A charging/discharging circuit comprises a PNP transistor controlled by a diode, an NPN transistor arranged on a discharging path and controlled by at least one photodiode, and a capacitor arranged between a photodiode and the NPN transistor. During charging stage, a charging current flows through the diode to produce a voltage drop, which cuts off the transistors to prevent leakage current. The photodiode generates a photo current such that the capacitor presents a negative voltage on one contact thereof, which facilitates the discharging process.