The circuit shown in the figure makes it possible to trigger a load after times that can extend up to 4 hours. The first half of the 556 that works as a stable whose frequency basically depends on RA, RB and C works as a clock, triggering a long interval counter of type N8281. Depending on the selected output, we will have intervals that can range from half an hour to 4 hours. The pulses of this output trigger the monostable with the other half of the 556 that drives a load for a time interval given by R and C. For a 1 M ohm resistor and a 1,000uF capacitor, we have a drive for approximately 15 minutes. The accuracy of the circuit fundamentally depends on the accuracy of the timing network RA, RB and C. The circuit can be implemented with two separate 555s. The N8281 is not an easy component to obtain.
