Audio Oscillator circuit

  
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The first was a simple single transistor RC audio oscillator built for a ham buddy. The breadboard circuit was literally a ball of components. Everything was just tack soldered together without a supporting circuit board. When my buddy asked for the breadboard, I figured he wanted the parts so he could rebuild the circuit in a more permanent form. Being a frugal Pennsylvania Dutchman, he merely soldered on
Audio Oscillator circuit - schematic

some connecting leads, plopped the circuit into a cardboard cup and filled it with a foam potting compound. It`s probably still working! The circuit here is a step up from the original quickie. Figure 1 shows the circuit. It`s a bare bones twin-tee audio oscillator adapted from a variety of circuits in the last 20 years of hobbyist and ham publications. My wrinkle is that you really don`t need to put it on a printed circuit board and enclose it in a fancy case. Mine is built on a scrap of Vector board. See Figure 2 for parts layout. Nothing is critical about the construction. Using a snap-on 9V battery connector almost eliminates the power switch. It`s been kicking around the shack for better than 10 years. Usually it`s used as an audio source for testing homebrew receiver audio stages. The maximum output level is about one volt RMS at 500Hz. My ear peaks at this frequency, so I use it for my testing. Simply scale the capacitor values for other frequencies and, for accuracy, measure the output level with a scope or DVM. To use the quickie, just tack solder on a resistive divider to get the level you need. Resistor values for various levels are given in Table 1. Connect the signal to the circuit you`re checking with short leads and a 0. 5uF Mylar coupling capacitor. Start with a level of about 10mv at the input of the audio level of only a microvolt. Try doing that with a commercial AC line powered audio oscillator! To check...



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