The circuit shown here produces a pure square tone of 440 Hz that is suitable for musical instrument tuning. The tone can be applied to the input of an amplifier or to a drive stage using a BD135 or another transistor as Ioad a Ioudspeaker The trimmer capacitor adjusts the point for a correct start for the oscillator when the power is tirned on. The diodes are gerenal-purpose types, such as the 1N914 or 1N4148.
