Display Contrastvoltage for 4*20

[Blubb]

Hallo,

i have problems to find the right resistor values for the temperature
kompensation.

The Range ist -20 to 70° and a voltage doubler is on board.

Can i use a pwm to set the contrast.

thank you

 

[ted]

Hi,

Your msg does not tell too much about the general configuration, type display, etc.

For sure, you can use PWM from a microcontroller to generate a contrast voltage for LCD. The power consumption is so small, that a simple RC filter between uC and LCD should be sufficient. Of course, the intendend contrast voltage range must be within the uC's GND and operating voltage window. There are ways to exceed that range: I have even once used a voltage tripler from uC PWM to a bias voltage (in that case it was a varicap, but the principle is very similar). Then the tricky part was that the voltage is not simply an average of ON/OFF ratio of the PWM output, when doubling/tripling etc.

Good luck

Ted

 

[Robby]

Contrast

Hello Blubb,

You only need 2 Restistors & 1 Capacities. Use an ordinary Portbit (or pwm-out). Place a RC between port and Contrast-Input & the second resistor as input burden to GND. The burden is for high impedance and against loading latchup (OV-Input). Take some experiments to find out the rules in pwm-clock RC-time-constant and burden. It'll flicker if the clock is too low! No need for double voltage or others!
The Values at my project: C -> 6.8µ, R -> 4.7k, Burdenresist -> 10k. The application consists of: Controller C164, LCD 4052 from D_E_N_S_I_T_R_O_N (standard controler), but it should work also with other components.
In this way you can control the Contrast completely by Software with a minimum in addition hardware.

greetings

Robby

 

[Blubb]

Hallo,

thank you for answering me.
My Problem ist not the RC - Filter. That works good.
My problem is to use a thermistor for temperature compensation.

the schematic are:

thre resistors in serie.
the resistor to GND are a NTC.
between the first and second resistor i use the
VEE voltage.

But i don´t know the right Values for the Offset.

I have a 20 * 4 display with kS0076 controller in
extendet temperature range

thankyou

 

[BENOIT]

To control the contrast of the LCD, you can do it with the micrcontroller:
and a digitally controlled potentiometer as the X9103 from Xicor.
There was an article with all the description a for 8051 (but can be
adapted to any CPU) in EDN Europe February 1999, Page 86. Surely
you should still be able to download the diagram &source code on the
ednmag website.
In the example, the X9103 allow you to regulate the contrast of a standard
Digital pot adjusts LCD's contrast
Jef Collin, CSE Systems, Turnhout, Belgium
You can use a digitally controlled potentiometer for many purposes. In this example, you can use the device to regulate the contrast of a standard (such as two lines by 40 characters) LCD. You can use the circuit in Figure 1 in a portable test system, in which you need to change the contrast of the LCD as a function of the viewing angle. You choose the contrast setup from a menu and then use up or down buttons with µC IC1 to adjust the contrast. The µC stores the contrast value in the digital potentiometer, IC2. This design uses a Xicor 10-k unit (dubbed "EEPOT"), but you could use other devices in the design.

The EEPOT connects to the LCD's VO line. You could connect the other side of the potentiometer to ground, but, for better contrast, you can apply a negative voltage to this terminal. This circuit has a serial interface, so it uses the negative-voltage generator in the MAX232 RS-232C/TTL converter. Listing 1 is the subroutine for the 8051 µC. The program calls the routines for contrast-up or -down. (DI #2314).

--SOURCE FOR 8051----- CUT HERE---------------------------------------
;********************************************
;* *
;* ROUTINES FOR EEPOT LCD CONTRAST CONTROL *
;* *
;* J COLLIN *
;* *
;********************************************



; *** BIT DEFINITIONS ***

EEPOT_INC BIT P1.4 ; EEPOT CONNECTIONS
EEPOT_UD BIT P1.5
EEPOT_CS BIT P1.6


; *** INITIAL SETTINGS ***

SETB EEPOT_CS
SETB EEPOT_INC
SETB EEPOT_UD



; *** UP CONTRAST ***

CONT_UP: NOP
SETB EEPOT_UD
SETB EEPOT_INC
NOP
CLR EEPOT_CS
NOP
CLR EEPOT_INC
NOP
SETB EEPOT_INC
NOP
NOP
CLR EEPOT_INC
NOP
SETB EEPOT_INC
NOP
SETB EEPOT_CS
RET



; *** DOWN CONTRAST ***

CONT_DWN: NOP
CLR EEPOT_UD
SETB EEPOT_INC
NOP
CLR EEPOT_CS
NOP
CLR EEPOT_INC
NOP
SETB EEPOT_INC
NOP
NOP
CLR EEPOT_INC
NOP
SETB EEPOT_INC
NOP
SETB EEPOT_CS
RET
------------------------------END OF CUT------------------------------------



On the other hand for the backlignht of the LCD there are two
ways of powering the backling :
one diagram would be a power by Current
the other would be power by Voltage.
in both case you have to find out the right resistor Value in
order to have a good backlight.
MB.