Timer 555
- 2. 55 5.2 555 timer; - What is the 555 timer? - Pin configurations of the 555 timer ; - 555 timer in astable operation ; - 555 timer in monostable operation ; - What are the 555 timer applications?
- 3. 55 5.3 555 Timer The 555 timer is one of the most remarkable integrated circuits ever developed. It comes in a single or dual package and even low power cmos versions exist - ICM7555. Common part numbers are; LM555, NE555, LM556, NE556.
- 4. 55 5.4 The 555 timer consists of; two voltage comparators a bi-stable flip flop a discharge transistor and a resistor divider network
- 5. 555 Layout 555.5 Also available: •556 (two-555’s in one DIP package) •555 in a “metal can” configuration 1 2 3 4 8 7 6 5 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. 55 5.6 Functional Diagram of 555 Timer
- 7. Reference and Comparators 55 5.7 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
- 8. Reference and Comparators 55 5.8 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)
- 9. Latch and Output 55 5.9 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.
- 10. Latch and Output 55 5.1 0 Pin 4: Reset connection (active low reset) Pin 7: “Discharge” connection Pin 3: Digital Output
- 11. 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 (R1 and R2) 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 .11
- 12. Calculations: Astable C)RR(693.0T CR693.0T BAH BL 555 .12 TH TL 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
- 15. 55 5.1 5 Monostable Characteristics Generates a single pulse of a fixed time duration each time it receives an input trigger pulse Length of pulse is given by: T= 1,1 RC
- 16. 555.16 Sample Calculation Design an oscillator with a frequency of 200Hz with a duty cycle of 78%. .1Determine Period (T): .2Determine TH and TL: s005.0 Hz200 1 F 1 T ms1.1s0011.0s005.0%22T ms9.3s0039.0s005.0%78T L H Time High, Time Low Set
- 17. © 55 5.1 7 END