A condenser microphone was used to sense a bell ringing. Since the microphone was going to be very close to the bell, a small amount of amplification was needed. After looking at the output of the bell ringing, we found a 50x amplifier to do the job nicely.
The lower opamp was used as a low current voltage divider.
The signal from amplification was sent to a comparator with a threshold at 3V. Although the comparator would output an oscillating signal, we found this to be acceptable since the bell was used for mutiny, and it would be a while before another mutiny would occur.