Urgent help please--need to make sense of a circuit

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  reachrkata

  This is how I understood the circuit -
  
  1) R1-C1-D1-D2 form a clipping circuit such that, before diode D3, we get a clipped sine wave of +5V to -5V p-p waveform.
  2) R3-Q1-R4 acts as the inverter for this wave and we get a 0V-5V signal at Collector of Q1 corresponding to +5V -5V at the base. Note that D3 prevents the negative part. Strictly speaking D3 is unnecessary because the transistor itself would take care of that.
  3) R7-Q2-R8 inverts this once again so that the 5V-0V signal here is in phase with the +5V -5V input to Q1.
  4) Both these out-of-phase signals are given to CR circuits which act as differentiators giving -ve and +ve pulses at the corresponding edges of both signals.
  5) These Pulse waveforms are combined using the OR logic via diodes such that only the positive going pulses of both waveforms add and pass through to Q3. Note that the pulses coming here correspond to the Zero-crossings of the +5V -5V signal.
  6) Q3 is in inverter configuration such that, at its collector, there is 5V whenever the AC signal is above/below 0. It gives a short pulse of 0V at the zero crossover regions.
  
  Hope this clarifies !
  
  -Karthik
  😁