1. 555 Timer EEE DEPARTMENT KUMPAVAT HARPAL(130560108002)
PATEL RAGHAV( )
GANDHI VIVEK( )

2. Oscillators
We have looked at simple oscillator designs using an inverter, and had a brief look at crystal oscillators.
In this presentation, we introduce the 555 timer; a versatile device that is easier to calculate, design and configure in a variety of ways.

3. A Versatile Device The 555 Timer is one of the best known IC’s.
The 555 is part of every experimenter's tool kit
Capable of creating a wide variety of circuits, including:
Oscillators with adjustable frequency and Duty Cycle
Monostable Multivibrators
Analog to digital Converters
Frequency Meters
Many other applications….
The clock on the Vulcan Board is generated by a 555 timer.

4. 555 Timer Configurations
In this course we will use the 555 timer in 2 modes:
Astable
With some calculations, we can determine the values of the capacitor and the 2 resistors (Ra and Rb) for astable operations.
Monostable
We can determine the value of the resistor and the capacitor with a simple formula for one-shot operations.
There are many other configurations and applications for this device.

5. 555 Layout 1 8 2 7 3 6 4 5 Also available:
Ground
Trigger
Output
Reset
Vcc
Discharge
Threshold
Control
Also available:
556 (two-555’s in one DIP package)
555 in a “metal can” configuration 1 2 3 4 5 6 7 8
Ground
Trigger
Output
Reset
Vcc
Discharge
Threshold
Control

6. Astable Multivibrator
Operation of the 555 Timer

7. Oscillator Configuration
Externally, the 555 requires an RC circuit to create the time delays required for the time high and the time low.
Standard configuration requires
common capacitor
a resistor for the charge cycle
a resistor for the discharge cycle

8. General Configuration
Basic connections:
Ground
Vcc
Note: some 555 timers may function at voltages other than 5 volts.
Reset (active low)
Output 1 2 3 4 8 7 6 5
Ground
Trigger
Output
Reset
Vcc
Discharge
Threshold
Control

9. General Configuration
Specialized connections:
Trigger
monitors low voltage
Threshold
monitors high voltage
Discharge
path to ground, to discharge the capacitor
Control
specialized input
filtering
special applications 1 2 3 4 8 7 6 5
Ground
Trigger
Output
Reset
Vcc
Discharge
Threshold
Control

10. Astable Configuration #1 (“Standard” Configuration)
Vcc
Vcc
Vcc Ra
Discharge 1 2 3 4 8 7 6 5
Ground Rb
Output
Threshold
Reset
Trigger
Control C

11. Note: Duty Cycle must be > 50%
Astable
Calculated
Values
Filter Cap
0.01μF
Note: Duty Cycle must be > 50%

12. Astable Configuration #1 (“Standard” Configuration)
Minimum duty cycle > 50%

13. Calculations: Astable
Time High, Time Low Set TH TL
Notes:
The value is a factor associated with the
charge/discharge cycle of the 555 timer.
Duty Cycle must be > 50%

14. Sample Calculation
Time High, Time Low Set
Design an oscillator with a frequency of 200Hz with a duty cycle of 78%.
Determine Period (T):
Determine TH and TL:

15. Sample Calculation Time High, Time Low Set
Since there are 2 variables in the TL equation, select C:
Determine RB by using the TL equation:
C=10μF

16. Sample Calculation
Time High, Time Low Set
Determine the value for RA:

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

18. 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.

19. 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):

20. Frequency, Duty Cycle Set
Design Solution
Frequency, Duty Cycle Set
3- Solve for Rb:
4-Solve for Ra:
5-Use standard values (optional step):
Ra=100k
Rb=47k

21. Frequency, Duty Cycle Set
Design Solution
Frequency, Duty Cycle Set
6- Calculate actual frequency and DC:

22. Frequency, Duty Cycle Set
Design Solution
Frequency, Duty Cycle Set
7- Create the circuit diagram using EWB:
555 Timer, 75.7% D.C.

