Four ways of connecting the 555 in Astable mode
Frequency =
1.44 / (R1+2×R2) × C1
1.44/(10,000+(2*10,000))*.010=4.8Hz
The diode in the top right circuit,
is in parallel with the resistor
to allow a duty cycle of less than 50%
So it is not always needed
for the circuit to function properly.
