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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.
My system counts minutes, it uses 2 bytes of RAM to make a 16 bit down counter. It is preloaded to 30,000 Dec. (30,000 minutes = 500 hours) Each minute the motor is running it decrements the counter, when it reaches zero it turns on the led. It only saves the state of the counters to EEPROM if there is a power failure. One input of the PIC is used to detect power loss.Thanks Brevor, I think I will also follow the same solution because it seems most cost effective solution probably with a coil meter as dr pepper's suggestion. One question, does your system write the data to eeprom after each completed hour or does it make a calculation between the switch on and off time and write the final value at the end?