555 Timer in Astable Mode

A free-running square wave generator.

With two resistors and a capacitor, the 555 oscillates by itself. The capacitor charges through both resistors up to two thirds of the supply, then discharges through only the second one down to one third, and the cycle repeats.

Because charging takes longer than discharging, the output is high longer than low. For example, 1 kΩ, 10 kΩ and 10 µF give about 6.9 Hz, which would blink an LED about seven times a second.

• High time depends on R1 + R2.
• Low time depends on R2 alone.
• Duty cycle is always above 50 percent in the basic circuit.

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