hey ,
please help me in designing a circuit which remaind us if the door of refrigrator is opened for more than particular time how to do this any ideas
dependent on the fridge wiring: put a small timer powered by the fridge internal lamp. If the lamp is on for more than, let's say, 5 minutes, a buzzer will beep. When you close the door, the lamp will shut-off and the timer will reset.
independent of the fridge wiring: a small timer circuit with a CMOS 555 with an LDR that triggers the timer. When the door is open, the LDR sees the light while when the door is closed, the LDR sees dark. The circuit can be battery powered but remember that batteries don't work well when it's cold.
A magnet is attached to the door.
A reed switch (closes when the door opens) inside the fridge
connects a 9v battery to a c555 timer IC.
If the timer times out, a piezo sounder does it's thing.
Well if I was building it I would design it to screw into the inside lamp socket and then have the bulb screw into it. No batteries required and lots of power for a loud alarm after time limit is reached.