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Design Project: Pulse-Width Modulation (PWM) Signal Generator
6 questions By Tony R. Kuphaldt
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Question 1 of 6
What shape of voltage waveform would you expect to measure (using an oscilloscope) across capacitor C1? How does this waveform interact with the DC reference voltage at the wiper of Rpot2 to produce a pulse-width modulated square wave output?
Reveal answerThe waveform will be a “sawtooth” shape. When compared against the DC reference voltage at Rpot2‘s wiper by the LM339 comparator IC, the result is a square wave of varying duty cycle.
Notes:Many students will initially be puzzled by the operating principle of this circuit. The best way I have found to answer their questions, I have found, is with a multi-trace oscilloscope (preferably one that can show three traces simultaneously). Connect one channel to the top of C1, the next channel to Rpot2‘s wiper, and the third to the comparator’s output terminal. A picture, as they say, is worth a thousand words.
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Question 2 of 6
Which direction would you have to move the wiper on potentiometer Rpot2 to increase the duty cycle of the output waveform? Explain your answer.
Reveal answerMoving the wiper up (as drawn in the schematic) produces greater duty cycles.
Notes:Notice that I did not even hint at why this is. Ask your students to fully explain their answers!
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Question 3 of 6
The design recommendation for this circuit is to make resistors R2 and R3 both equal to the resistance of the potentiometer Rpot2. This way, the full mechanical range of the potentiometer will be useful for adjusting duty cycle: fully turning it one way will just produce a 0% duty cycle, while fully turning it the other way will just produce a 100% duty cycle.
Explain why those resistor values need to be equal to achieve this optimum usage of pot range. Hint: it has something to do with the internal workings of the 555 timer IC.
Reveal answerThe sawtooth signal seen at capacitor C1 (with reference to ground) oscillates between 1/3 and 2/3 supply voltage.
Notes:This question forces students to explore what the 555 is really doing, and to recognize the comparator’s function in generating PWM square waves.
