Section 24.4 The 555 Timer. Rectangular Wave In the RC circuit from last week, we charged and...
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Transcript of Section 24.4 The 555 Timer. Rectangular Wave In the RC circuit from last week, we charged and...
Section 24.4The 555 Timer
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
• 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
100(%)cycleduty XT
PW
Rectangular Wave Response• The voltage across the capacitor will behave as
below in response to such a wave:
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
Red arrows indicate voltages during charging phases
Vcc
0
Current Flow During Capacitor Discharging
The capacitor discharges through just RB until vc = 1/3 Vcc
Capacitor Voltage and Output Voltage
Blue arrows indicate voltages during discharging phases
Vcc
0V
Timing
CRRtc BAeVcctv )/(1)(
During the charging phase, the capacitor voltage can be written as:
CRtc BeVcctv /)(
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 = (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 + 2RB)C ln(2) sec
• Frequency – 1/Period
f = 1/(RA + 2RB)C ln(2) = 1.443/(RA + 2RB)C Hz
555 Duty Cycle
• Duty Cycle = 100 PW/T %or
%2
100
%)2ln(2
)2ln(100
BA
BA
BA
BA
RR
RR
CRR
CRRDutyCycle
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