# DIGI260 Lab5

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```					Lab #5: 555 Timer                                              SAIT- ENT-DIGI260

Lab #5: 555 Timer
Updated February 2011

Print pages 3 to 5

Objectives:
1. Observe and analyze the operation of a 555 timer as an Astable
Multivibrator.
2. Observe and analyze the operation of a 555 timer as a Monostable
Multivibrator.

Pre-Lab Preparation:
1. Attend lectures and review the theory of operation of the 555 timer.
2. Read the textbook chapters, on-line materials and specification sheets
that relate to the 555 timer as a monostable and as an astable.
3. Calculate the predicted values for each procedure.
4. Prewire the labs described in each procedure.

Equipment Required:
1.   Experimenter’s board with 5V power supply
2.   Function Generator
3.   Oscilloscope
5.   Parts kit and misc. resistor and capacitor values

1. Marks will be deducted for failing to offset the voltage on the function
generator. At this stage of the program this is inexcusable.
2. There is a recommended 100 Ω minimum value for Ra.
3. EWB 5.12 doesn’t handle the 555 timer as a monostable very well, and
sometimes causes the program to become unstable. If you wish to
simulate it, ensure that it is the only circuit in the simulation window.

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Lab #5: 555 Timer                                                           SAIT- ENT-DIGI260

Procedure 1:

Design and construct an astable multivibrator using the 555 timer, and
observe the effect of changing the values of Ra, Rb and C on the output
frequency and duty cycle.

1. Build an astable multivibrator using the 555 timer with the following values:
a. Ra = 1 k
b. Rb = 4.7 k
c. C = 0.1 F
2. Measure the output values and record these in Table #1. Sketch the
waveform in Diagram #1. Measure the rise and fall times, Pulse Width
and Pulse Separation and record these below Table #1.
3. Calculate the frequency and duty cycle for the 555 utilizing the above
values. Record the calculation steps in the Calculation #1 area further in
this lab (only record for the first set of values).
4. Record the calculated values in Table #1.
5. Record the measured values from the physical circuit in Table #1.
6. Repeat steps 4 and 5 for the Ra/Rb/C values listed in Table #1.
7. Answer the questions at the end of this lab, and demonstrate the
simulation and functional circuit to your instructor.

Procedure 2:
Design and build a monostable multivibrator using a 555 timer.

1. Design a monostable multivibrator using the 555 timer with a pulse width
of 20 ms. Select the appropriate capacitor and calculate the required
resistance. Record the calculations in the Calculation #2 area further in
this lab. Note: This configuration in EWB 5.12 is sometimes unstable. If you choose to
simulate it, ensure this is the only circuit in the simulation and that there are no extra
components.

2. Build the circuit, apply the appropriate input triggering pulse and record
the measured values in Table #2.
3. Answer the questions at the end of this lab, and demonstrate the
simulation and functional circuit to your instructor.

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Lab #5: 555 Timer                                          SAIT- ENT-DIGI260

Output waveform (Procedure 1)

Diagram #1: 555 Timer output

Voltage Values (555 Astable):

Rise and Fall Times: Rise: ________________       Fall: _____________

Pulse Width and Pulse Separation: Pw: ____________ Ps: ____________

Calculations #1 (555 as astable Multivibrator):

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Lab #5: 555 Timer                                           SAIT- ENT-DIGI260

Values            Calculated             Measured (actual)
Frequency/Duty            Frequency/Duty
Cycle                     Cycle
Ra: 1 k
Rb: 4.7 k                   /                        /
C: 0.1 f
Ra: 1 k
Rb: 2.2 k                   /                        /
C: 0.1 f
Ra: 1 k
Rb: 4.7 k                   /                        /
C: 0.01 f
Ra: 2.2 k
Rb: 4.7 k                   /                        /
C: 0.1 f
Table#1: 555 Timer as an astable multivibrator

Calculations #2 (555 as monostable Multivibrator):

Specified Pw Circuit Pw

20ms
Table #2: 555 as a monostable

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Lab #5: 555 Timer                                                SAIT- ENT-DIGI260

1. For the 555 as an astable multivibrator, what impact did increasing Ra
have on duty cycle and frequency? Explain why.

2. For the 555 as an astable multivibrator, what impact did decreasing Rb
have on duty cycle and frequency? Explain why.

3. For the 555 as an astable multivibrator, what impact did increasing C have
on duty cycle and frequency? Explain why.

4. For the 555 as a monostable multivibrator, what happens if the input
trigger time low exceeds the 20 mS output pulse width? Explain.

Signatures:

Procedure 1 physical circuit, diagram, calculation, table, and questions: ___________

Procedure 2 calculation, table, circuit and questions: _____________

Student Name: ____________________________ Date: _______________

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