How the 555 Timer IC Operates Internally

Figure 1. Inside the 555 Timer IC

Figure 1 shows the main internal circuit components of the 555 timer IC.

Through the 555's internal resistors, the (-) input of comparator 1 is pegged at 2/3 Vcc while the (+) of comparator 2 is pegged at 1/3 Vcc (i.e., if pin 5 is not used). This establishes the internal voltage thresholds to which the externally connected inputs of the comparators will be compared. The behavior of the voltage levels of the external inputs of the comparators are determined by the external components connected to them, which are usually resistors and capacitors.

The comparators are able to set/reset a flip-flop, which in turn determines whether the discharge transistor Q1 is 'on' or 'off'. A negative going pulse at pin 2 makes comparator 2's output go high, which causes the flip-flop to set, which outputs a '1' at pin 3. The flip-flop is reset in two ways: by a negative-going pulse at pin 4 or by allowing the (+) input of comparator 1 to exceed 2/3 Vcc, usually by allowing a capacitor connected to pin 6 to charge up.

The discharge path connected between pin 7 and pin 6 and the the flip-flop state determine when and how the pin 6 capacitor discharges and, consequently, how the (+) input voltage of comparator 1 decays so that a new timing cycle may begin.

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