The base frequency is set by R1 and the capacitor; resistor R2 determines the sweep- it is typically driven by a positive decaying voltage from the trigger generator.
The output side of the capacitor is charged negatively by the integrator, and the Schmitt trigger switches on, turning on the transistor and rapidly discharging the capacitor, in readiness for another cycle. As the opamp output does not quite swing down to the negative supply, a negative bias at the transistor base prevents it from staying permanently on.
The circuit is conveniently powered from two 4.5 Volt batteries, but will withstand three times that. Any dual opamp similar to the TL072 will do for the integrated circuit. A BC549 was used for the transistor, but this device is not critical, either. The sawtooth output from the integrator is probably more appropriate for the percussion synthesiser.
This circuit will probably do for a percussive synth, but there are certainly more ways to design a low frequency VCO.
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