Electrical – LCD 16×2: circuit to adjust the LCD backlight by a PWM signal

backlightlcdmsp430pwm

I am testing a lcd 16×2 whith msp430g2553. I am controlling the backlight by PWM signal. I am using the P2.4 as output generating a PWM signal. I have connected the anode directly to pin P2.4 of the microcontroller and the cathode directly to GND. In the datasheet, (msp430g2553 on page 24) specifies that the maximum output is 6 mA and, on the other hand, on the LCD datasheet, LCD Datasheet, on page 32, the typical consumption is 32 mA. So … Why it works if you can only drain 6 mA? Would it be more correct to put a transistor as shown in this circuit?

schematic

^{simulate this circuit – Schematic created using CircuitLab}

What should be the value of R1?(The battery voltage varies from 4.2 to 3.3V) I can not find that information in the LCD datasheet, surely because I do not know what parameters I have to analyze.

Best Answer

The correct way to connect up the back light is the way that you show with the MOSFET. The IRLML2502 N-FET may be overfill in the max current ratings category but is still in a nice small SMT style case and should work very well for this application with a logic driving signal from an MCU with a 3.3V to GND swing.

Select the R1 value to limit the total current to the LCD back light to around 30mA to 35mA when the backlight is continuously on. Then the variable duty cycle of the PWM signal can reduce average current to a lower value. If you do not have data on the forward voltage drop of the backlight from A to K you can try connecting the backlight up to a variable bench supply and a series resistor. You can slowly increase the voltage whilst measuring the A to K voltage to see where it levels out as the supply voltage continues to increase.

To calculate R1's value you subtract the A-k voltage from your 3.6V battery voltage and then divide that by the 0.035A LED current. This will give the value in ohms for the series resistor.

The reason that the backlight seemed to work when you connected it directly to P2.4 was that the port pin was able to source some current to the backlight and the LEDs were able to light. But keep in mind that this was probably over stressing that port pin beyond its maximum ratings. That is why the MOSFET driver circuit is highly recommended. Once you get the PWM connected up in a manner so that the back light current can be at its max at near 100% duty cycle you will notice that the overall brightness of the backlight will not be at all linear with the PWM duty cycle.

Related Solutions

Electronic – arduino – Debugging a 16×2 LCD Display on an Arduino

Do you have a diode across your relay? If not the inductive spikes on closing/opening will likely cause problems.

When you e.g. stop the current flowing through a relay, it tries to keep it going, if there is no route of discharge it will create a large voltage (essentially it will keep rising until it finds a route).
Specifically the formula is V = L(dI/dT). This means the inductor opposes changes in current through it by developing a voltage across it proportional to the rate of change of current.

Try placing a diode across the relay, orientated to oppose normal current flow.

EDIT - checking the product page it seems it is a "ready rolled" relay in a box with peripheral components added, so it's unlikely to be the above. Even so I would try placing the diode across the power to the relay.
Also make sure your supply is well filtered as mentioned in the comments. Place a few capacitors of at least 1uF next to the ICs and LCD, and a bulk cap of >100uF somewhere (all from power to ground)

Electronic – Controlling an LCD device using serial I2C module

#define ON  1
#define OFF 0

//#define __LCD2__  // 2 row display
#define __LCD4__  // 4 row display

#ifdef __LCD2__
  #define LCD_MAX_LINES  2  // usually a 1602 2 X 16
  #define LCD_DISPLAY_WIDTH 16
  #define LCD_ID 0x4E       // I2C Base address for this interface
#endif

#ifdef __LCD4__
  #define LCD_MAX_LINES  4  // usually a J204A 4 X 20
  #define LCD_DISPLAY_WIDTH 20
  #define LCD_ID 0x40       // I2C Base address for this interface
#endif

#define RS          0b00000001  //P0 - PCF8574T Pin connected to RS
#define RW          0b00000010  //P1 - PCF8574T Pin connected to RW
#define EN          0b00000100  //P2 - PCF8574T Pin connected to EN
#define BACKLIGHT   0b00001000  //P3 - PCF8574T Pin connected to BACKLIGHT

#define LCD_LINE1   0x80        // LCD RAM address for line 1
#define LCD_LINE2   0xC0        // LCD RAM address for line 2
#define LCD_LINE3   0x94        // LCD RAM address for line 3
#define LCD_LINE4   0xD4        // LCD RAM address for line 4

int LcdBacklight = ON;

void InitLCD(void)
{
    printf("Initializing LCD...\r\n");

    delay_ms(500); //LCD power up delay

    LcdSendByte(0x00);
    LcdSendByte(0x10);
    LcdSendByte(0x00);
    LcdSendByte(0x00);
    LcdSendByte(0x10);

    //First state in 8 bit mode
    LcdSendByte(0x30);
    LcdSendByte(0x30);

    //Then set to 4-bit mode
    LcdSendByte(0x30);
    LcdSendByte(0x20);

    //mode 4 bit, 2 lines, characters 5 x 7
    LcdSendByte(0x20);
    LcdSendByte(0x80);

    //no need to have the cursor on. 
    //but if you want then '0Ch' is the addr
    LcdSendByte(0x00);
    LcdSendByte(0xC0);

    //the cursor moves to the left (06h)
    LcdSendByte(0x00);
    LcdSendByte(0x60);

    printf("LCD   Initialized...\r\n");
}

Best Answer

Related Solutions

Electronic – arduino – Debugging a 16×2 LCD Display on an Arduino

Electronic – Controlling an LCD device using serial I2C module

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