Hello Ozgur84,
Not really much to explain, Where I work we have several "air handlers" large blowers that circulate the air.
The devices I made simply connect to the blower motor power source (120 V 60Hz) and counts how many hours the blower motor runs. At 500 hours it lights an led to alert a maintenance worker to replace the air filters.
When the filters are replaced the maintenance worker pushes a reset button that turns off the led and resets the hour counter to zero. It uses a Microchip PIC 12F629 to count the runtime. It saves the registers to EEPROM in the event of a power failure and then restores them when power returns.