When a current of air hits tbe piezo element, a small signal is generated and is fed through Cl and Rl to inverting input pin 2 of one section of the LM324. That causes output pin 1 to increase. Resistor R3 is used to adjust the sensitivity of the detector. The circuit can be set so high as to detect the wave of a hand or so low that blowing on the element as hard as you can will produce no output. Resistor R2 is used to adjust the level of the output voltage at pin 1.
The detector circuit can be used in various control applications. For example, an SCR can be used to controlll7-Vac loads as shown. Also, an npn transistor, such as a TIP29, can be used to control loads as shown.