PWM and Brightness Level

Now for the complex circuit with external power source. (One LED is a poor application when come to lighting a room) Configured the circuit. Plugged one unit of LEDs we bought early.

And nothing happened!!! Well I am in a familiar ground again.

Had a hard look at the circuit. Looked for the pin layouts. Found out my knowledge of transistors are only valid for some simpler dimension. It's totally different when it comes to PNP and NPN!!!

Rewired the circuit.

Nope. Seems the transistor pin layout was not the only issue.

I started the piscope(It gives a live graph of GPIO volt levels) Well there is a problem there. It's not showing the volt level changes for the pin which circuit was connected.

I changed the GPIO pin. Updated the script. Executed it.

Houston, we have landed!!!

At first configurations I was using software GPIO and it was showing some jitters in the light. That's because the Linux is not a real time OS and it uses scheduling technique to operate in multi process environment. So when come to sending signals via GPIO in a high frequency it's possible to miss some due to scheduling.

But the raspberry pi is capable with hardware PWM and i found a nice library with python interface. It mentioned there are few GPIO pins which are capable driving hardware PWM. I tried with GPIO pin 18 and it worked.

With PWM it adjust the brightness by changing the duty cycle(in 50% duty cycle, one half of it is in hight volt state and other half is in low state, 75% is 3/4 in high and 1/4 in low) Also you can change the frequency. It defines how long is a duty cycle(high+low) in time dimension.

With low frequencies(~sub 100Hz) the blinking is visible to naked eye. When it increases(~100Hz - 1kHz) it's become invisible to naked eye but can be seen from camera. This the frequency range being used by the frequency in embedded circuit the LED set came with.

Bellow is a video recording(high frame rate) of the LED brightness being adjusted via circuit it came with.