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PICList Thread
'Help -- need power on circuit for LCD'
1999\04\15@222254 by Brian Gracia

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face
Hi All,

I want to controll my 4x20 LCD from a pic.  When the pic has not received
any keyboard input in say 5 min., I want it to power down (completly) the
LCD.  Blanking the screen may save some power, but being able to turn the
LCD on and off at will would help to prolong the life of it.

Any suggestions?

Brian Gracia

1999\04\16@002239 by Corey Drechsler

picon face
Well, a PIC can supply a fair amount of current from their output pins
-  I know the 16C84 could supply about 25 mA, which is more than enough
for most LCD panels.  So, you could power the LCD from one of the pic's
output pins, and then you could turn the LCD on and off at will..

Corey Drechsler

1999\04\16@010404 by Des Bromilow

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You will need to run the power to the LCD via some kind of gate (logic level) and control that gate via the PIC or a timer.
Controlling it via a timer would be accomplished by having a retriggerable countdown based on the data lines. Once no data has appeared on the data line for a specified time.... turn off the power to the LCD.

The timer cct option would be a solution if you're running short of pins on the PIC

Does this help?
Des

>>> Brian Gracia <spam_OUTbgraciaTakeThisOuTspamRTRIPP.COM> 4/16/99 12:17:02 pm >>>
Hi All,

I want to controll my 4x20 LCD from a pic.  When the pic has not received
any keyboard input in say 5 min., I want it to power down (completly) the
LCD.  Blanking the screen may save some power, but being able to turn the
LCD on and off at will would help to prolong the life of it.

Any suggestions?

Brian Gracia

1999\04\16@043858 by Windows-1252?Q?Sebasti=E1n_Dols?=

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face
What about to use a TRN in non-linear-mode to interrupt the power of the
LCD? I think that the cost is only one PIC pin..
----
Hi All,

I want to controll my 4x20 LCD from a pic.  When the pic has not received
any keyboard input in say 5 min., I want it to power down (completly) the
LCD.  Blanking the screen may save some power, but being able to turn the
LCD on and off at will would help to prolong the life of it.

Any suggestions?

Brian Gracia

1999\04\16@084254 by Bernhard Kraft

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face
On Thu, 15 Apr 1999, Brian Gracia wrote:

>Hi All,
>
>I want to controll my 4x20 LCD from a pic.  When the pic has not received
>any keyboard input in say 5 min., I want it to power down (completly) the
>LCD.  Blanking the screen may save some power, but being able to turn the
>LCD on and off at will would help to prolong the life of it.
>
>Any suggestions?
>
>Brian Gracia
>

You could use a transistor, connected to the PIC to apply power to the
display. I prefer the following circuit :

             /----- Vdd
------|     |/
     |-----|
     |     |\          |-----------|
PIC   |      _\|---------| LCD +     |
     |                 |  LCD -    |
     |                 |-----------|
------                       |
                            |
                            |
                           ---

It's called "emitter folger" In German. You do not need any resistors
cause the base current couldn't become bug cause it has also to go through
the display.

The voltage on the LCD + pin depends on the voltage drop between the base
and the emitter. Usual the voltage drop is 0,7V so you have 4,3 V for the
display, I think this should be enough. The current you can draw depends
on the current the PIC can supply (about 20mA).
The current you can draw is the maximal base current (20mA from the PIC)
multiplied by the current gain of the transistor (for normal transistors
between  100 and 300) so you could at least draw
I = 20mA * 100 = 2000mA = 2A !!!
                        =======
2 Ampere ! But i don't know how well it would work if you take such a
large current. If you draw more than 100mA you would need a transistor in
a TO200 case, because most of the "small" ones in the TO92 case have a
maximum collector to emitter current of 100mA


mfg.
===================================================================
Kraft Bernhard aka. Krufti              /"\
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                                       / \
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1999\04\16@104134 by mwalsh

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Brian Gracia wrote:

> Hi All,
>
> I want to controll my 4x20 LCD from a pic.  When the pic has not received
> any keyboard input in say 5 min., I want it to power down (completly) the
> LCD.  Blanking the screen may save some power, but being able to turn the
> LCD on and off at will would help to prolong the life of it.
>
> Any suggestions?
>
> Brian Gracia

I use a P-Channel MOSFET with the source connected to Vcc to control the
power
to an LCD in one of our battery powered products. The lower the rds(on) the
better.
Just remember to either tristate or write lows to all of the signal lines
when the power
is switched off or the LCD will remain powered through the signal lines.

