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555 Astable Multivibrator

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 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.

555 Astable Multivibrator circuit schematic
555 Astable Multivibrator

555 Astable Pulse Adjustment

555 Astable Multivibrator, with Pulse Width Adjustment
555 Astable Pulse Width Adj

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 Oscillator is also possible using a different circuit arrangement. Although as the other page suggests a flip flop could be added to the output of the 555 to even out the duty cycle, which also happens to produce a binary divider. Clocking a 7473, with both the J & K inputs tied high, toggles the output each time the clock goes high [the clock to the flip flop is the output from the timer].

Circuit Description:
The 555 integrated circuit should have a 0.1uF ceramic bypass capacitor [C3] between Vcc pin and ground; however the capacitor is not a requirement, just good design practice.
In addition the Control Voltage [Pin 5] of the 555 is not used in this application so an optional 0.01uF bypass capacitor [C2] is used between pin 5 and ground. This bypass capacitor is also optional, and pin 5 may be left open if required.
Output: Pin 3 is the output of the circuit. The output will toggle or oscillate between ground and [near] Vcc. With Vcc set at 5 volts the output will switch to a minimum voltage of 2.75 volts or a typical voltage of 3.3 volts. Using a 15 volt Vcc as another example the 555 will typically reach 12.5 volts [200mA source], 13.3 volts [100mA source]. The rise and fall times of the output pulse will be about 100nS.
Trigger: Pin 2 is connected to C1 which develops the voltage to the pin. The Trigger is also connected to the Threshold pin which also sees the same voltage. As the Trigger voltage [Vc] falls below 1/3 Vcc the internal comparator within the 555 will trigger.
Threshold: Pin 6 is also connected to C1 which develops the voltage to the pin. The Threshold is also connected to the Trigger pin which also sees the same voltage. As the Threshold voltage [Vc] rises above 2/3 Vcc the internal comparator within the 555 will trigger.
Discharge: Pin 7 is connected between resistors R1 and R2. The capacitor C1 will charge through R1 and R4, but will only discharge through R2 and R3 into pin 7. This allows for a duty cycle that may be adjusted via the two potentiometers through the blocking diodes.

555 Astable Multivibrator free running Frequency Graph
555 Astable Multivibrator Frequency

Generic Part Numbers: LM555, SE555, NE555, TLC555
Generic Packages: SOIC, SOP, DIP
Check the data sheet for the complete part numbers.
The 555 Timer has an operating temperature range of 0C to 70C
The 555 Timer has an storage temperature range of -65C to 150C
Also refer to How to Derate an IC.

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