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Contrast Control for LCDs Schematic Circuit Diagram

The usual contrast adjustment for an LC-Display involves a 10-k potentiometer. This method works well under stable power supply voltage conditions. However, if the voltage fluctuates, as is the case with battery power, the potentiometer needs frequent readjustment, making it inconvenient. The circuit explained here provides a solution to this issue. The mentioned potentiometer is designed to maintain a consistent current flow from the contrast connection (typically pin 3 or Vo) to ground. For instance, a common green display with 2×16 characters requires approximately 200 μA. Additionally, at a 5 V power supply voltage, there is an extra current of 500 μA flowing through the potentiometer itself.

Contrast Control for LCDs Schematic Circuit Diagram

Introduction to Energy-Efficient Design

Previously, energy efficiency was not a strong point of electronic circuits. However, the introduction of the LM334 IC revolutionized this aspect. When paired with a single resistor, it functions as a constant current source. This circuit guarantees a consistent current of 200 μA to ground, regardless of the power supply voltage. Adjustments are possible by replacing R1 with a 2.2-kΩ potentiometer, allowing the current to be tailored as needed. The calculation for R1 is straightforward, considering temperature and current values.

Calculating and Adjusting Current: Practical Implementation

The formula for R1, R1 = 227×10^-6 x T / I, is vital, where T represents the temperature in Kelvin and I stands for the current in amperes. In a practical scenario, substituting the appropriate values yields a specific resistance. Noteworthy is the LM334’s current dependency on temperature, although this isn’t necessarily a concern, especially with slight temperature fluctuations. This relationship doesn’t have to be strictly linear, ensuring flexibility in real-world applications.

Significant Power Savings: Impact on Battery-Powered Devices

Implementing this circuit results in a remarkable 25% power saving, especially with LCDs consuming 1.2 mA. For battery-powered devices, this energy conservation is highly advantageous. Furthermore, the circuit eliminates the need for contrast adjustments, remaining stable even as the battery voltage decreases. Although, for newer LCD technologies like OLED and PLED, thorough testing is recommended to ascertain the circuit’s suitability for brightness adjustments, ensuring optimal functionality.


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