The 555 Oscillator
In the previous tutorial we saw that the 555 Timer IC can be connected either in its
Monostable mode thereby producing a precision timer of a fixed time duration, or in its Bistable mode to produce a flip-flop
type switching action. But we can also connect the 555 timer IC in an Astable mode to produce a very stable
555 Oscillator circuit for generating highly accurate free running waveforms whose output frequency can
be adjusted by means of an externally connected RC tank circuit consisting of just two resistors
and a capacitor.
The 555 Oscillator is another type of relaxation oscillator for generating stabilized
square wave output waveforms of either a fixed frequency of up to 500kHz or of varying duty cycles from 50 to 100%. In the
previous 555 Timer tutorial we saw that
the Monostable circuit produces a single output one-shot pulse when triggered on its pin 2 trigger input. In order to get
the 555 Oscillator to operate as an astable multivibrator, it is necessary to continuously re-trigger the 555 IC after each
and every timing cycle.
This retriggering is basically achieved by connecting the trigger input (pin 2) and the
threshold input (pin 6) together, thereby allowing the device to act as an astable oscillator. Then the 555
Oscillator has no stable states as it continuously switches from one state to the other. Also the single timing resistor
of the previous monostable multivibrator circuit has been split into two separate resistors, R1
and R2 with their junction connected to the discharge input (pin 7) as shown below.
Astable 555 Oscillator
In the 555 Oscillator above, pin 2 and pin 6 are connected together allowing the circuit
to re-trigger itself on each and every cycle allowing it to operate as a free running oscillator. During each cycle capacitor,
C charges up through both timing resistors, R1 and R2
but discharges itself only through resistor, R2 as the other side of R2
is connected to the discharge terminal, pin 7.
Then the capacitor charges up to 2/3Vcc (the upper comparator limit) which is determined by the
0.693(R1+R2)C combination and discharges itself down to 1/3Vcc (the lower comparator limit)
determined by the 0.693(R2.C) combination. This results in an output waveform whose voltage
level is approximately equal to Vcc - 1.5V and whose output "ON" and "OFF" time periods are
determined by the capacitor and resistors combinations. The individual times required to complete one charge and discharge
cycle of the output is therefore given as:
Astable 555 Oscillator Charge and Discharge Times
Where, R is in Ω's and C in Farads.
When connected as an astable multivibrator, the output from the 555 Oscillator will
continue indefinitely charging and discharging between 2/3Vcc and 1/3Vcc until the power supply is removed. As with the
monostable multivibrator these charge and discharge times and therefore the frequency are independent on the supply voltage.
The duration of one full cycle is therefore equal to the sum of the two individual times that the capacitor charges and
discharges added together and is given as:
555 Oscillator Cycle Time
The output frequency of oscillations can be found by inverting the equation above for the total cycle
time giving a final equation for the output frequency of an Astable 555 Oscillator as:
555 Oscillator Frequency Equation
By altering the time constant of just one of the RC combinations, the
Duty Cycle better known as the "Mark-to-Space" ratio of the output waveform can be accurately set and is
given as the ratio of resistor R2 to resistor R1. The Duty Cycle for
the 555 Oscillator, which is the ratio of the "ON" time divided by the "OFF" time is given by:
555 Oscillator Duty Cycle
The duty cycle has no units as it is a ratio but can be expressed as a percentage ( % ).
If both timing resistors, R1 and R2 are equal the output duty cycle
will be given as 2:1 or 33%.
An Astable 555 Oscillator is constructed using the following components, R1 = 1kΩ,
R2 = 2kΩ and capacitor C = 10uF. Calculate the output frequency
from the 555 oscillator and the duty cycle of the output waveform.
t1 - Charge "ON" time is calculated as:
t2 - Discharge "OFF" time is calculated as:
Total periodic time is calculated as:
The output frequency, ƒ is therefore given as:
Giving a duty cycle value of:
As the timing capacitor, C charges through resistors R1 and
R2 but only discharges through resistor R2 the output duty cycle can be
varied between 50 and 100% by changing the value of resistor R2. By decreasing the value of
R2 the duty cycle increases towards 100% and by increasing R2 the duty
cycle reduces towards 50%. If resistor, R2 is very large relative to resistor
R1 the output frequency of the 555 astable circuit will determined by R2 x C
The problem with this basic astable 555 oscillator configuration is that the duty cycle, the "mark-to-space"
ratio will never go below 50% as the presence of resistor R2 prevents this. In other words we
cannot make the outputs "ON" time shorter than the "OFF" time, as (R1 + R2)C will always be greater
than the value of R1 x C. One way to overcome this problem is to connect a signal bypassing diode
in parallel with resistor R2 as shown below.
