Technique to trigger oscilloscope on peak to peak value
Accepted comment from mkeith as answer:


  Use the delay trigger. Trigger on the rising edge at 9V, with a 250 ms
  delay. OR set the trigger for 5V with the trigger mode set to normal.
  The last wave to trigger the scope will be near the middle, and it
  will stay on the scope in normal trigger mode. Auto mode will
  retrigger the scope even if there is no trigger present, but normal
  mode won't do that.