ground, usually through a transistor (I am thinking back to my days as a mechanic) even those pretty excuse me lights on the top of opieDog's G ride are done that way. What you can do rather than using PORT HIGH to turn it off is tristate the port by TRIS 1 . The downside to this is you now have a noise antenna, so use plenty of bypass filters, caps, choke . . .

[Melanie]

The way you have it wired is fine... providing you have a current limiting Resistor in series somewhere.


Backlight var PortD.0

LightON con 0
LightOFF con 1

.. ..

Backlight=LightON

[Darrel Taylor]

Some of the LCD backlights I've come across will draw 100ma or more depending on brightness. Big 4x20 displays.

You might want a transistor in there, since the PIC pin can only sink 25ma.
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[DavyJones]

Thanks everyone.
Odd the specs don't say what ma the backlight will draw/require or I am blind, probably blind....

It just says typical voltage 4.2 to 4.6v

Without knowing the current it will draw what would you suggest for a limiting resistor value?
thanks
David

[DavyJones]

I just got off the phone with New Haven display and they said that it is typically going to draw between 120 and 150ma for this backlight. He also said that on the back if R7 has a resistor then I don't need a current limiting resistor just hook up 5vdc to it and I will be fine. So I guess that answers that?
Thanks
David