I am planning to change the brightness of an LED using a PIC 16F877A. I get suggestions that the brightness can be changed by changing the time interval between two pulses. But the brightness of the LED depends on its power, and that depends only on voltage and current (which we can't change in the PIC controller). Please explain how this works, or if the suggestion is wrong please suggest some other ideas.
Electrical – Changing LED brightness using PIC 16F877A
ledpic
Related Solutions
PICs have the annoying gotcha of having ANSEL default to ones, which means that ports that can act as analog inputs are so by default. This will force digital reads to zero (see port diagram below taken from the PIC16F833 datasheet to understand why), and that is probably why RB0 gets cleared when setting RB1 with a Read-Modify-Write operation.
In main, clear ANSEL before accessing PORTB and try again.
By the way, LEDs usually drop more than 0.7V, ~2V is common, so it is possible that the first experiment succeeded for the same reason that the second one failed.
Here is the forward conduction characteristic of a typical LED: -
At 2V forward voltage the LED consumes 20mA and is quite bright. At 1.8V forward voltage the current has fallen to 5mA and the LED will be somewhat dimmer than the 20mA scenario. At 1.6V the LED is taking almost zero current and below this voltage it is doubtful that it illuminates at all.
If you have 10 similar LEDs in series, to obtain a forward current of 20mA requires a forward voltage of 20V.
If you are happy with a forward current of 5mA then 18 volts is all that is needed. And, if your power supply is (say) 20V, a series resistor is needed that drops 2 volts at 5mA therefore its resistance will be about \$\dfrac{2V}{5mA}\$ = 400 ohms.
If you have a limited supply voltage, you can wire the LEDs in parallel banks or use a boost converter to lift your 5V to about 20 volts. The CAT4238 device from Motorola can do this: -
Best Answer
Look up something called pulse width modulation, usually abbreviated PWM.
With PWM, you always switch the LED either full on or full off. However, you do this so fast that a human looking at the LED only perceives the average brightness.
It only takes a few 10s of Hz to get to the point where you can't see the individual pulses when looking directly at the LED. However, it will still flicker noticeably when you move your eyes. It is usually good to use a few 100 Hz minimum PWM frequency for brightness-adjusting LEDs. You can also soften the on/off transitions with added electrical components like capacitors and inductor.
The 16F877 has PWM generators built in. Take a look at the CCP modules. Once a CCP module is set up, all the firmware has to do is write a new duty cycle into the right register or registers to set the brightness.