1. Unit IV
555 Timer

2. Timer?
An automatic mechanism for activating a device at a preset time.
Used to indicate how many times someone has done something.
A Timer is used for producing precise time delay. Secondly, it can be used to repeat or initiate an action after/at a known period of time. This feature is very commonly used in several applications.
By function timers can be categorized to two main types. A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch; a device which counts down from a specified time interval is more usually called a timer or a countdown timer

3. Block Diagram of a basic Timer
The charging unit consists of RC circuit
When the Trigger input is activated the capacitor can be discharged completely so that voltages across the capacitor can be made zero.
Immediately after the switch is released, the capacitor starts charging through resistance R, with the time constant RC.

4. IC 555
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications.
The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element.
The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555 and was called "The IC Time Machine" and was also the very first and only commercial timer IC available.

5. 555 Pinout

6. The 555 timer IC consists of Two analog voltage Comparators C1 and C2
Block Diagram of IC 555 Timer
The 555 timer IC consists of
Two analog voltage Comparators C1 and C2
RS-Flip Flop
A discharge Transistor &
Three 5K resistors.
The comparator drives the F/F, which in turn drives the transistor.
The output of the timer is generally taken at Q' output terminal.
AC1
AC2

7. 555 Timer Pinout
Pin 1. – Ground, The the 555 timer to the negative (0v) supply rail. Pin 2. – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state. Pin 3. – Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or motors can be connected directly to the output. Pin 4. – Reset, This pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic “1” level when not used to prevent any unwanted resetting of the output.

8. Pin 5. – Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise.
Pin 6. – Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.
Pin 7. – Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.
Pin 8. – Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.

9. MULTIVIBRATOR
A Multivibrator is an electronic circuit that generates square, rectangular, pulse waveforms, also called nonlinear oscillators or function generators.
Multivibrator is basically a two amplifier circuits arranged with regenerative feedback (+VE feedback).
There are three types of Multivibrator:
Astable Multivibrator: Circuit is not stable in either state—it continuously oscillates from one state to the other. (Application in Oscillators)
Monostable Multivibrator: One of the state is stable but the other is not. (Application in Timer)
Bistable Multivibrator: Circuit is stable in both the state and will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. (Application in Flip flop)
9/19/2018

10. 555 as Astable Multivibrator
Assume the Capacitor C is discharged when power is on. (ie) Vc(t=0)= 0.
Therefore, Threshold = Trigger = 0.
Hence R=0, S=1 which Set the F/F; Q=1,Q’=0, 555 output is HIGH and the discharge transistor will be in OFF state.
Therefore Capacitor will charge through (Ra+Rb) to Vcc.
When the voltage across C Vc(t) become greater than Vcc/3 (Trigger>Vcc/3) the comparator 2 o/p goes low and the S input =0, which keep the F/F in previous state and the capacitor continue charging.
When Vc(t) > 2/3 Vcc, comparator 1 output goes high, (ie) R=1, and it reset the F/F : Q=0,Q’=1.
The transistor is turned on, and the capacitor begins discharging through Rb and Ton.
Vc(t)

11. The output wave form is shown in figure.
When the Vc(t) becomes less than the 1/3Vcc the lower comparator o/p is high and S input to FF is High , which set the F/F.
Therefore, Q=1 and Q’=0 and the transistor is turned off. The capacitor starts charging through the resistors Ra and Rb .This process repeats.
The output is a square wave.
The output wave form is shown in figure.

12. Charging and Discharging Times
The voltage across the capacitor oscillates between Vcc/3 and 2Vcc/3.
The capacitor charges through Ra+Rb and discharges through Rb only.
Therefore, the discharging time (T1) and charging time (T2) are different.
Hence the output is not a symmetric square wave. TH!=TL

13. Calculation of T1 and T2
During the time period T1, C is discharging. (ie) from initial voltage
Vc(t)= 𝟐 𝟑 Vcc to final voltage 𝑽𝒄𝒄 𝟑
Thus Vc(t) can be written as,
Vc(t) = 𝟐 𝟑 Vcc 𝒆 ( −𝒕 𝑹𝒃𝑪 ) ; discharging through Resistor Rb
(ie) at t =0, Vc(t) = 𝟐 𝟑 Vcc and at t=T1 Vc(t) = 𝑽𝒄𝒄 𝟑
At t =T1, Vc(T1) = 𝑽𝒄𝒄 𝟑 = 𝟐 𝟑 𝐕𝐜𝐜 𝒆 ( −𝑻𝟏 𝑹𝒃𝑪 )
(ie) 0.5 = 𝒆 −𝑻𝟏 𝑹𝒃𝑪
Taking log on both sides, ln 0.5 = -T1/(RbC)
= -T1/RbC , (TL) T1 = 0.693* Rb*C
2VCC/3
VCC/3