23. Minimum Value for Ra
The discharge transistor causes the capacitor to discharge to ground.
Ra must have a minimum value of 25 to prevent a short circuit of the power supply through the discharge transistor.
Minimum Value

24. In-Class Practice Problem
Design an oscillator with a frequency of 500Hz and a duty cycle of 80%. Use a 10μF Capacitor. Calculate using each of the equation sets.

25. Other Astable Configurations

26. Astable Configuration#2
Rb must be < .5 Ra

27. Astable Configuration #3

28. 555 Timer Operation

29. Internal Operation of the 555 Timer
Understanding the internal operation of the 555 timer is important
The device combines various circuit theories.
Various aspects provide a preview of other circuits, such as the Analog to Digital decoders.
Understanding its internal function makes it easier to create new designs with the device.

30. Comparator
The comparator is an operational amplifier (op-amp) configuration.
The comparator compares 2 analog voltages and provides a digital output. + -
If V+ > V-, the output is a digital 1
If V- > V+, the output is a digital 0

31. Reference and Comparators
The Comparators provide digital logic to an SR Latch
A 3-resistor voltage divider provides reference voltages
Comparators compare voltage levels.
If “+” is higher, output = 1
If “-” is higher, output = 0

32. Reference and Comparators
Pin 6: “Threshold” connection (high Comparator)
Pin 8: Vcc connection
Pin 5: “Control” connection, used with filter cap.
Pin 2: “Trigger” connection (low Comparator)
Pin 1: Gnd connection

33. Latch and Output
The Q’ output controls the transistor (“on” or “off”).
The transistor acts like a switch.
The SR Latch holds its output states
The reset input can be used to enable or disable the timer.
The buffer has important electrical functions.

34. Latch and Output Pin 7: “Discharge” connection
Pin 4: Reset connection (active low reset)
Pin 3: Digital Output

35. Charge Animation
Discharge Animation

36. What is a disadvantage of the 555 as a timing device?
Specification sheet
From the specification sheet for the LM555, determine the following:
Operational voltage range
Maximum current output for each state
Frequency Range
Output rise and fall time
What is a disadvantage of the 555 as a timing device?

37. Function of the Control Input (Pin5)
Pin 5 is the control voltage connection, and is used to access the 2/3 Vcc point of the voltage divider.
Normally, a 0.01F capacitor is connected to ground to provide output voltage stability.
Used as an input for some applications.
AC to pulse modulation
voltage controlled oscillator

38. Design Exercises

39. Design Exercises
Using a 555 timer, design an oscillator with an output of 6Hz with a duty cycle of 60%. Use a 100μF capacitor. Build in EWB.
Using a 555 timer, design an oscillator with an output of 10KHz with a duty cycle of 50%. Use a 3.3μF Capacitor. Build in EWB.

40. Animated Slides
The following slides contain animations to demonstrate the operations of:
555 as an astable: charge cycle
555 as an astable: discharge cycle

41. Capacitor Charges via Ra and Rb Latch in a set state
+ < -
- < + 1
+ > - 1 1 1
- > + Vc
Capacitor Charges via Ra and Rb
Latch in a set state
Q’ is low; Q output is high
Capacitor continues to charge
Lower comparator provides logic low. Latch in hold state.
Upper comparator + input is greater than 2/3 Vcc reference
Latch receives a reset state
Q’ is high; Q output is low
Transistor is “on” and a connection to ground is made.
Capacitor begins to discharge
Animated charge cycle

42. Capacitor is discharging. Q output is low.
- > +
- < + 1 1 1
+ > - 1
+ < - Vc
Capacitor is discharging. Q output is low.
Upper comparator + voltage less than reference voltage.
Latch in a hold state.
Lower comparator + voltage is greater than – voltage.
Latch is set. Q’ is low.
Q output is high.
Transistor is “off”. Capacitor begins to charge.
Animated discharge cycle

43. END