Design and configure a 555 timer in the astable mode. You will need to calculate related component values and predict the output signal. To confirm your predictions, use an oscilloscope to measure the output. Record your findings in the lab report.

The 555 timer configured in the astable mode is a free running oscillator with a square wave output that has a duty cycle of greater than 50%. There are astable multivibrators with duty cycles equal to or less than 50%.If a duty cycle equal to or less than 50% is desired, the output must be inverted, or the chip reconfigured. There are other configurations that will allow for a duty cycle equal to or less than 50%. Most applications use a duty cycle of greater than 50%. This lab only covers those applications. The waveform is defined as a square wave pulse train with different high and low times. The high and low times are generally referred to as a mark and space and are defined by the external timing capacitor. To produce this mark and space, the capacitor charges to 2/3 Vcc, the threshold voltage, and then discharges to 1/3 Vcc, the trigger voltage. The output is high while the capacitor is charging to the threshold voltage, then low while the capacitor discharges to the trigger voltage. The capacitor charges and discharges at the same rate, but the charge and discharge times are different amounts. The time constant is longer for the charge cycle than the discharge cycle because the timing capacitor charges through Ra and Rb, but discharges through Rb and an internal transistor. Ra and Rb provide more resistance than Rb and the discharge transistor. The longer charge time means the mark will be of longer duration than the space, which means the duty cycle will be greater than 50%. In the astable mode, the duty cycle is also known as the mark to space ratio.

The initial charge at turn on will make the first mark a little longer because the capacitor will be charging from 0 volts instead of 1/3 Vcc.

Figure 1 defines the basic idea.

The first part of the design process requires you to consider and determine operating parameters. The initial consideration is a frequency of operation and a duty cycle. These parameters will determine the related component values for the 555 timer. Like any electrical component, the 555 timer has minimum and maximum electrical and operating characteristics. These minimum and maximum characteristics are basic design criteria and are outlined in the linear circuits data book. You must stay within the design criteria when determining your operational parameters.

There is considerable information about the 555 timer in the data book. To design your 555 timer you will not have to use all of this information, only the information for the characteristics mentioned in the lab report. There is also some special consideration for Ra, Rb and the duty cycle.

1)Note 6, under threshold current, defines maximum values for Ra and Rb in terms of Vcc. Ra and Rb limit threshold current. This current is critical in the charging of the timing capacitor. Small values for Ra and Rb work better than large values.

2)The duty cycle must be greater than 50%. Prudent values are around 70% to 80%.

Design a 555 astable multivibrator

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Introduction

Purpose

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Equipment

  1. electronics simulator
  2. one 555 timer
  3. one Ra
  4. one Rb
  5. load resistor
  6. one timing capacitor
  7. one filter capacitor
  8. one load capacitor (optional)

Procedure

Design an astable multivibrator that has a pulse output with a duty cycle of greater than 50%.

  1. Choose a frequency of operation
  2. Calculate period
  3. Choose a duty cycle
  4. Calculate the time for the space, Tlow.
  5. Choose an amount for Ra+Rb, (as a guideline)
  6. Calculate for the timing capacitor
  7. Calculate for Ra
  8. Calculate for Rb
  9. Calculate for Ra+2Rb
  10. Choose values for the load resistor, load capacitor, and filter capacitor. Nominal values are 1K, 10pF, and 0.1μF.

Construct the circuit and measure the output with an oscilloscope. Record the values as measured data. Compare the measured data with the calculated data.

SCHEMATICS

EXPERIMENT DATA

Expected results (calculations)

Frequency ______

Period ______

Duty cycle ______

Tlow ______

Ra+Rb ______

Timing capacitor ______

Ra ______

Rb ______

Ra+2Rb ______

ACTUAL RESULTS (measurements)

Frequency ______

Period ______

Duty cycle ______

Tlow ______

FORMULAE AND PROCEDURES FOR DESIGN

  1. Choose a frequency. Use the free running frequency chart in the data book as a guide. Be reasonable and prudent in your choice.
  2. Calculate the period, (time), of your frequency.
  1. Choose a duty cycle greater than 50%. 70% or 80% is prudent.
  2. Calculate Tlow. Tlow is the amount of time the pulse stays low.
  1. Choose an amount for Ra+Rb. This amount is a guideline, it won’t

be exact and will change when you factor in other values. The

purpose of choosing this value at this point is to have a resistance

value to use in the calculation of the timing capacitor. As you

complete your design you will need to calculate individual Ra and Rb

values.

  1. Calculate the timing capacitor value. Capacitance is Time divided

by resistance. The time is the period of your frequency. The

resistance is Ra+Rb that you chose in step 5.

  1. Calculate Rb.
  1. Calculate Ra
  1. Calculate the free running frequency to verify your R and C

Calculations in steps 6, 7 and 8.

The number .693 comes from the function of the timing capacitor.

At turn on, the output is high and the capacitor starts charging

toward Vcc. Once it reaches 2/3 Vcc, the output goes low and the

capacitor starts to discharge. When the capacitor discharges

down to 1/3 Vcc, the output goes back to high and the capacitor

starts to charge again toward Vcc. 2/3 Vcc is exactly half way

between 1/3 Vcc and Vcc. Therefore, charge time is

-ln(1/2)(Ra+Rb)C. The expression -ln(1/2) = .693 and -ln(1/2) is

significant of 2/3 Vcc being halfway between 1/3 Vcc. This idea

is seen again when the capacitor starts to discharge from 2/3 Vcc.

It will try to discharge to 0 volts, ground. 1/3 Vcc is halfway

between 2/3 Vcc and ground therefore, the discharge time is -

ln(1/2)(Rb)C.

The number 1.44 comes from the charge and discharge of the

timing capacitor also. The total charge and discharge time is

0.693(Ra+2Rb)C. The output frequency is the inverse of this time,

or 1.44/(Ra+2Rb)C.