going to use 6 interrupts
Exactly what wires in the old machine do you need to interface to? Presumably some sort of temperature sensor. What kind?And some kind of humidity sensor. What kind?And something to run the egg-turning motor, and something to run the ventilation fans.And a switch to sense when the door is opened. And something to control the lights?What controls were on it originally? What user-interface readouts, etc?It would be almost trivial to add logging, and a graphic readout of current status and a log of temp, humidity, egg turns, vent, door opens, etc. With microprocessor power, you can do a lot more than back when the machine was designed.
That is why I suggested storing the start time in non-volatile memory (NVM) somewhere so you can restart the cycle if you haven't lost too much time. There are also tricks of saving "checkpoints" to NVM every so often (like every 60 seconds). So that if you lose power, you can check how long the machine has been down and take the ambient readings to see if the batch is still OK or doomed.
Consider getting a suitable shield and using a SD card for this - it'll mean you can do huge amounts of logging, which will be invaluable while you're debugging your build, or diagnosing a failure when you have deployed it for real.
For your door open/close you should just have your fan wired through a contact on the door. I.e. door open, contact is open, fan shuts off. Safer than relying on on programming.
nah, what if door contact malfunctions , the arduino should be able to trigger the high alarm
and as for the LCD i have thought of that but discarded it due to the complexity of the amount of buttons needed, but the rotating encoder revives the concept again.
can u further elaborate on the opto-isolator concept to detect if the relay is activated, as i can't imagine that in action.