IC 555 timer/Multivibrator/Pulse generator.pptx

Regarding introduction and modes of operation of IC-555

1. Basics of Electronics Engineering (BEE-EC101) IC 555 timer and its modes of operations Batch-2019 CSE By: Harpreet Kaur

2. Overview of Topics IC 555 timer Description Applications of IC 555 Pin diagram of IC 555 Functional block diagram of IC 555

3. 5 Pin diagram of IC555

4. Pin description Pin 1 :  Grounded Terminal:  All the voltages are measured with respect to the Ground terminal . The trigger pin is used to feed the trigger input hen the 555 IC is set up as a monostable multivibrator . This pin is an inverting input of a  comparator  and is responsible for the transition of  flip-flop  from set to reset

5. Pin description Pin 3:   Output Terminal:  Output of the timer is avail­able at this pin. There are two ways in which a load can be connected to the output terminal. One way is to connect between output pin (pin 3) and ground pin (pin 1) or between pin 3 and supply pin (pin 8). The load connected between output and ground supply pin is called the  normally on load  and that connected between output and ground pin is called the  normally off load .

6. Pin description Pin 4:   Reset Terminal:  Whenever the timer IC is to be reset or disabled, a negative pulse is applied to pin 4, and thus is named as reset terminal. The output is reset irrespective of the input condition. When this pin is not to be used for reset purpose, it should be connected to + V CC  to avoid any possibility of false triggering.

7. Pin description Pin 5:   Control Voltage Terminal:  The threshold and trigger levels are controlled using this pin. The pulse width of the output waveform  is determined by connecting a POT or bringing in an external voltage to this pin.  The external voltage applied to this pin can also be used to modulate the output waveform Pin 6 :  Threshold Terminal:  This is the non-inverting input terminal of comparator 1, which compares the voltage applied to the terminal with a reference voltage of 2/3 V CC .

8. Pin description Pin 7  :  Discharge Terminal:  This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground. It is called discharge terminal because when transistor saturates, capacitor discharges through the transistor. When the transistor is cut-off, the capacitor charges at a rate determined by the external resistor and capacitor . Pin 8:   Supply Terminal:  A supply voltage of + 5 V to + 18 V is applied to this terminal with respect to ground (pin 1).

9. Functional block diagram of IC 555

10. Block diagram of IC555 it consists of 2 comparators, a flip-flop, a voltage divider, a discharge transistor and an output stage . The voltage divider consists of three identical 5k resistors which create two reference voltages at 1/3 and 2/3 of the supplied voltage, which can range from 5 to 15V . The Trigger and the Reset pins of the IC are connected to VCC through the two resistors, and it that way they are always high. The two pushbuttons are connected between these pins and the ground, so if we hold them pushed the input state will be low.

11. About IC-555 A 555 Timer is an 8 pin mini dual-in-line package IC. The 555 IC is capable of producing accurate time delays and/ or oscillations. Introduced in 1972 by SigNetics . ( Sig nal Net works Electron ics ) The 555 is still in widespread use to generate clock pulse or square or rectangular waveforms A single 555 timer can provide time delay ranging from microseconds t o hours. It operates from a wide range of power supplies ranging from + 5 Volts to + 18 Volts supply voltage.

12. Reality

13. Pin Configuration The function of each pin of the IC is given below – Pin–1: Ground Pin–2: Trigger Pin–3: Output Pin–4: Reset Pin–5: Control Voltage Pin–6: Threshold Pin–7: Discharge Pin–8: Vcc

14. Features High Current Drive Capability (200mA) Monostable operation Astable operation Bistable operation Generates clock pulses for the operation of sequential circuits Ability to operate from a wide range of supply voltages Trigger and reset inputs are logic compatible

15. Internal Block Diagram

16. what is inside 555? A functional block diagram of 555 timer is given below. The device consists of two comparators ,2 transistors, a flip-flop and buffered outputs stage. The reference voltages for the two comparators inside the 555 are produced across a voltage divider consisting of three equal resistors of 5K ohms each.

17. Look at the given block diagram of the IC, to see that there are three resistors of 5k ohm each connected in series. These three resistors produce 1/3 and 2/3 voltage levels for controlling the action of trigger and threshold comparators inside the IC. Due to this arrangement of the three resistors, the IC has a typical code number as IC555 . The threshold comparator is referenced at 2/3 Vcc and the trigger comparator is referenced at 1/3Vcc. The two comparators control the flip-flop which, in turn , controls the state of the output i.e. either ON or OFF states.

