The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555 and was called "The IC Time Machine" and was also the very first and only commercial timer ic available. It provided circuit designers and hobby tinkerers with a relatively cheap, stable, and user-friendly integrated circuit for both monostable and astable applications. Figure 1a. NE555 (Top View)
ASTABLE OPERATION
When the circuit is connected as shown in figure 1b (pin 2and 6 connected)it triggers itself and free runs as a multivibrator. The external capacitor charges throughR1 and R2 and discharges throughR2 only.
Thus the duty cyclemay beprecisely set by the ratio of these two resistors. In the astable mode of operation, C1 charges and discharges between 1/3 Vcc and 2/3 Vcc. As in the triggered mode, the charge and discharge times and therefore frequency are independent of the supply voltage.
Figure 1b. Astable Oscillator
50% DUTY CYCLE OSCILLATOR
For a 50% duty cycle the resistors RA and RE may be connected as in figure1c.The time preriod for the output high is the same as previous,
t1 = 0.693RA C.
For the output low it is
t2 =[(RARB)/(RARB)] CLn [(RB-2RA)/( 2RB-RA)]
Thus the frequency of oscillation is:
F=1/(t1+t2)
Note that this circuit will not oscillate if RB is greater than 1/2 RA because the junction of RA andRB can not bring pin2 down to 1/3 VCC and trigger the lower comparator.
Figure 1c. 50% Duty Cycle Oscillator
