PROCEDURE
  Astable Multivibrator
  • Use the 555 timer IC to build an astable multivibrator, as shown in Figure 2.1.  Use a potentiometer for R1 and a fixed resistor for R2.  Using three different capacitor values and a range of values for R1, verify the timing diagram, shown in Figure 2.2, provided by the manufacturer.  View the output on an oscilloscope.
  • Connect a speaker to the output of the 555 and to the 5-volt power supply.  If the output is too loud you can add a resistor between the speaker and the power supply.  The frequency of the tone can now be varied with the potentiometer.
  • Connect Pin 5 of the timer to a variable voltage supply.  Vary the voltage to a maximum of the supply voltage.  What happens to the output?

Figure 2.1  Astable Multivibrator

 

Figure 2.2  Astable Timing Diagram
 Monostable Multivibrator
  • Build a monostable multivibrator as shown in Figure 2.3.  Use a potentiometer for R.  Using three different capacitor values and a range of values for R, verify the timing diagram, shown in Figure 2.4, provided by the manufacturer.  View the output on an oscilloscope.

Figure 2.3  Monostable Multivibrator

Figure 2.4  Monostable Timing Diagram
Use the "Output" on the benchtop function generator, set at a low frequency square wave, to provide the input pulse.  The pulse input should be a narrow pulse that goes from 5V to 0V.   To get a narrow pulse output:
  • Pull out the "SYM" button and turn it completely to the right.  This should give a very wide positive-going pulse and a very narrow negative-going pulse.
  • Adjust the Amplitude to produce a 5V peak-to-peak wave.
  • Pull out the "DC OFFSET" button and adjust it so that the output on the oscilloscope goes from 0V to 5V.
For your first reading:
  • Use a 1.0 microF capacitor and a 10K resistor.
  • Adjust the frequency to about 10 Hz.
  • The output pulse from your circuit should be a positive going pulse, approx. 10 ms wide, that goes positive when the input goes negative.  The output pulse should be longer that the input pulse.
For subsequent readings, you will need to adjust the frequency to make the negative pulse narrow enough, or to make the pulses far enough apart to allow for the length of the output pulse (for example, with a 1K resistor and 1.0 microF capacitor you will need a frequency of about 120 Hz in order to see the time delay)