555 timers make good square wave oscillators. The challenge becomes trying to get a 50/50 duty cycle since the capacitor charges through both resistors(R1 & R2) but discharges through R2 alone. Signal bypassing diodes can be used to achieve a 50/50 duty cycle. These ensure that capacitor charging only goes through R1 from Vcc and discharging through R2 at pin 7. When R1 = R2, you get a 1:1 ratio.
