Expecting replies by tonight, eh? Is that because the assignment is due by tomorrow? :wink:
Now, looking at this circuit, I'm sure you can work it out yourself, you get the credit for it, anyway.
I do not know what pwm stands for (do not expect anybody else to know the same acronym's as you do) but looking at it I could work out what the circuit does.
A few hints; the 555 is a timing/ oscillator chip, check up what exactly it does in that configuration.
The other chip is a binary counter with a tri state output (I looked that up with "google"). So, what does a binary counter do?
You'll also see it has a clock (CLK) input. Where does that clock pulse come from? An oscillator perhaps? So you have part of the inverter chip performing that function, the other part is conditioning the signal from the 555 output.
Over to you, have fun :lol:
Klaus