A_555_USED_AS_AN_RC_AUDIO_OSCILLATOR

Published:2009/7/6 21:57:00 Author:May | From:SeekIC

Circuit Notes Transistor Q5 and the 1000 ohm resistor form the variable element needed for controlling the frequency of VDO by limiting the charging current flowing into the 0.15μF timing capacitor according to the forward bias being applied to Q5. As the voltage on pins 2 and 6 of U1 reach 2/3 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. The discharge time provided by this resistor assures a reasonable, although asymmetrical, waveform for the aural signal generated by U1. At 1/3 VCC the intemal 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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