Posted on Oct 23, 2012

This circuit mimics the dual-tone drone sound that`s produced by the unusual wind instrument. Urnjunction transistors Ql and Q2 are connected in similar audio-oscillator circuits. Each of the oscillator frequencies is determined by one of the two resistors selected by one of the pushbutton switches, S4 through Sll. The odd-numbered resistors in R7 to R21, determine the frequency for the Ql oscillator circuit and the even-numbered resistors in R8 through R22, determine the frequency for Q2`s circuit.

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When S4 is pressed, the positive supply is connected to both R7 and R8 through isolation diodes Dl and D2, causing both oscillators to operate. A narrow, fast-rising positive pulse is developed at Bl of both Ql and Q2 for each cycle of operation. Transistors Q3 and Q4 serve as a simple audio mixer, which is used to combine the pulses from each oscillator. The mixed signal at the collectors of Q3 and Q4 is coupled through R6 to the base of Q5, which amplifies and drives an 8-0 speaker, SPKRl. Switches, S2 and S3 are used to reduce the oscillator"s frequency by about 50% when closed, to produce a new group of tones.

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