Internal Block Diagram of IC 555

  IC 555

  • The IC 555 is a more versatile IC, it basically contains built-in timer circuit hence widely known as timer IC.
  • It is introduced by Signetic Corporation in early 1970.
  • IC 555 is basically a monolithic timer circuit which can be used in many applications.
It is a low-cost timer capable of producing precise time delays with a wide range.





Fig. - Pin diagram  of IC 555

  • Above fig. shows a pin diagram  of IC 555.  It is an 8 pin IC. Pin 1 is ground , pin 2 is trigger, pin 3 is output, pin 4 reset, pin 5 is control voltage, pin 6 threshold, pin 7 discharge and pin 8 is positive supply voltage.

The IC 555 timer will work with any supply voltage between 3 Volt to  18 Volt. (max. load current 200 mA)

NOTE : Symbol and Pin Diagram of OPAMP IC 741


NOTE : Working of RSFF


 

BLOCK DIAGRAM OF IC 555

  • The IC 555 combines the following elements as shown in the following block diagram.
  • 1. A potential divider arrangement
  • 2. RS flip flop
  • 3. Two Comparator
  • 4. Discharge transistor

Fig. – Simplified functional block diagram of IC 555 timer

■       It consists of two comparators, voltage divider network flip-flop and output stage.

■       The upper comparator is a threshold comparator, pin (8) and pin (5) as control input in most of the applications, pin (5) is connected to ground through capacitor.

■       The lower comparator is called trigger comparator with trigger input pin (2).

■       Three resistors each of 5KΩ are used as potential divider. These resistors are connected between +Vcc and ground, therefore each resistor has voltage drop of 1/3 Vcc. The reference voltage of lower comparator is 1/3 Vcc and of upper comparator is 2/3Vcc.

■       Whenever Threshold voltage becomes greater than the control voltage (2/3 Vcc), the output of upper comparator will be high and sets the flip-flop making discharge transistor ON while the o/p of IC 555 low because Q is high and Q’ is low.

■       Whereas, when the trigger input goes below 1/3 Vcc, the output of trigger comparator will be high.

■       The output of comparator goes to the SR flip-flop. The output of upper comparator goes to S input and it sets the flip-flop whereas the output of lower comparator goes to R input and it resets the flip-flop.

■       The output of flip-flop Q goes to the discharge transistor, the collector of discharge transistor is pin 7.




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