555 Timer with a 50% duty cycle
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 &ndash; 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.