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Astable Circuits

Introduction

An astable circuit is one that oscillates continuously between two states, it cannot settle into just one state. If the output is Logic 1 it will become Logic 0, if the output is Logic 0 it will become Logic 1.

The period of oscillation is determined by the timing components used. The output is high for some period of time (called the mark) and low for some period of time (called the space) - the mark-space ratio is determined by the choice of timing components and the design of the circuit.

For a square wave, a mark-space ratio of 1:1 is required

The period (T) of oscillation is the time for one complete cycle - this is related to the frequency.

mark space ratio

Frequency = 1 ÷ Time Period

f = 1 ÷ T


555 Astable Circuit

555 astableThis circuit uses the 555 chip as an astable oscillator. This is an important circuit that you should learn.



555 Astable Time Period

The time period depends in Ra, Rb and C. The output only oscillates when pin 4 is held high. If pin 4 is low (connected to 0V) then the output is simply 0V.

The time period of the 555 astable is given by the equation

T = 0.7 (Ra + 2Rb) C

555 Astable Mark - Space Ratio

SPACE: In operation the output Q is low whilst the capacitor discharges - as this happens pin 7 is held low by the internal circuitry of the 555 IC and so the capacitor discharges through Rb only. The time for this discharge is Tspace = 0.7 Rb C (the voltage across the capacitor halves).

MARK: When the capacitor is sufficiently discharged the output goes high and pin 7 is allowed to float. The capacitor now charges through Ra and Rb and so the time to charge is Tmark = 0.7 ( Ra + Rb ) C.

Therefore the space is always shorter than the mark. The mark-space ratio obviously cannot be 1:1. The total time period is given by adding the charging and discharging times together:

Tperiod = Tmark + Tspace
T = 0.7 (Ra + 2Rb) C

555 Astable Considerations

The 555 astable is quick and easy to build. However, the following points should be noted:

555 Astable - How to calculate timing component values

There are three components that determine the time period. The best way to approach the problem of what components to use is to (a) always use Ra as 1k unless there is a good reason not to do so, (b) make an informed intelligent guess for the value of C and then (c) calculate the value of Rb.

The value of Rb must the greater than 1kΩ and less than 1MΩ.

As an example, this is how to calculate the values required for a 555 based astable with a frequency of 100Hz.

  1. f=100Hz requires T=0.01s
  2. No requirement for mark-space ratio is required and so we are free to choose.
  3. Start with Ra=1k (just a free choice but always a sensible option)
  4. It is easier to choose the capacitor value as we have a wider choice of resistors.
  5. Choose C=10µF (just a guess)
  6. Calculate Rb=115 ohms. This is too small, go back and guess a better capacitor
  7. Choose C=10nF (an informed guess)
  8. Calculate Rb=714k. This is better but not convenient.
  9. Choose C=22nF (a better guess)
  10. Calculate Rb=324k, use 330k - good enough!

NOR Gate Astable

There are many different ways to make an astable circuit using transistors or logic gates. One example is an astable made from NOR gates as shown below. The astable has a resistor and capacitor to determine the time period and an enable to allow the astable to be controlled. This astable produces a clean square wave (mark-space ration = 1:1). The calculation of the time period depends on the types of logic gates used as different families of logic gate have different threshold voltages.

NOR astableFor CMOS gates, the time period T is given by

T = 1.6 R C

The equation for the time period is just an approximation. It is best to make R variable so that the required time period can be achieved!

In operation, Q will be permanently high when the Enable is held high. When Enable falls low, the astable will oscillate.