555 astable mode

*In this circuit, the charge time constant is (R1+R2)*C and the discharge one is R2*C; *When the Threshold pin (THR) voltage (connected to the capacitor) reaches (2/3)*VCC, the internal flip-flop SR is set and the capacitor discharges through R2; * When the capacitor voltage falls below VCC/3, the internal flip-flop SR is reset and the capacitor charges through R1 and R2; *The capacitor voltage varies between (2/3)*VCC and VCC/3, and the 555 output shows a rectangular signal that varies between 0 and VCC; *Depending on the R1 and R2 values, the width pulse (W) can be modified between 50% and 100% of the signal period (T); -> W = 0,693*(R1+R2)*C -> T = 0,693*(R1+2*R2)*C -> f = 1/T