1 / 19

Section 24.4 The 555 Timer

This section explores the fundamentals of the 555 Timer in astable mode, particularly focusing on rectangular waves and duty cycle calculations. It discusses the relationship between pulse width (PW) and cycle time (T) to determine duty cycle, along with practical applications involving a capacitor in an RC circuit. The content includes activities that require calculating the time constants for charging and discharging, as well as building a simulation in Multisim. Gain insights into frequency, pulse width, and duty cycle calculations critical for timer applications.

jacqui
Télécharger la présentation

Section 24.4 The 555 Timer

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Section 24.4The 555 Timer

  2. Rectangular Waves- Reminder: Duty Cycle – ratio of pulse width to cycle time where PW= the pulse width of the circuit input T= the cycle time of the circuit input 2

  3. Rectangular Wave If you repeatedly switch between the battery and the short you are effectively applying a rectangular time pulse to the RC circuit. b) If SW = PW, what is the duty cycle? a) If SW = .5 PW, what is the duty cycle?

  4. Rectangular Wave Response The voltage across the capacitor will behave as below in response to such a wave:

  5. In-Class Activity • For the circuit above, what value of C will allow the capacitor to “fully” charge and “fully” discharge if the square wave has a period of 2ms? • What is the time constant for this circuit?

  6. In-Class Activity • Determine the charging time constant and the discharging time constant in the circuit below:

  7. 555 Integrated Circuit Timer

  8. Inside the 555 is a voltage divider

  9. The 555 Compares the voltage at pin 6 to 2/3 Vcc The 555 Compares the voltage at pin 2 to 1/3 Vcc

  10. Typical Wiring for a 555(astable mode) Notice that the boxed area is just an RC circuit

  11. Current Flow During Capacitor Charging The capacitor charges via RA and RB until vc = 2/3 Vcc

  12. Capacitor Voltage and Output Voltage Vcc 0 Red arrows indicate voltages during charging phases

  13. Current Flow During Capacitor Discharging The capacitor discharges through just RB until vc = 1/3 Vcc

  14. Capacitor Voltage and Output Voltage Vcc 0V Blue arrows indicate voltages during discharging phases

  15. Timing During the charging phase, the capacitor voltage can be written as: During the discharging phase, the capacitor voltage can be written as:

  16. In-Class Activity • How long does it take to:a) charge up from 1/3 Vcc to 2/3 Vcc? • Hint: Calculate time to reach each voltage first then subtract • b) discharge from 2/3 Vcc to 1/3 Vcc

  17. 555 Timing • Pulse width – capacitor charging time:PW = (RA + RB)C ln(2) sec • Space width – capacitor discharging time:SW = RB C ln(2) sec • Period = Pulse width + Space widthT = PW + SW = (RA + 2RB)C ln(2) sec • Frequency – 1/Periodf = 1/(RA + 2RB)C ln(2) = 1.443/(RA + 2RB)C Hz

  18. 555 Duty Cycle • Duty Cycle = 100 PW/T %or

  19. In-Class Activity • In Multisim, build this 555 circuit, use RA = RB = 1kΩ and C = 1µF. • 555 can be found in Place-Mixed-Timer-LM555CM • What are the following: T, PW, SW, duty cycle, f? • Show them in hand calculations and record what you see on Multisim oscilloscope

More Related