555-Timer is one of the most popular and mostly used ICs. It best suits for timing/timekeeping related circuits. It consists of two operational amplifiers operated in an open loop or comparator mode, RS Latch with additional Reset input, a discharge transistor, an inverting buffer and an amplifier in the output stage. It has a voltage divider circuit with three 5K Ohm resistors in series. 556 is a dual timer IC. The internal block diagram of 555 is as follows;
Internal Block diagram of 555-Timer
The Upper comparator-UC is a Non-Inverting comparator, compares the input signal with 2/3Vcc and Lower comparator-LC is an Inverting comparator, compares input with 1/3 Vcc. The output of UC is given as R input of RS Latch and output of LC is given as S input of RS Latch. The RS Latch has an external Reset input which provides the option to reset the output instantaneously. The inverted output of Latch turns ON/OFF the discharge transistor. Inverting Buffer in the output stage inverts the output of Latch and drives the load.