1. 555 Timer ©Paul Godin Updated February 2008

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. 555.2

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. 555.3

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. 555.4

5. 555 Layout Also available: 556 (two-555’s in one DIP package) 555 in a “metal can” configuration 555.5 12341234 87658765 Ground Trigger Output Reset Vcc Discharge Threshold Control 1 2 3 4 5 6 7 8 Ground Trigger Output Reset Vcc Discharge Threshold Control

6. Operation of the 555 Timer Astable Multivibrator 555.6

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 555.7

8. General Configuration ◊Basic connections: ◊Ground ◊Vcc ◊Note: some 555 timers may function at voltages other than 5 volts. ◊Reset (active low) ◊Output 555.8 12341234 87658765 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 555.9 12341234 87658765 Ground Trigger Output Reset Vcc Discharge Threshold Control

10. Astable Configuration #1 (“Standard” Configuration) 12341234 87658765 Ground Trigger Output Reset Vcc Discharge Threshold Control Vcc Ra Rb C Vcc 555.10

11. Astable Filter Cap 0.01μF Calculated Values Note: Duty Cycle must be > 50% 555.11

12. Astable Configuration #1 (“Standard” Configuration) Minimum duty cycle > 50% 555.12

13. Calculations: Astable THTH TLTL Notes: The value 0.693 is a factor associated with the charge/discharge cycle of the 555 timer. Duty Cycle must be > 50% Time High, Time Low Set 555.13

14. 555.14 Sample Calculation ◊Design an oscillator with a frequency of 200Hz with a duty cycle of 78%. 1.Determine Period (T): 2.Determine T H and T L : Time High, Time Low Set

15. 555.15 3.Since there are 2 variables in the T L equation, select C: 4.Determine R B by using the T L equation: C=10μF Sample Calculation Time High, Time Low Set

16. 555.16 5.Determine the value for R A : Sample Calculation Time High, Time Low Set

17. Calculations: Astable Notes: The value 0.693 is a factor associated with the charge/discharge cycle of the 555 timer. Duty Cycle must be > 50% Frequency, Duty Cycle Set 555.17

18. Sample Design ◊Build an oscillator using a 555 timer with a frequency of 72kHz at 75% D.C. Use a 100F capacitor. Frequency, Duty Cycle Set 555.18

19. Design Solution 1- Determine the ratio of the resistors Ra and Rb: 2- Use the ratio in the frequency equation (substitution): Frequency, Duty Cycle Set 555.19

20. Design Solution 3- Solve for Rb: 4-Solve for Ra: 5-Use standard values (optional step): Ra=100k Rb=47k Frequency, Duty Cycle Set 555.20

21. Design Solution 6- Calculate actual frequency and DC: Frequency, Duty Cycle Set 555.21

22. Design Solution 7- Create the circuit diagram using EWB: 555 Timer, 74.2kHz @ 75.7% D.C. Frequency, Duty Cycle Set 555.22

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 555.23

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. 555.24

25. OTHER ASTABLE CONFIGURATIONS 555.25

26. Astable Configuration#2 Rb must be <.5 Ra 555.26

27. Astable Configuration #3 555.27

28. 555 Timer Operation 555.28

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. 555.29

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 0 555.30

31. Reference and Comparators The Comparators provide digital logic to an SR Latch Comparators compare voltage levels. If “+” is higher, output = 1 If “-” is higher, output = 0 A 3-resistor voltage divider provides reference voltages 555.31

32. Reference and Comparators Pin 8: Vcc connection Pin 5: “Control” connection, used with filter cap. Pin 1: Gnd connection Pin 6: “Threshold” connection (high Comparator) Pin 2: “Trigger” connection (low Comparator) 555.32

33. Latch and Output The Q’ output controls the transistor (“on” or “off”). The SR Latch holds its output states The transistor acts like a switch. The reset input can be used to enable or disable the timer. The buffer has important electrical functions. 555.33

34. Latch and Output Pin 4: Reset connection (active low reset) Pin 7: “Discharge” connection Pin 3: Digital Output 555.34

35. Charge Animation Discharge Animation 555.35

36. 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? 555.36

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 555.37

38. Design Exercises 555.38

39. Design Exercises 1.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. 2.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. 555.39

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 555.40

41. + < - 0 - < + 1 Vc 0 1 Animated charge cycle + > - 1 - > + 01 0 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 555.41

42. Vc 1 0 1 0 + < - - < + Animated discharge cycle 0 1 + > - 0 - > + 1 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. 555.42

43. ©Paul R. Godin prgodin ° @ gmail.com 555.43 END