Lab #6: 555 Timer SAIT- ENT-DIGI260

Lab #6: 555 Timer

Updated January 2012

Objectives:

  1. Observe and analyze the operation of a555 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
  4. PC with access to specification sheets and other reference material
  5. Parts kit and misc. resistor and capacitor values

Additional Notes:

  1. Marks will be deducted for failing to offset the voltage on the function generator for procedure 2. At this stage of the ENT program failing to offset voltage is inexcusable. Develop a good habit of checking the offset voltage before connecting the generator to the circuit.
  2. Electronic simulation software is not required for this lab.
  3. Remember, there is a recommended 100 Ω minimum value for Ra.
  4. 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.

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. Note: Electronic simulation software is not required.

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

Procedure 2:

Design and build a monostable multivibrator using a 555 timer.

Note: Simulating this circuit using EWB 5.12 software is not recommended.

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 pulses 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.

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):

Values / Calculated Frequency/Duty Cycle / Measured (actual) Frequency/Duty 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

Answer the following questions based on your observations.

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 1physical circuit, diagram,calculation, table, and questions: ______

Procedure 2 calculation, table, circuit and questions: ______

Student Name: ______Date: ______

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