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