# Section 24.4 The 555 Timer. 2 Rectangular Waves - Reminder: Duty Cycle – ratio of pulse width to cycle time where PW = the pulse width of the circuit.

## Presentation on theme: "Section 24.4 The 555 Timer. 2 Rectangular Waves - Reminder: Duty Cycle – ratio of pulse width to cycle time where PW = the pulse width of the circuit."— Presentation transcript:

Section 24.4 The 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

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

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

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?

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

555 Integrated Circuit Timer

Inside the 555 is a voltage divider

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

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

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

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

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

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

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

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

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

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

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

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