Improved 555 Oscillator Duty Cycle
By connecting this diode, D1 between the trigger input and the
discharge input, the timing capacitor will now charge up directly through resistor R1
only, as resistor R2 is effectively shorted out by the diode. The capacitor discharges as normal
through resistor, R2.
Now the previous charging time of t1 = 0.693(R1 + R2)C is modified
to take account of this new charging circuit and is given as: 0.693(R1 x C). The duty cycle is
therefore given as D = R1/(R1 + R2). Then to generate a duty cycle of less than 50%, resistor
R1 needs to be less than resistor R2.
555 Oscillator Applications
We said previously that the maximum output to either sink or source the load current via pin 3 is about
200mA and this value is more than enough to drive or switch other logic IC's, a few LED's or a small lamp etc and that we
would need to use a bipolar transistor or MOSFET to amplify the 555's output to drive larger current loads such as motor
But the 555 Oscillator can also be used in a wide range of waveform generator circuits
and applications that require very little output current such as in electronic test equipment for producing a whole range
of different output test frequencies from very accurate sine, square and pulse waveforms or as LED or lamp flashers and
dimmers to simple noise making circuits such as metronomes, tone and sound effects generators and even musical toys for
We could very easily build a simple 555 oscillator circuit to flash a few LED's "ON" and "OFF" or to
produce a high frequency noise. But one very nice and simple to build science project using an astable based 555 oscillator
is that of an Electronic Metronome.
Metronomes are devices used to mark time in pieces of music by producing a regular and recurring musical
beat or click. A simple electronic metronome can be made using a 555 oscillator as the main timing device and by adjusting
the output frequency of the oscillator the tempo or "Beats per Minute" can be set. A tempo of 60 beats per minute means that
one beat will occur every second and in electronics terms that equates to 1Hz. So by using some very common musical
definitions we can easily build a table of the different frequencies required for our metronome circuit as shown below.
Metronome Frequency Table
The output frequency range of the metronome was simply calculated as the reciprocal of 1 minute
or 60 seconds divided by the number of beats per minute required, for example (1/(60 secs / 90 bpm) = 1.5Hz)
and 120bpm is equivalent to 2Hz, and so on. So by using our now familiar equation above for calculating the output frequency
of an astable 555 oscillator circuit the individual values of R1, R2 and
C can be found.
The time period of the output waveform for an astable 555 Oscillator is given as:
For our electronic metronome circuit, the value of the timing resistor R1
can be found by rearranging the equation above to give.
Assuming a value for resistor R2 = 1kΩ and capacitor
C = 10uF the value of the timing resistor R1 for our frequency
range is given as 71k6Ω at 60 beats per minute to 23k5Ω
at 180 beats per minute, so a variable resistor (potentiometer) of 100kΩ would be more
than enough for the metronome circuit to produce the full range of beats required and some more. Then the final circuit
for our electronic metronome example would be given as:
555 Electronic Metronome
This metronome circuit demonstrates just one simple way of using a 555 oscillator to produce an
audible sound or note. It uses a 100kΩ potentiometer to control the full range of output pulses or beats, and as it
has a 100kΩ value it can be easily calibrated to give an equivalent percentage value corresponding to the position
of the potentiometer. For example, 60 beats per minute equals 71.6kΩ or 72% rotation.
Likewise, 120 beats per minute equals 35.6kΩ or 35% rotation, etc. Additional resistors or trimmer's
can be connected in series with the potentiometer to pre-set the outputs upper and lower limits to predefined values, but
these additional components will need to be taken into account when calculating the output frequency or time period.
While the above circuit is a very simple and amusing example of sound generation, it is possible to use
the 555 Oscillator as a noise generator/synthesizer or to make musical sounds, tones and alarms by
constructing a variable-frequency, variable-mark/space ratio waveform generator.
In this tutorial we have used just a single 555 oscillator circuit to produce a sound but by cascading
together two or more 555 oscillator chips, various circuits can be constructed to produce a whole range of musical and
sound effects such as the police car "Dee-Dah" siren given in the example below.
555 Oscillator Police "Dee-Dah" Siren
The circuit simulates a warble-tone alarm signal that simulates the sound of a police siren.
IC1 is connected as a 2Hz non-symmetrical astable multivibrator which is used to frequency
modulate IC2 via the 10kΩ resistor. The output of
IC2 alternates symmetrically between 300Hz and 660Hz taking 0.5 seconds to complete each