The 555 Oscillator

The 555 Oscillator

 

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 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 of 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%.

 

Example No1

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.C only. 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 "ON" time shorter than the "OFF" time as (R1 + R2)C will always be greater than R1.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.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.