Section 24 4 The 555 Timer Rectangular Waves

• Rectangular Waves - Reminder: Duty Cycle – ratio of pulse width to

Rectangular Wave • If you repeatedly switch between the battery and the short you

Rectangular Wave Response • The voltage across the capacitor will behave as below in

In-Class Activity • For the circuit above, what value of C will allow the

In-Class Activity • Determine the charging time constant and the discharging time constant in

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

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

Current Flow During Capacitor Charging The capacitor charges via RA and RB until vc

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

Current Flow During Capacitor Discharging The capacitor discharges through just RB until vc =

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

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

In-Class Activity • How long does it take to: a) charge up from 1/3

555 Timing • Pulse width – capacitor charging time: PW = (RA + RB)C

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

In-Class Activity • In Multisim, build this 555 circuit, use RA = RB =

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Section 24. 4 The 555 Timer

• 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

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 2 ms? • 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 RA and RB until vc = 2/3 Vcc

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

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

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

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 voltage first then subtract • b) discharge from 2/3 Vcc to 1/3 Vcc

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 width T = PW + SW = (RA + 2 RB)C ln(2) sec • Frequency – 1/Period f = 1/(RA + 2 RB)C ln(2) = 1. 443/(RA + 2 RB)C Hz

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

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