Circuit for Interfacing LCD

[gauravkothari23]

Hi all...
i want to interface 16X2 Character LCD with microcontroller, and some other peripherals, and will be operated using 4 x AA battery.
My problem is when the battery is full and voltage around 6.4V the brightness of LCD is also good and very bright and contents can be viewed very easily, but as the battery starts draining and voltage level reaches 4V the brightness gets completely dull, so again i have to adjust the brightness from brightness pin of LCD, which is not reliable in my case. so i need the solution where i can clearly view the LCD in all voltage conditions.

 

[KlausST]

Hi,

The datasheet should tell you. --> read the datasheet.
Also read about valid VCC range.

Most displays need a contrast voltage (VEE) that is related to +supply (VCC) instead of ground (GND)
In this case you need to keep (VCC - VEE) constant.
You may build an analog circuit to keep it constant.
With low VCC it may be (datasheet) that VEE needs to be negative.
Then a capacitive voltage inverter circuit should work.

********
I usually use stable VCC.
And I usually use the microcontroller PWM to adjust VEE voltage. Thus I'm able to compensate for diplay production tolerance and temperature (contrast voltage depends on temperature).

Klaus

 

[gauravkothari23]

Hi,

The datasheet should tell you. --> read the datasheet.
Also read about valid VCC range.

Most displays need a contrast voltage (VEE) that is related to +supply (VCC) instead of ground (GND)
In this case you need to keep (VCC - VEE) constant.
You may build an analog circuit to keep it constant.
With low VCC it may be (datasheet) that VEE needs to be negative.
Then a capacitive voltage inverter circuit should work.

********
I usually use stable VCC.
And I usually use the microcontroller PWM to adjust VEE voltage. Thus I'm able to compensate for diplay production tolerance and temperature (contrast voltage depends on temperature).

Klaus

Thanks KlausSt
i have checked that when voltage at pin 3 of LCD or contrast pin is approx 0.18V the LCD brightness is absouletly clear and can easily viewed. so am planning to add AMS-1117 1.5V Voltage regulator and then adding a voltage divider to get 0.18V at pin 3. so that at any voltage between 4V to 6.5V the contrast or brightness would not change.

 

[bobTPH]

Is 0.18V the difference between Vcc and Vee, or are you measuring it referenced to ground? As Klaus said, Vee is referenced to Vcc, not to ground.

Bob

 

[dreamlover]

For LCD:
Pin three (3) is Vee and is the difference in voltage between VCC (+Ve) and VSS (Ground). This LCD voltage is adjusted to provide the sharpest contrast. Vee must be lower then Vcc and higher then ground

 

[KlausST]

Hi,

Vee must be lower then Vcc and higher then ground

For some 3V3 operated LCDs VEE need to go negative.


From post#2:

In this case you need to keep (VCC - VEE) constant.

Exampke: Let's say you need a contrast voltage of 4.2V.
And contrast voltage = Vcc - Vee,
Then for VCC = 5V: Vee = 5V - 4.2V = 0.8V
For VCC = 3.3V: Vee = 3,3V - 4.2V = -0.9V (= negative = below GND)

Klaus

 

[niciki]

Think about VCC, VEE and GND as the potentials and not the voltage.
The voltage is the difference between potentials, but it doesn't tell us what potential is the reference (the common reference potential is GND which is not the case this time).

In this case, whatever the potential of GND, you need to measure the voltage between VCC and VEE only.