Low duty cycle PWM and LED brightness

[ccurtis]

Heya, Audioguru. Well, I bought it (and several others I tried) new from Digi-Key about a month ago. It's not the "high efficiency" type that puts out thousands of mcd, but it's not the old 1970s kind either. You can see the data here: https://www.digikey.com/product-detail/en/sunled/XSDG23MB/1497-1042-ND/4745857

 

[Pommie]

I currently have a 3W RGB LED hooked up to a 16F18326 and tried the following code using just the red LED.

int16_t bright=0;  
    while(1){
        while(bright<0b0000001111111111){
            if(bright==0)
                bright=1;
            else
                bright=(bright<<1)+1;
            PWM1_LoadDutyValue(bright);
            __delay_ms(250);
        }
        while(bright>0){
            bright>>=1;
            PWM1_LoadDutyValue(bright);
            __delay_ms(250);
        }
    }

You definitely see distinctive steps even when the value is going from 1 to 0 using 10 bits (0.1%).

Mike.
BTW, I used the code configurator for the first time yesterday to setup the chip. Very impressed - quick and easy.

 

[ccurtis]

Hey, Pommie, so would you say your LED is pretty darn bright for 0.4%? Subjectively speaking, of course.

 

[Pommie]

It's hard to tell as it's a 3W LED so anything is bright. However, the final step (0.1% as 10 bit used) is very noticeable.

Mike.

 

[ccurtis]

Amazing!

 

[gophert]

Your LED is low enough current to allow sourcing and sinking from pins on the PIC. So you can simply apply power from your PWM pin and select a ground pin connected to a different current-limiting resistor that suits your needs.

This way you get 1-255 duty cycle on a 100 ohm resistor (30 mA), then switch the ground pin to an input, then switch another pin from input to and output low, that is connected to a 1000 ohm (or 22k ohm) resistor (3mA) for 1-255 duty cycle.

In pictures,

Or

 

[audioguru]

So you are using a modern 3.3V to 4.1V green LED that is extremely bright. I have a solar garden light like that that lights up my entire neighbourhood at night.