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Laboratory Tasks

When making connections, please take care that the power supply is not reversed and that components are not loosened.

1.

Connect the oscillator circuit to a 15VDC power supply. The LED should begin flashing at a rate determined by potentiometer R4. Adjust R4 and note the effect.

2.

Adjust R4 to obtain an oscillating frequency of about 10Hz. Use an oscilloscope to monitor the voltage waveforms at LM311 pins 7, 2 and 3. Sketch these accurately in your notebook using a common time axis. What is the duration of the LED flash? Is it affected by the setting of R4?

3.

Determine the minimum and maximum rising and falling RC time constants which may be obtained at C1 by adjusting R4. Determine the minimum and maximum frequency of oscillation. Deduce an approximate expression for the relationship between the frequency of oscillation and the value of R4.

4.

Slowly decrease the DC supply voltage from 15V to the point where oscillation ceases. (Do NOT apply a supply voltage above 20V!) Does the frequency of oscillation change significantly? Explain.

5.

What would you expect to happen if diode D1 were reversed or removed? Try it and see. Please replace the diode and return it to its original orientation when finished.

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