The time delay, T, in seconds is: T = 1.1 x Cl x (Rl + R2). The resistances are in megohms and capacitances in microfarads. The sum of R1 and R2 should not be Jess than 1000 0 nor higher than 20 MO. Pressing S1 starts the timing cycle. A low-going pulse, instead of S1 can also be used to initiate the timing cycle. With the values shown and allowing for the tolerances of the 200-JtF capacitor, the delay will range from 4 minutes and 50 seconds to 7 minutes and 26 seconds.
The output terminal, pin 3, of 555, is normally low and switches high during the timing cycle. The output can either sink or source currents up to 200 mA. Therefore, a load such as a relay coil can be connected between pin 3 and Vccc or between pin 3 and ground, depending on circuit requirements. When the relay is connected between pin 3 and ground, it is normally de-energized so it is energized only during the timing cycle. Connecting the relay to ground will. save power and allow the IC to run cool.