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555 Multivibrator 50% Duty Cycle

Multivibrator definition. 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. 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].

555 50% duty cycle generator
555 Timer 50% Duty Cycle

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.

Circuit Description:
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 comparator.
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, but the output signal level will reflect this. That is, if your interfacing to TTL ICs than the 555 should be power off of 5 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 R2. The capacitor C1 will charge through R1, but will only discharge through R2 into pin 7. This allows for a duty cycle that may be adjusted, because resistors R1 and R2 are not series a 50% duty cycle is possible.

Timing Equations:
Charge Time; t1 = 0.693*R1*C1
Discharge Time; t2 = [(R1*R2/(R1 + R2)]*C1 lin [(R2 - 2*R1)/(2R2 - R1)]
Total period; T = t1 + t2
Oscillation frequency; f = 1/T
Duty cycle = D = 50% with R1=51k, R2=22k ohms, C1=0.01uF

Generic Part Numbers: LM555, SE555, NE555
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.

Related Function; 555 Decreasing Frequency, Astable Multivibrator
Related Circuit; 555 Monostable multivibrator, standard circuit

Return to the main 555 Functional Description page.

Flip Flop Introduction
Another alternative to forcing the 555 to output a 50 percent duty cycle is to feed the output of the 555 into a flip flop. A JK Flip Flop will toggle its output each time the clock input is applied, while both the J and K inputs are tied high. So regardless of the duty cycle being output from the timer, a flip flop will transform the duty cycle into a square wave. The only draw back to this approach is that the circuit requires an additional flip flop.

Because the flip flop only toggles on the rising edge of the clock signal, the output from the flip flop is half the frequency of the input [which is the output of the 555 timer]. So when a flip flop is used to clean up the duty cycle the timer needs to be configured to operate at twice the frequency. When the 555 is operated off 5 volts a TTL flip flop may be used, or a CMOS flip flop may be used when the supply is something other than 5 volts.

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