Easiest way is to use a micro-controller, an 8 pin PIC would be ideal (or even a 6 pin one) - how accurate does it need to be? - presumably 'not very' as you mention 555's?.
I don't need a high accurate timing, just that. My only worrying is about current leaking while using high valued (1000uF+) capacitors or 1Meg+ resistors
I don't need a high accurate timing, just that. My only worrying is about current leaking while using high valued (1000uF+) capacitors or 1Meg+ resistors
If you don't have or can't get PICs, you should not be surprised if 555's and large value capacitors are equally hard to come by, unless you can scavenge the parts you need. The problem with this is that big capacitors can rapidly deteriorate with age and could pose safety problems. If you've never seen a big capacitor fail, and shoot a plume of steam about 20 feet in the air, you're in for a treat.
Do you want to use these in some quantity or do you just want to build one?
If you wanted a few hundred I was going to offer to do it for a small fee. If you want just one I still think a PIC is the way to go, but you're on your own, with a little help from your friends. I know I'll get no argument from Nigel.
whenever dealing with long times, counters come to mind. if using programmable device such as pic is not possible, external counter could do.
for example you could cascade two 4017 counters to get 19 step counter. your 555 chip would only need to provide clock of 1 minute period. instead of 555 you could also use use cmos counter chip with built in oscillator and get very good precission, specially if you use xtal. it's 3 chips but no programming....
Over the years I have built many timing circuits with 555 timers. For timing the most critical component is the capacitor. With film capacitors the timers are very stable and can be real accurate. For long times the CMOS is a good choice as resistor values up to 20M ohms can be used.
I recently needed an adjustable timer in 1/2 hour increments from 4 hours to 8 hours. I used a 555, a film capacitors, some 20 turn trim pots and a pair of CD4060 CMOS binary counters in cascade. I calculated what frequency the
555 needed to be at for each time. Adjusted the trimpot while monitoring the frequency of the 555 on a frequency counter. Then let the dividers do there
thing.