The application circuit shows astable mode.
Pin 6 (threshold) is tied to Pin 2 (trigger) and Pin 4 (reset) is tied to VCC (Pin 8).
The external capacitor C1 of Pin 6 and Pin 2 charges through RA, RB and discharges through RB only.
In the internal circuit of the LM555 one input of the upper comparator is the 2/3 VCC (∗R1 =R2=R3, another input if it
If it is connected Pin 6.
As soon as charging C1 is higher than 2/3 Vcc, discharge transistor Q1 turns on and C1 discharges to collector of
Therefore, the flip-flop circuit is reset and output is low.
One input of lower comparator is the 1/3 VCC, discharge transistor Q1 turn off and C1 charges through RA and RB.
Therefore, the flip-flop circuit is set and output is high.
So to say, when C1 charges through RA and R1 output is high and when C1 discharges through RB output is low.
The charge time (output is high) T1 is 0.693 (RA+RB) C1 and the discharge time (output is low) T2 is 0.693 (RB C1).
Thus the total period time T is given by
T=T1 +T2 = 0.693 (RA +2RB) C1.
Then the frequency of astable mode is given by
(RA + 2RB)C1
The duty cycle is given by
D.C = =TT2
RA + 2RB
If you make use of the LM556 you can make two astable modes.