18. Pin configuration of IC 555 : Pin-1, GROUND : It is the GROUND PIN of the IC. The negative terminal of DC power supply or battery is connected to this pin. Note that this pin should be connected directly to ground and NOT through any resistor or capacitor. If done so, the IC will not function properly and may heat up and get damaged. This happens because all the semiconductor blocks inside the IC will be raised by certain amount of stray voltage and will damage the IC.IC 555 Pin-2, TRIGGER It is known as TRIGGER PIN. As the name suggests in triggers i.e. starts the timing cycle of the IC. It is connected to the inverting input terminal of trigger comparator inside the IC. It triggers when the voltage at this pin LESS THAN 1/3 of the supply voltage (Vcc). The triggering circuit inside the IC is very sensitive and may be accidently activated due to surrounding noise. To avoid this, the pin is always connected to a pull-up resistor (10k-ohm), if this pin is used separately. Pin-3, OUTPUT This is the OUTPUT PIN of the IC. It can SINK or SOURCE a maximum current of 200mA. Sinking the current means, when the output of the IC is at logic-0 state i.e. LOW and so it can absorb current into its output. Similarly sourcing the current means, when the output of the IC is at logic-1 i.e. HIGH and so it can give out current from its output.

19. Pin-4, RESET It is the RESET PIN of the IC. When it is connected to positive terminal of battery, the IC works normally. However, when it is grounded (either directly or through a maximum of 100k-ohm resistor), the IC stops its working completely and its timing cycle stops .However in general applications, this pin is always connected to positive terminal so that the IC works normally. Pin-5 , C ontrol VOLTAGE This is known as the CONTROL VOLTAGE pin. The 2/3 of supply voltage point on the terminal voltage divider is brought out to pin-5, known as the control terminal of the IC. The timing cycle can be modified by applying external DC control voltageto this pin. This allows manual or electronic remote controlling of the time interval of the IC. But if you are NOT using this pin for any such purpose, then this pin MUST BE GROUNDED THROUGH A CAPACITOR OF 0.01uF. This prevents the time interval from being affected by picking up of stray AC or RF noise from the surrounding. Pin-6, THRESHOLD This is known as the THRESHOLD PIN. It finalizes the timing cycle of the IC, when its voltage is equal to or greater than 2/3Vcc, the output is at logic-0 state.Note that the typical value of threshold current is 0.1mA, just like the RESET PIN. The time width of this pulse should be greater than or equal to 0.1uS. Pin configuration of IC 555 :

20. Pin-7, DISCHARGE It is known as DISCHARGE PIN. It discharges the external capacitor into itself, but when fully charged…! It is connected to the collector of an NPN transistor inside the IC. Due to this, the discharging current going into this pin MUST NOT EXCEED 50mA, otherwise the internal transistor may get damaged. Pin-8, +Vcc It is known as the +ve supply terminal of the IC. The battery voltage connected across this pin and ground pin SHOULD NOT EXCEED 18V. Generally the range of operating voltage of the IC is 3V–18V. Pin configuration of IC 555 :

21. Monostable Operation In this mode, the timer generates a fixed pulse whenever the trigger voltage falls below Vcc/3. When the trigger pulse voltage applied to the #2 pin falls below Vcc/3 while the timer output is low, the timer's internal flip-flop turns the discharging Tr. off and causes the timer output to become high by charging the external capacitor C1and setting the flip-flop output at the same time. Astable Operation In astable operation, the trigger terminal and the threshold terminal are connected so that a self-trigger is formed, operating as a multivibrator. When the timer output is high, its internal discharging Tr. turns off and the VC1 increases by exponential function with the time constant (RA+RB)*C. When the threshold voltage, reaches 2Vcc/3, the comparator output on the trigger terminal becomes high , resetting the F/F and causing the timer output to become low . 3. Bistable operation Modes of operation

22. Functional diagram of 555 timer + Threshold R + - ( 2/3)Vcc Threshold > (2/ 3)Vcc => R=logic ‘1’ Threshold < (2/3)Vcc => R=logic ‘0’ ( 1/3)Vcc S - Trigger Trigger <(1/ 3)Vcc => S=logic ‘1’ Trigger >(1/ 3)Vcc => S=logic ‘0’

23. Modes of Operation 55 5 I C T imer ap p lica t i o ns can b e cl a ssified i n to two main categories: Mon o sta bl e Mu l ti v i b rators : Pro d uci n g a si n gle pulse when triggered. Astable Multivibrators : Producing a square wave . Bistable Multivibrators : It has two stable states when two input short duration pulses are applied at trigger and threshold inputs

24. 1. Monostable Multivibrator Monostable multivibrator is often called as one- shot multivibrator.

25. 2. Astable Multivibrator Astable multivibrators are also known as Free-running Multivibrator . Astable do not need trigger pulse for external to change the output. The period for LOW and HIGH can be calculated based on resistor and capacitor value that connected at outside of timer. C charges through R A + R B and discharges through R B .

26. Applications Precision Timing Pulse Generation Time Delay Generation Sequential Timing