Mark Walsh

1999\04\16@105553 by Justin Crooks

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Your LCD may use less than 20 mA(peak).  If it does, just supply power from
the PIC.

----------
{Quote hidden}

1999\04\16@112035 by Barry King

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face
Brian,

My LCD only needs < 10 mA at 5 V.  So I just power my LCD from a port
pin directly.  To turn it on, set the port latch to 1 and then set
TRIS bit for that pin to 0 (enable output).  PIC I/O pins can source
the 10 or so mA needed for the LCD with only a 0.1 V or so drop, so
you get 4.9 V on a Vdd of 5 V.

When turning off the LCD, Set TRIS to 1. This turns off the output
driver, turning the LCD off.  Don't just set the port latch to 0,
because that asks the PIC slam the LCD power pin low, as hard as it
can, which is not needed and may cause noise problems.

I recommend that any time you use a PIC pin as a power on / off
supply, you use TRIS to control it, because any reactance of the load
(usually capacitive) will cause trouble when powering off.  You
will get noise on the chip when turning the load on, but that can't
be helped.

BTW, Berhards suggestion might work if the load is more than the PIC
pin can handle.  But, there is another way, using a PNP transistor
(like 2N2906):
                    Power
        |             |
        |             /
        |          ||/_emitter
PIC pin  |-/\/\ ----|
(output  |      base|\  collector
0       |            \---------
for ON) |                     |
        |                    load
                              |
                             ground

In this circuit, the PIC drives the base low and can saturate the
transistor so the power lost in the transistor is low.  The
disadvantages are that you need the resistor, and that the base
current is "wasted", that is, it does not contribute to the load
current.

But if the saturation hfe of the transistor is high enough,
you only need a tiny base current.  The resistor from the PIC pin
sets this current.  Check the load curves for your transistor to be
sure what the minimum hfe is, and the maximum base current needed,
Ib = load current / hfe. Then get the resistor value from (Vdd - 0.7
V)/ Ib.

Example: For a 5 V power supply, to supply 100 mA, if hfe (min) =
100, then Ib = 1mA, so resistor is (5 - 0.7)/1 mA = 4300 Ohms.
Standard values are 4700 or 3300.   It probably best to overkill by
using the smaller resistor, 3300 Ohms.

In the emitter follower, the transistor is in linear operation, it's
dissipating Vce (voltage drop) x Ie (load current).  So calculate for
your load which is better efficiency.

So its a classic engineering tradeoff- do you want low power
dissipation in the transistor, and small voltage drop?  OR do you
want highest efficiency?

Hope this helps.

Barry.
------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
.....barryKILLspamspam.....nrgsystems.com
"The witty saying has been deleted due to limited EPROM space"

1999\04\16@200108 by Wagner Lipnharski

picon face
20mA? what kind of LCD is that? Optrex units up to 4 x 44780 power
consume is less than 2mA. It is common to see it around 500uA (0.5mA)...
Are you sure is not your way to supply Vo (contrast adjustment voltage)
that is not so well dimensioned and it is consuming a lot of current?
Wagner.

Justin Crooks wrote:
>
> Your LCD may use less than 20 mA(peak).  If it does, just supply power from
> the PIC.
>

1999\04\20@162020 by John Payson

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|I want to controll my 4x20 LCD from a pic.  When the pic has not received
|any keyboard input in say 5 min., I want it to power down (completly) the
|LCD.  Blanking the screen may save some power, but being able to turn the
|LCD on and off at will would help to prolong the life of it.

|Any suggestions?

Sure.  On many LCD modules I've seen, a large portion of the display
current is used up in the bias resistors.  If there is no VEE (i.e. it's
allowed to float to VDD), there will be no display and current consump-
tion will be quite low.

What I'd probably suggest doing would be to wire a port pin (e.g. RA4)
to a moderate-sized cap to ground (e.g. 1uF) through a 220ohm resistor,
and then tie the cap to VEE.

Then, if you set RA4 to output low, the display's contrast will rise to
maximum while if you float RA4 it will go to blank.  If you PWM port pin
RA4 between "low" and "float", then you'll be able to adjust the display
contrast at will.

Cute, eh?

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