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.

Frequency = 1 ÷ Time Period

f = 1 ÷ T

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

The time period depends in R_{a}, R_{b} 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 (R_{a} + 2R_{b}) C

**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 R_{b} only. The time for this discharge is *T _{space} = 0.7 R_{b} 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 R_{a} and R_{b} and so the time to charge is *T _{mark} = 0.7 ( R_{a} + R_{b} ) 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:

T_{period} = T_{mark} + T_{space}

T = 0.7 (R_{a} + 2R_{b}) C

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

- The 555 takes a lot of current as the output changes state. This can affect other ICs using the same power supply. A large value capacitor (47µF) connected near to the 555 IC will help to reduce the impact of this problem
- The mark-space ratio cannot be 1:1 - for a true square wave a different circuit is required
- If R
_{a}<< R_{b}the mark-space ratio is almost 1:1 and T = 1.4 R_{b}C (e.g R_{a}= 1k, R_{b}= 47k etc) - The output of the 555 can source or sink up to 100mA

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 R_{a} 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 R_{b}.

The value of R_{b} 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.

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

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.

For 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.

© Paul Nicholls

April 2018

Electronics Resources by Paul Nicholls is licensed under a Creative Commons Attribution 4.0 International License.