A single op amp, one fourth of an LM324 quad op amp, is configured as a standard inverting amplifier. At power up, a positive voltage is applied to the noninverting input of Ul, via R3, forcing its output high. That high output travels along three paths.. The first path is the tone output. Along the second path, by way of R5, that high is used as the drive Ul is fed back, via R4, to the inverting input of Ul. That forces Ul `s output to go low.
Fixed-frequency-generator - schematic

And that low, when fed back to the inverting input of Ul, causes the op amp output again to a high, and the cycle repeats itself. As configured, Ul provides a voltage gain of4.7 (gain= R4/Rl). The outer ring of the piezo element is usually connected to the circuit ground. The large inner circle serves as the driven area, and the small elongated section supplies the feedback signal. Resistor R5 sets BZl"s output-volume level. That level can be increased by decreasing R5 for example, to 470 ohm. To decrease the volume, increase R5 to about 2.2 Kohm, or so. Resistors R2 and R3 set the bias for op amp Ul"s positive input pin 3 to half of the supply-volt ·age level. That allows fora maximum voltage swing at U1 "s output. Although a quad op amp is specified, almost-any similar low cost single or dual op amp will work for U1a.

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