S.A electronics

The 555ic  has a ability to be used in astable  and monostable circuits.the astable mode of 555 is also called as free running oscillator because no external trigger is required to generate  a output frequency(HIGH. LOW) The output frequency can be  changed by changing the the value of resistors R1&R2 and value of capacitor c1 In astable mode the output frequency alternates between vcc &0v on a continous bias Working In the above  circuit trigger & threshold pins are connected each other which allows the circuit re-trigger itself on each cycle and opreates a free running oscillator. the capacitor gets charged through both resistors but discharges only through resistance R2 the discharge  pin of timmer ic is connected to other side of resistance R2 The capacitor charges  upto 2/3 vcc (upper comparator limt)which is determined by the 0.693(R1+R2)C the capacitor discharges itself down to 1/3vcc(lower comparator limt)and is determined b...