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Hi guys,   I’m interfacing with a LCD 2×16 display for my Final Degree Project and I need a way to control the contrast voltage of the LCD. Contrast voltage range of a typical LCD 2×16 display is  between 5V and GND. A typical voltage value for the contrast is 0.5V so… how can we achieve this value ? Simply, always there are two ways, the “easy” way and the “pro” way !

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The “easy” way usually consists in a potentiometer connected to 5V and GND with the middle pin as the output variable voltage.

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The “pro” way consists in a low pass filter connected to a microcontroller pin pwm. The contrast voltage value is selected changing the duty cicle of the pwm. With a 50KHz frequency output we can select a resistor of 15k and a capacitor of 100nF and check it with a LTspice simulation.

To obtain 0.5V output we have to set a duty cicle of 10%, Because 5V*0.1 = 0.5V. The output with this duty cicle is:

simulation

 

In this simulation we can see that the output is approximately 0.5V . We cam see that the output waveform takes 4.5 ms to get the desired value, this time is aproximately 3*R*C . To calculate the RC filter cutoff is necessary to do the next formula:

Freq = 1/(2*pi*R*C)  = 1/(2*pi*15k*100n) = 106 Hz.

This formula say to us that at 106 Hz the AC signal will be reduced by 3dB. We can check it in the bode diagram:

Bode

 

We can see that for small frequencies we get the same value at the output ( DC frequency value is 0 ) and for high frequencies we get smaller values from input to output, when this happens we can say that this is a RC Low pass filter. To obtain a DC output from pwm we have just to change the duty cicle of the 50Khz signal. For example .. with 90% duty cicle we get 5V*0.9 = 4.5V DC at output.

Comment ! see you in the next post !

One thought on “Low Pass Filter to Control LCD Contrast

  1. As you can see, this seems to be working but the display is all flickery. The reason for this is that the display expects a fixed voltage but instead it gets a PWM signal from the Arduino. To fix this we add a very simple low pass RC filter so the output voltage can be filtered and we get a relatively stable output.

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