Download presentation

Presentation is loading. Please wait.

1
Section 24.4 The 555 Timer

2
Rectangular Wave In the RC circuit from last week, we charged and discharged the capacitor by moving a switch. If you repeatedly switch between the battery and the short you are effectively applying a rectangular time pulse to the RC circuit.

3
3 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

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

5
555 Integrated Circuit Timer

6
Inside the 555 is a voltage divider

7
The 555 Compares the voltage at pin 6 to 2/3 Vcc The 555 Compares the voltage at pin 2 to 1/3 Vcc

8
Typical Wiring for a 555 (astable mode) Notice that the boxed area is just an RC circuit

9
Current Flow During Capacitor Charging The capacitor charges via R A and R B until v c = 2/3 V cc

10
Capacitor Voltage and Output Voltage Red arrows indicate voltages during charging phases Vcc 0

11
Current Flow During Capacitor Discharging The capacitor discharges through just R B until v c = 1/3 V cc

12
Capacitor Voltage and Output Voltage Blue arrows indicate voltages during discharging phases Vcc 0V

13
Timing During the charging phase, the capacitor voltage can be written as: During the discharging phase, the capacitor voltage can be written as:

14
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

15
555 Timing Pulse width – capacitor charging time: PW = (R A + R B )C ln(2) sec Space width – capacitor discharging time: SW = R B C ln(2) sec Period = Pulse width + Space width T = PW + SW = (R A + 2R B )C ln(2) sec Frequency – 1/Period f = 1/(R A + 2R B )C ln(2) = 1.443/(R A + 2R B )C Hz

16
555 Duty Cycle Duty Cycle = 100 PW/T % or

17
In-Class Activity In Multisim, build this 555 circuit, use R A = R B = 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

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google