Sudden disturbance detector.The circuit will always try and keep zero volts at the output, regardless of the input voltage.But when a sudden disturbance happens at the input ,the effect will be multiplied at the output, making it easier to trigger an alarm signal.Apply about 2 volt to the input and try changing the input slowly and you will see it will attempt to maintain 0 volt at the output.Then make a sudden 0.5 volt change at the input and you will notice an amplification of the sudden change.You can change the value of the bottom capacitor to decrease / increase the response delay.
