i was trying to build a timer with a 30 second on and a 3 minute off, i tried using the circuit below on kpsec.com, the one using a diode to get less than a 50% duty, i used r2=1.2M, r1=0.2M and c1=220uF according to the calcs on that page. The output stays on, does not cycle, whats wrong? i also tried it with c1=47uF, same result
if i change out c1 to 10uf the timer will cycle about 1 sec on and 8 secs off
555 timers are very limited as to maximum times, in this case the leakage in the 220uF is probably more than the current available through the 1.2Mohm - you could try using a tantalum capacitor?, this is an application where they really are better.