As discussed in previous tutorial, IC 555 can be used in three different modes. The most common use of IC 555 is to provide pulses at regular intervals, i.e., act as a clock. This can be achieved by using resistors and capacitors with IC 555, as will be explained in this tutorial. The time period and duty cycle of the clock cycle depends on the values of resistors and capacitor used.
The resistors RA and RB, and capacitor C are connected externally with the IC 555 at the pins specified below:
C (+) -> Pin 2 or 6
C (-) -> GND
Pin 2 -> Pin 6
RB between Pin2/6 and 7
RA between Pin 7 and Vcc
Pin 8 -> Vcc
Pin 1 -> GND
When the circuit is turned on and the capacitor is initially discharged, the output of comparator 1 and 2 will be LOW and HIGH respectively, because the pins 2 & 6 are LOW.
Now, the capacitor will be charged through resistors RA and RB.
Once the voltage difference across capacitor increases upto 2/3 Vcc, the outputs of comparator 1 and 2 will become HIGH and LOW respectively, which will eventually make the Q pin of SR Flip-flop HIGH and the transistor will get turned ON. Now, the capacitor will start discharging through RB via transistor.
When the voltage difference across capacitor reaches 1/3 Vcc, the pins 2 and 6 will become LOW and HIGH respectively, which will make the output Q of SR Flip-Flop to go LOW, hence turning the transistor OFF.
These stages can be summarized as in the table below:
Voltage difference across capacitor
Comparator 1 Output
Comparator 2 Output
Output of SR FF
Slightly greater than 1/3 Vcc (from 0)
Slightly greater than 2/3 Vcc
ON (discharging starts)
Slightly less than 1/3 Vcc (from 2/3 Vcc)
OFF (charging starts)
The capacitor will continue to charge and discharge (stage 3 & 4 in the table) hence giving a rectangular pulse of HIGH and LOW.
The TON can be determined by calculating the time for which the capacitor charges from 1/3 Vcc to 2/3 Vcc which comes out to be as:
Similarly, the TOFF can be determined by calculating the time for which the capacitor discharges from 2/3 Vcc to 1/3 Vcc, i.e.,
Note that the term RA appears only in the expression of TON. The reason is that the capacitor is charged through RA and RB both, but, it discharges through RB only. It is also worth to note that we cannot replace RA with wire because when the transistor will get turned ON, the Vcc and GND will get shorted.