MFOS Function Generator

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Often in synth-DIY trouble-shooting and/or circuit experimentation, a signal source with variable frequency and amplitude is just what you need to test a circuit’s behavior. If the signal source has a variety of wave-shapes, it will serve in more scenarios. This article will present a 9V battery-powered function generator that outputs sine, square, and triangle waveforms. The frequency is adjustable from about 20 Hertz to about 11KHz and the output level can be varied from 0 to about 3V, peak to peak. I kept the output level within this range as the circuit is essentially powered by a virtual +/-4.5V power supply and I wanted the unit to continue to function as long into the 9V battery’s depletion as possible. The circuit drains about 11mA when used with the LED shown in the schematic. However, if you buy a power switch with an obvious passive indication of state, you can leave the LED off and the circuit will draw about 9mA. In either case, you can count on a good long life on a 9V alkaline battery. The signal can be injected into the input of a VCF or VCA circuit under development or a mixer or an amplifier input you want to test. Since it’s battery powered you can leave one in your toolbox.