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created: 11 Feb 2013 05:25
tags: 3.2" geeetech lcd module screen tft
The backlight for the geeetech HY-TFT320 3.2" TFT LCD Screen module can't be controlled through software programming. Therefore hardware modification is our only way.
Here is a simple guide on how to incorporate a hardware backlight control for the GETech 3.2" TFT LCD Screen module
Normally, we connect the LCD (pin 2 LED_A) to 5V to turn the backlight on.
The amount of current that flows from 5V pin to LEDA pin is approximately 130mA.
The Atmega 2560 chip (Mega ADK Board) has a maximum output current of 40mA for each of its I/O pin. Therefore, we can't just directly connect it to the backlight pin (LED_A) for electronic switching.
Atmega Maximum Ratings* Datasheet: Pg.367
So, if we want to control it electronically we need to use a transistor that is capable of switching currents greater than 130mA.
The BC337 Transistor has a maximum collector current (Ic) of 500mA which suits fine for the job.
BC337 Transistor Limiting Values: Pg. 2
The component will now be connected in this manner:
As the schematic diagram above shows, we need to get the value for the base resistor (Rb) for the transistor. We can do that by following the transistor current equation.
Base Current (Ib) = Collector Current (Ic) / Beta (Hfe)
Where:
- Beta (Hfe) can be found in the BC337 datasheet = 100
- Collector Current (Ic) = 130mA
- Base Current (Ib) = Unknown.
Following the transistor current equation
Base Current (Ib) = Collector Current (Ic) / Beta (Hfe)
Base Current (Ib) = 130 mA / 100
Base Current (Ib) = 1.3 mA
Now that we have the Base Current, we can now get the Base Resistor (Rb). To solve of the Base Resistor (Rb) value, we'll need to apply basic Ohm's law where:
Resistance (R) = Voltage (V) / Current (I)
Resistance (R) = ( 5V - 0.7V ) / 1.3 mA
Resistance (R) = 3300 Ohms or 3.3 kOhms
*Note that the 0.7V is the voltage drop for the transistor which is standard for solving the base current.
Now because of the Base Resistor, the digital output pin of the Arduino board will only produce 1.3mA which is totally fine!
We can also incorporate a PWM on the output pin to produce a fading effect.
Dimming effect on TFT Screen
Links:
Atmega2560 Datasheet http://www.atmel.com/images/doc2549.pdf
BC337 Datasheet http://www.datasheetcatalog.org/datasheet/philips/BC337_3.pdf
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