In this mode, the 555 work as a free running mode. The output of Astable multi-vibrator will continuously toggle between low and high, there by generating a train of pulse, which is why it is known as pulse generator. It is a best example for a perfect square wave generator. They are used as an inverter and also used in many of the internal part of the radio. Selecting a Thermistor as a timing resistor allows the use of the 555 in a temperature sensor.
This type of circuit could be used to give a mechanism intermittent motion by switching a motor on and off at regular intervals. It can also be used to flash lamps and LEDs, and is useful as a 'clock' pulse for other digital ICs and circuits.
In this mode; the output is stable neither in ‘high’ state nor in ‘low’ state. Hence it oscillates from one state to another giving us a square wave or clock. We can set the clock frequency ‘F’ and Duty Cycle ‘D’ by the formula-
Capacitor 0.1nf is just to filter the noise and its value can be suitably chosen to be 0.01μF. It can also be neglected.