RC audio oscillator with 555

  
Transistor Q5 and the 1000 ohm resistor form the variable element needed for controlling the frequency of VCO by limiting the charging current flowing into the 0.15 timing capacitor according to the forward bias being applied to Q5. As the voltage on pins 2 and 6 of Ul reach 2A Vcc (about 6 volts with a 9-volt supply) the timer will fire and pin 3 will be pulled low. Pin 7, an open collector output, goes low and begins to discharge the timing capacitor—through the 3.3 kilohm resistor.
RC audio oscillator with 555  - schematic

The discharge time provided by this resistor assures a reasonable, although asymmetrical, waveform for the aural signal generated by Ul. At lA Vcc the internal flip-flop resets, the output on pin 3 goes high, the open collector output on pin 7 floats, and the timing cycle begins again.




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