A square wave oscillator using 555 IC can be configured to give symmetric oscillation (50% duty cycle). Other circuit uses diodes to split the charging and the discharging paths through different resistors, but here we need no such diodes. Here is the schematic diagram:
The time period for the output high is the same as regular configuration,
t1 = 0.693 RA C,
but for the output low, the period is
Thus the oscillator frequency is equal to 1/(t1+t2).
Please remember this oscillator circuit will not oscillate if RB is greater than 1/2 RA because the junction of RA and RB cannot bring pin 2 down to 1/3 VCC and trigger the lower comparator. [Circuit’s schematic diagram source: National Semiconductor Application Notes]