This circuit is intended to switch 230VAC-12VDC power supply of LED on and simultaneously dim it by single PWM signal from digital output (Arduino, RPi). As long as the PWM is fed to the circuit, the relay is switched on and dimming is controlled by the very same PWM signal using MOSFET. Stoping the PWM signal, the power supply is switched off within a few miliseconds to save energy. The intention is to spare one GPIO output pin so only one pin per LED is needed. Otherwise you would need one pin to switch the power supply on and another one to dim it by PWM. Digital output is protected by 1.6k resistor, therefore only hundreds of microamperes are drawn from it. Proper function is maintained by BJT transistors and separate 3V power supply.
