555 is used for producing a clock (square wave) at a desired frequency. It can be used in various ways like the astable mode, monostable mode etc. Here, we deal with the astable operation of 555. Astable mode ensures that the 555 is self-triggered & so, it acts as a multi-vibrator. Let us look into the working of 555 in astable mode:
These are the connections needed to make the 555 chip run in the ‘astable’ mode. The pin numbers are given in circles.
Note the right-most side of the figure, and consider all the connections with the pins as ‘open-circuits’. Ignore the rest of the circuit for a while.
D1 - Circuit Connections to be made
This over-familiar circuit will majestically jump up. The Vc1 is the driving signal for this 555, connected at PIN 6, which is the THRESHOLD PIN. As soon as Vc1 reaches ⅔Vcc, then the output at PIN 3 goes low, and the capacitor starts discharging, via PIN 7 with Rb as the Resistor and ground as the other terminal. (Refer to the first image to see how it MIGHT happen.) When it reaches ⅓Vcc, the output at PIN 3 goes high, and the DISCHARGE PIN’s connection with ground is broken. The capacitor again starts charging, and the cycle is repeated. You need not bother yourself with how the Circuit is broken or established.
This graph gives the OUTPUT at PIN 3 and the input voltage at PIN 7