Electronic – independent control of frequency and duty cycle in 555 timer (formulas)

If you need an accurate 50% duty cycle, a simple 555 circuit is not the way to go. A better method is to have an oscillator run at double frequency, where duty cycle is unimportant. Then feed that signal in a divide-by-two circuit and you'll have a perfect 50% duty cycle.

A simple divider can be made using a JK-flip flop where the J and K are high, like in this circuit:

^{simulate this circuit – Schematic created using CircuitLab}

There are countless ways to create a 2-divider, all with their own advantages and disadvantages. A simple Google image search will give you pages full of them.

If the duty cycle itself does not matter and the requirement is for pulses of, say, 0.5ms, then you could send the output of your circuit to a monostable 555 circuit. Dual-555 chips are available: the 556.