The 555 timer IC is a TTL
compatible device that can operate in several different modes.
FIGURE 1 shows how external
components can be connected to a 555 timer to produce a square wave clock
signals or a free running oscillator.
The frequency of oscillations
and its duty cycle are determined by the value of the external resistors,
RA and RB
and the external capacitor, C.
The formulas to obtain the
values of the frequency and its duty cycle are given in the DIAGRAM.
You could see that it is
impossible to produce a perfect 50% duty cycle square wave output.
However, it is possible
to get very close to a 50 percent duty cycle by making RB
>> RA while keeping RA greater
than 1kohm so that t1 is approximately
equal to t2.