The stage involving the three 5 kohm resistors work like a voltage divider stage which produces 1/3rd voltage level at the non-inverting input of the trigger comparator op amp and a 2/3 voltage division on the inverting input of the threshold comparator op amp. We can see that it us made up of 21 transistors, 4 diodes, and 15 resistors. The image shown below represents the internal schematic of a standard IC 555. The time interval can be set by selecting appropriate values for an external resistor, capacitor network. The time intervals can be used for keeping a relay controlled load ON or activated for the desired amount of time and an automatic switch OFF once the delay period has elapsed.
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