555 Individual Pulse Width circuit

Posted on Feb 6, 2014

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 a square-wave oscillator with a oscillation frequency dependent on the value of the

555 Individual Pulse Width circuit
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resistor and capacitors used in the circuit. The basic Astable Multivibrator circuit is shown below; however the description of the circuit operation is provided on the 555 Astable Circuit page. This circuit arrangement allows for independent charge and discharge times by forcing the capacitor [C1] charging through R1 and R4 [potentiometer] and the discharging current through R2 and R3 [potentiometer]. The diodes [D1 and D2] control the flow of current through the individual resistors; however both resistor paths use the same capacitor [C1]. In this case potentiometers are added to the circuit to allow for adjustment of the resistance in the timing path. However the potentiometers are not required and could be replaced with fixed resistors to control the charge and discharge times of the capacitor. Except for the addition of the potentiometers and the separate current paths created by the individual diodes, this circuit functions just like an Astable Multivibrator. The circuit could be simplified further by replacing the two variable resistors [R3 & R4] with a single variable resistor. The two end of the resistor would be connected to each of the diodes and the center tap would connect to pin 2 of the 555. The only issue with using one potentiometer is that the pulse widths are fixed together. So that as one period is increased [or decreased] the other period is decreased [or increased]. In addition a 50% Duty Cycle...

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