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555 Astable Multivibrator

MULTIVIBRATOR. 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. A multivibrator is basically two amplifier circuits arranged with regenerative feedback. One of the amplifiers is conducting while the other is cut off. In this case both amplifier circuits are contained within a single 555 IC.

There are 3 types of multivibrators: Astable, Monostable, Bistable.

Graphic 555 IC Astable multivibrator circuit schematic
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 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 oscillates based on the values of the components used, as defined by the equations shown below.
A related transistor circuit configured as an Transistor Astable multivibrator.

Circuit Description:
Power: With any integrated circuit always Bypass the Vcc pin to ground via a capacitor, normally a 0.1uF ceramic capacitor [not shown in the circuit above]. Pin 8 is Vcc and Pin 1 is ground; however the capacitor should be connector to Vcc and taken to the closest ground possible, which may not necessarily be pin 1 of the IC. The 555 IC may be operated with any voltage between 4.5 volts and 16 volts.
Control Voltage: Pin 5 of the 555 is an input and is not used in this application. However is board space allows the control voltage should be bypassed using a 0.01uF between pin 5 and ground. The capacitor will filter noise on the internal resistor ladder and fix the voltage level on pin 5 so the input does not float to a different voltage.
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.
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.
Discharge: Pin 7 is connected between resistors R1 and R2. The capacitor C1 will charge through R1 and R2, but will only discharge through R2 into pin 7. This allows for a duty cycle that may be adjusted, but because resistors R1 and R2 are in series a 50% duty cycle is not possible. However using a different circuit arrangement a 50% Duty Cycle Oscillator is possible. However other circuit arrangements are possible, this example allows for either the high or low pulse to be adjusted separately; 555 Individual Pulse Width Adjustment.


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)




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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