This is an adjustable frequency , 50% duty cycle astable multivibrator using the 555 timer IC.
An approximately 50% duty cycle is achieved by using a single timing resistor. In contrast to the conventional 555 astable circuit, which uses a discharge resistor to the discharge pin and additional resistor to Vcc for charging, this circuit has almost symmetrical charge and discharge periods. To obtain this balance, the circuit around the open-collector DISCHARGE terminal is modified by adding external BJT and diode. When the discharge transistor is off, the BJT in emitter follower configuration drives the timing resistor high to charge the timing capacitor. The voltage level at the emitter of the transistor is about one diode drop less than the power supply voltage. To match this voltage loss, a silicon diode is employed between the timing resistor and the discharge pin. When the discharge transistor is on the timing resistor discharges the timing capacitor through the diode to the discharge terminal. The voltage level at the anode of the diode is just a little higher than one diode drop. Hence, symmetry and consequently 50% duty cycle can be achieved.
To make the frequency adjustable, the timing resistor is split into a fixed and variable resistor. The variable resistor provides frequency adjustment. The fixed resistor can serve to limit the maximum frequency allowed. It can also serve as a current limiter, useful when large capacitor is used in the circuit.