Question 16F84A & LED on LCD (See Schematics)

jahan · Joined: 04 Apr 2005 Posts: 63

Simple question that I cannot figure out.
I have 16F84A connected to and LCD and Keypad. (see schematics below)

Is there a simple way to turn ON the LED on LCD after first key entry,
and turn OFF the LED on LCD 2 seconds after the last key entry?

I have the keypad and the lcd part working OK. It does echo any entry from keypad to the LCD.
It would be nice to turn OFF the LED on LCD after 2 second inactivity.

Thankx

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Well first thing you had better do is put a resistor between the base and the pic.

Your task is quite simple. On each keypress, turn on the LED and start a timeout value. This value gets reset each time a key is pressed. If you don't press a key, the timer will eventually expire or exceed a limit and at that point, turn off the LED.

Guest · Posted: Thu Jul 14, 2005 12:27 pm

Possibly even more important than putting a resistor in series with the base lead of the transistor switch, is putting a current limiting resistor in series with the backlight LED line. Most panels do not include the current limit on the display. Without it the backlight will be very bright (and hot) for a short period of time. If you don't have the display data sheet try a 33 or 22 Ohm, half Watt resistor as a starting point for a 5V supply.

jahan · Joined: 04 Apr 2005 Posts: 63

jahan · Joined: 04 Apr 2005 Posts: 63

newguy · Joined: 24 Jun 2004 Posts: 1907

MikeValencia · Joined: 04 Aug 2004 Posts: 238 Location: Chicago

I think you're asking the same question on two parallel threads.

http://www.ccsinfo.com/forum/viewtopic.php?t=23163

Newguy is telling you to turn on the lcd in one isr, and to turn it off in another isr. As for me, i choose to set flags in an isr, and then take action out in my main loop if that flag is set.

Both methods work; there is no right or wrong place to turn the LCD on or off.

jahan · Joined: 04 Apr 2005 Posts: 63

newguy · Joined: 24 Jun 2004 Posts: 1907

Most LCD backlights have a forward voltage drop of about 3.5V - 4V or so. This means that the light emitting diodes that make up the backlight have been matrixed - usually two in series, and a whole bunch of these pairs paralled.

So, when you apply 5V across the backlight, the LEDs themselves account for about 4V of the drop, leaving you to drop the remaining 1V across a resistor. The purpose of this resistor is to limit the current through the LEDs so that they don't burn out, and don't get blindingly bright.

If you're shooting for about 50 mA through your backlight when it's on, then you need a series resistor of a size R = 1V/0.05A = 20 ohms.

You can adjust the brightness of your backlight by varying this series resistor. A smaller value will mean more current and a brighter display, and a higher value will mean the opposite.

The datasheet for your LCD should say something about the maximum backlight current and its' forward voltage drop. If you can't find the datasheet or these parameters if you do have the sheet, then you're probably safe to assume a 4V forward voltage drop and about 100 - 150 mA max through the backlight.

Hope this helps.

newguy · Joined: 24 Jun 2004 Posts: 1907

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA