Perhaps a 555 circuit

[stuee]

Hi, where i work at a large format printers, printing the billboards seen on roads / buildings, im the techie, im trying to make a timer to get people to take more care of the machines, each machine has 24 xaar heads about 600 usd each and they need looking after.
I want to make a circuit so when the carrage goes to the park possision the timer starts counting, when it gets to mabey 15 mins a buzzer comes on, now they can either spray the heads and press reset or put the nightcover on, i want a main on/off switch but also the night over has a reed switch on it so the heads stop firing, can i somehow hook it to that so if they put the cover on the timer turns off, till its taken off again.
Any help would be of use, thanks
Stu

 

[audioguru2]

Hi Stu,
15 minutes is a long time for a 555 timer. It will need a high value capacitor with extremely low leakage.
Instead, I suggest using a CD4060B oscillator/counter IC. A timer project that uses it is here:
http://www.mitedu.freeserve.co.uk/Circuits/Timing/24hour.htm