Posted on Apr 28, 2012

A CA3240 dual MOSFET-input device is used as a white-noise source. Op amp IC2 is used as a driver stage for the push-pull output stage formed by Q5 and Q6. Transistors Q2, Q3, and Q4 form a variablefrequency multivibrator. Rll, the speed control, is used to control the multivibrator`s frequency. The output is differentiated by C8 and applied to modulator transistor Ql, through Dl and R7. Transistor Ql modulates the gain of the output amplifier stage by changing the impedance to ground, through R6 and C4. When the multivibrator`s frequency is reduced using Rll, C8 discharges slowly, creating a sound similar to escaping steam from a stopped locomotive.

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To find the proper value for R3, short Ql "s collector to ground. Then, increase the value of R3 until the current drain from the power supply is less than 60 mA. Then remove the short from Ql. To see if the device is operating properly, close switch Sl and reduce the resistance of Rll. Wait 10 seconds, then rotate Rll slowly. You should hear a sound similar to a steam locomotive picking up speed.

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