Posted on Jun 22, 2012

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.

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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.

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