The circuit on a breadboard for test

Most of you probably know the popular HD44780 character LCD controller from Hitachi. Usually you find it on LCD display boards with 16 pins. One of these pins is called "Contrast Adjustment" and does exactly this: It adjusts the LCD contrast. This pin requires a voltage level. If a microcontrollers has a Digital to Analog module, then this pin can be directly interfaced with this module. But the D/A module is not very common, instead, the PWM module is.

In such applications, the backlit is usually done with LEDs, so a simple PWM driver can directly adjust the brightness. Unfortunately, the PWM module cannot be directly used to adjust the contrast. So, i made a very simple and small circuit to interface the PWM output of a microcontroller to the contrast adjustment pin of an HD44780 Character LCD.

Here is the circuit schematic: (click to enlarge)

There is a red rectangle at the bottom right corner of the schematic. This is the interesting part. The rest of the circuit (the PIC along with the LCD) is only added to demonstrate the circuit operation. The PIC initializes the LCD and then sends a simple string to be displayed. The potentiometer R1 is used to generate a control voltage from 0 to 5 volts, and it is directly interfaced to an analog input of the PIC. The PIC then translates this control voltage into a binary number with the internal A/D module. Finally, it generates a PWM signal with a duty cycle from 0 to 100% according to the analog control value taken from the R1 potentiometer.

The PWM output from the PIC (RB3) is then driven to the interface circuit. This circuit is composed by a transistor amplifier (T1) and a low-pass filter R3/R4 - C1. This low pass filter will convert the PWM signal into a voltage level. This voltage level is higher when the PWM duty cycle is closer to 0%, and lower when the duty cycle is closer to 100%. This signal can be directly used for the contrast adjustment of the LCD.

PWM LCD Contrast Adjustment - Assembly listing - V1.0

PWM LCD Contrast Adjustment - Hex file - V1.0

Resistors
R1	Potentiometer 10 KOhm Linear Rotary 1/2W
R2	Resistor 22 KOhm 1/4 Watt 5% Carbon Film
R3	Resistor 4.7 KOhm 1/4 Watt 5% Carbon Film
R4	Resistor 680 Ohm 1/4 Watt 5% Carbon Film
Capacitors
C1	Electrolytic Capacitor 47 uF 16 Volts
Transistors
T1	BC548 Switching and Applications NPN Epitaxial Transistor
Integrated Circuits
IC1	PIC16F88 Microcontroller
Misc
LCD1	LCD 16x2 Character LCD w/ HD44780U1 controller

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