14. The general expression for the voltage across the capacitor during charging from initial voltage Vi to final voltage Vf is given by
Vc(t) = Vf + (Vi-Vf) 𝒆 ( −𝒕 𝑹𝑪 )
During time period T2, (TH)
Capacitor charges from initial voltage Vi= 𝑽𝒄𝒄 𝟑 to final voltage Vf = Vcc through Ra + Rb but when it reaches 𝟐 𝟑 Vcc discharge takes place.
Substituting relevant values,
Vc(T2) = Vcc + (Vcc/3 – Vcc) exp(-T2/((Ra+Rb)*C))
2Vcc/3 =Vcc + (Vcc/3 – Vcc) exp(-T2/ ((Ra+Rb)*C))
2/3 = 1 + (1/3 -1) exp( -T2/ ((Ra+Rb)*C))
1/3 = 2/3 exp (-T2/ ((Ra+Rb)*C))
(TH) => T2 = (Ra+Rb)*C

15. Total Time period T = T1 + T2
T= 0.693(Ra +2Rb)C
Therefore f = / [( Ra + 2Rb).C]
Duty Cycle
The Duty cycle of a square wave is defined as the ratio of the time the wave is high (TH), to the time period T.
TH Ra + Rb
DC = =
TL + TH Ra+2Rb

16. Calculations: AstableTH TL
Note that Duty Cycle D must always be > 0.5. To get 50% duty cycle, RA = 0, which would short out VCC

17. Monostable Multivibrator
When the o/p of RS f-f is high the circuit o/p is high, the transistor is off and the capacitor charges through R to VCC.
When this voltage crosses the 2/3VCC the upper comparator o/p is high and RS f-f o/p is low and the circuit o/p is low, the transistor is on and the capacitor discharges to ground level.
When the triggering input is applied at pin 2 and it crosses 1/3 VCC then the lower comparator o/p is high and RS f-f o/p is high the and the transistor is off then the capacitor charges through R to Vcc.
This process will repeat.

18. 9/19/2018

19. If vi =0,
Vc = vf + (0 -vf) exp (-t/RC)
Vc = Vf (1-exp (-t/RC)

20. Applications of Monostable MV
Frequency Divider
The application of trigger pulse to a monostable multivibrator based on 555 gives a positive going pulse on the output. The monostable circuit can be used as a frequency divider if the timing interval is adjusted to be longer than the period of input signal at the trigger input.
9/19/2018

21. Pulse width Modulator
9/19/2018

22. Internally, the control voltage is adjusted to 2/3 Vcc.
It is basically a monostable multivibrator with a modulating input signal applied at the control voltage input (pin no.5).
Internally, the control voltage is adjusted to 2/3 Vcc.
Externally applied modulating signal changes the control voltage and hence the threshold voltage of the upper comparator.
As a result the time period required to charge the capacitor upto threshold voltage level changes and hence width of the output pulse varies depending upon the input signal called pulse width modulation.
9/19/2018

23. 9/19/2018

24. 9/19/2018

26. Sample Calculation
Design an oscillator with a frequency of 200Hz with a duty cycle of 78%.
Determine Period (T):
Determine TH and TL:

27. Sample Calculation
Since there are 2 variables in the TL equation, select C:
Determine RB by using the TL equation:
C=10μF

28. Sample Calculation
Determine the value for RA:

29. Calculations: Astable
Notes:
The value is a factor associated with the
charge/discharge cycle of the 555 timer.
Duty Cycle must be > 50%

30. Frequency, Duty Cycle Set
Sample Design
Frequency, Duty Cycle Set
Build an oscillator using a 555 timer with a frequency of 72kHz at 75% D.C. Use a 100F capacitor.

31. Frequency, Duty Cycle Set
Design Solution
Frequency, Duty Cycle Set
1- Determine the ratio of the resistors Ra and Rb:
2- Use the ratio in the frequency equation (substitution):

32. Design Solution 3- Solve for Rb: 4-Solve for Ra:
5-Use standard values (optional step):
Ra=100k
Rb=47k

33. Design Solution
6- Calculate actual frequency and DC: