In the previous tutorial, we learnt what a 555 timer IC is, how it works and how to use it to design a monostable multivibrator. In this tutorial that comes with a video, we will learn how to design an astable multivibrator with a 555 timer IC.
An astable multivibrator is an oscillator that has no stable state. Unlike the monostable multivibrator that has one stable state, the astable multivibrator has no stable state, the oscillation does not stop at any state, it keeps vibrating until power is cut off from the design. apart from this written tutorial, we will also have a video tutorial on how to simulate an astable multivibrator with the 555 timer IC. The video tutorial is shown below.
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The astable multivibrator can be designed with a 555 timer IC to give output with duty cycle more than 50% and duty cycle less than 50% or 50%. We shall discuss what duty cycle is in a moment.
Astable multivibrator means that the device will produce an output that goes high and stays high for some time, and then go low and stay low for some time, the cycle repeats for as long as there is power supply to the device. See image below.
The image above shows the waveform of astable multivibrator output designed with a 555 timer IC. The output is on pin 3 of the 555 timer IC, the output oscillates from 0V to VCC-1.5, the time the output is high is ton and the time it is low is toff, the total time the oscillation is ton + toff = T.
Above is the circuit diagram of an astable multivibrator using 555 timer IC. The circuit is interfaced with a relay to switch a 12 volts bulb. With the values of the series network resistors and capacitor used, the circuit produces an oscillation that stays ON for one second and OFF for one second, we shall show see how to calculate this ON and OFF times in a moment.
When the 555 timer IC is used to design an astable multivibrator, there are some parameters that are calculated as well, these parameters include:
- ON time (ton)
- OFF time (toff)
- Vibration (oscillation) Cycle time
- Duty cycle
- ON time (ton): The period of time the output voltage produced on pin 3 of the 555 timer IC stays high before it goes low.
- OFF time (toff): The period of time the output voltage produced on pin 3 of the 555 timer IC stays low before it goes high.
- Cycle time: The sum of the ON time (ton) and OFF time (toff) of the astable multivibrator.
- Frequency: number of complete oscillations of the vibration in one second
- Duty cycle: The ratio of the ON time (ton) to the sum of the ON time (ton) and the OFF time (toff) time of the 555 timer IC.
To calculate these parameters, the formulas are below:
ON time (ton): 0.694(R1 + R2)C in seconds(s)
OFF time (toff): 0.694R2C in seconds (s)
Vibration cycle time (T): ton + toff in seconds(s)
= 0.694(R1 + R2)C + 0.694R2C = 0.694C(R1 + R2 + R2)
= 0.694C(R1 + 2R2) = 0.694(R1 + 2R2)C
Frequency (f): 1/T in hertz (Hz)
Duty cycle = ton/(ton + toff)
= 0.694(R1 + R2)C/[0.694(R1 + R2)C + 0.694(R1 + 2R2)C]
If you simplify that, you will get
(R1 + R2/ R1 + 2R2) X 100%
From this resultant formula of the duty cycle, we can see that, the duty cycle will never be less than 50%, this is because the capacitor C charges through the two resistors R1 and R2, and discharges only through R2. To understand vividly what happens inside the 555 timer IC when it is in action read our previous tutorial on how to use a 555 timer IC. This then makes the ON time of the 555 timer IC greater than the OFF time. Hence, we cannot obtain equal “mark-to-space” ratio.
We must redesign the circuit to be able to produce oscillation with duty cycle less than 50%.
To obtain such a circuit with the design feature stated above, we readjust the circuit, there are various ways to do that by changing the position of the series resistors, however, the design I prefer to use is the design shown below that uses two diodes.
From the design shown below, when the capacitor is charging, it charges through R1 and the diode D1, and when it discharges, it does so through R2 and D2. In this way, we can obtain duty cycle less than 50%. The formula for the ON and OFF times of the timer is shown below.
ON time (ton): 0.694R1C
OFF time (toff): 0.694R2C
Duty cycle = 0.694R1C/(0.694R1C + 0.694R2C)
With this, we can obtain a duty cycle less than 50% by making R1 smaller.
Vibration cycle time (T): ton + toff
= 0.694R1C + 0.694R2C = 0.694(R1+R2)C
Frequency (f): 1/T
Having shown the various parameters of the 555 timer IC, it’s time to calculate some of them. We shall use the design values of the components in the circuit above.
To calculate the on time of the astable multivibrator in figure 2 above, we have
Firstly, you convert the resistance and capacitor values to ohms and farads respectively.
ON time (ton): 0.694(R1 + R2)C
0.694 X (1000 + 150,000) X 0.00001 = 1.0second
OFF time (ton): 0.694 X 150,000 X 0.00001 = 1.0second
Total vibration time: ton + toff = 1+1 = 2seconds
Frequency: 1/T = ½ = 0.5 Hz
Duty cycle: ton/(ton + toff) = ½ X100% = 50%
This means that the bulb blinks every 1 second, that is, it goes ON for a second and goes OFF for a second. With the 555 timer IC, you can design a lot of circuit as stated in the previous tutorial. Subsequently, we will design various devices with this IC. However, before then, you can other wonderful tutorials listed below: