Square-wave oscillator using 555

The following fig shows the circuit dia of Square Wave Oscillator using 555. The capacitor C2 charges through Ra (Ra1+Ra2) and diode to approximately 2/3 Vcc and discharges through Rb and terminal 7 until the capacitor voltage equals approximately 1/3 Vcc; then cycle repeats. To obtain a square wave output (50% duty cycle) Ra must be a combination of a fixed resistor and potentiometer so that the potentiometer can be adjusted for the exact square wave.

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