555 IC Square Wave Oscillator circuit

  
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A form of relaxation oscillator which comprises two stages that are coupled so that the input of one is derived from the output of the other. Basically two amplifiers cross-coupled with regenerative feedback, in it`s most simplistic form. One of the amplifiers is always conducting while the other amplifier is in cut off.In this case both amplifier circuits are contained within a single
555 IC Square Wave Oscillator circuit - schematic

555 IC. However the 555 integrated circuit contains a great many transistors. In this case the 555 timer below is configured as an Astable multivibrator with the output having a 50% duty cycle, a square wave [t1 and t2 are equal time periods]. The Astable multivibrator circuit has no stable state. With no external signal applied, the internal transistors alternately switch from cutoff to saturation at a frequency determined by the RC time constants of the coupling circuits. So an Astable multivibrator is an oscillator which could either be used as a pulse generator or square-wave generator depending on the value of the resistor and capacitors used in the circuit. This particular circuit will produce a 50% duty cycle if R1 is set to 51k ohms and R2 is set to 22k ohms, because the capacitor will charge via R1 and discharge via R2. The capacitor C1 then sets the frequency of operation. The normally connected 555 acts as a Astable multivibrator but will not generate a 50% duty cycle because the capacitor charges through both resistors but only discharges through one of the resistors. For reference an Astable multivibrator is shown above. Note the difference in resistor connections. A related transistor circuit configured as a Transistor Astable multivibrator. Note that this circuit will not oscillate if RB is greater than 1/2 RA because the junction of RA and RB cannot bring pin 2 down to 1/3 VCC and trigger the lower...



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