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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 an oscillator which could be used as a square-wave generator. For more detail on the 555 Astable Circuit.

The frequency of the output voltage is completely dependent on the values of the resistors and capacitors used in the circuit. So the more accurate the value of components, and the less they drift, the more accurate the resultant output frequency. The actual output voltage is determined by the supply voltage [Vcc] and has nothing to do with the timing components.

555 Astable Multivibrator schematic
555 Astable Multivibrator

555 Reducing Frequency Multivibrator

Same basic circuit as before, except the path to Vcc for the RC network has been changed. By changing Vcc the timing is changed because the circuit is nothing more than a 'timer' measuring the amount of time it takes for the capacitor to charge to a 'set' threshold level.

555 Reducing Frequency Astable Multivibrator
555 Astable Multivibrator Reducing Frequency

In this particular case the RC supply voltage is based either on a discharging capacitor [C1] or the voltage divider circuit formed by resistor R1 and capacitor C1, depending on the position of the switch. While the 555 is still set to operate off Vcc [V].

Using a mechanical switch for SW1 does not really allow the circuit to be used for any meaningful application other than maybe some experimentation. However a transistor or FET could easily be used as a semiconductor switch and replace the mechanical switch. Of course adding a transistor and controlling it would be a completely different topic, as the control to the transistor could be a circuit in it self. A general purpose PNP 2N2904 Transistor or a general purpose NPN 2N2222 Transistor could be used.

Circuit Description:
Power: The 555 IC may be operated with any voltage between 4.5 volts and 16 volts. As with any integrated circuit always Bypass the Vcc pin to ground via a 0.1uF ceramic capacitor [not shown in the circuits above]. Pin 8 is Vcc and Pin 1 is ground.
Control Voltage: Pin 5 of the 555 is an input and is not used in this application. However if board space allows the control voltage should be bypassed using a 0.01uF capacitor between pin 5 and ground.
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 C2 which develops the voltage to the pin. The Trigger is also connected to the Threshold pin which also sees the same voltage.
Threshold: Pin 6 is also connected to C2 which develops the voltage to the pin. The Threshold is also connected to the Trigger pin which also sees the same voltage.
Discharge: Pin 7 is connected between resistors R2 and R3. The capacitor C2 will charge through R2 and R3, but will only discharge through R3 into pin 7. This allows for a duty cycle that may be adjusted, but because resistors R2 and R3 are in series a 50% duty cycle is not possible.

Timing Equations:
Charge Time; t1 = 0.693*(R1 + R2)*C1
Discharge Time; t2 = 0.693*R2*C1
Total period; T = t1 + t2 = 0.693*(R1 + 2*R2)*C1
Oscillation frequency; f = 1/T = 1.44/[(R1 + 2*R2)*C]
Duty cycle = D = R2/(R1 + 2*R2)

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.

Editor note; at least me in field the 555 is not widely used.

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