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.

Frequency = 1 ÷ Time Period

f = 1 ÷ T

555 Astable Circuit

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

555 Astable Time Period and Frequency

The time period depends in Ra, Rb and C. The OUTPUT only oscillates when RESET (Pin 4) is held HIGH.

If RESET (Pin 4) is momentarily LOW (connected to 0V) then the output is reset to LOW and if RESET is held LOW, the OUTPUT stays LOW (0V).

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

T = 0.7 (Ra + 2Rb) C

As f = 1/T, the frequency of the 555 astable is given by

f = 1.44 / (Ra + 2Rb) C

Note: The equations for Time Period and Frequency are equivalent because 1 / 0.7 = 1.44.

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.

MARK - SPACE RATIO: The ON time divided by the OFF time is called the Mark - Space ratio. The Mark - Space ratio is given by:

TON / TOFF = (Ra + Rb) / Rb

PERIOD: The total time period is given by adding the MARK (charging) and SPACE (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:

• 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 Ra << Rb the mark-space ratio is almost 1:1 and T = 1.4 Rb C (e.g Ra = 1k, Rb = 47k etc)
• The output of the 555 can source or sink up to 100mA

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 such as needing a particular mark-space ratio, (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 = 1kΩ (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 = 115Ω. This is too small, go back and guess a better capacitor
7. Choose C = 10nF (an informed guess)
8. Calculate Rb = 714kΩ. This is better but not convenient.
9. Choose C = 22nF (a better guess)
10. Calculate Rb = 324kΩ, use 330kΩ - good enough!

555 Astable RESET (Pin 4) used as an ENABLE

The RESET pin, Pin 4, on the 555 can be used as an enable

• When Pin 4 is HIGH, the 555 Astable will oscillate
• When Pin 4 is LOW, the output of the 555 Astable is 0V (LOW) and the circuit does not oscillate

A push button (or other similar input) or a different subsystem can be used to control the 555 Astable by making Pin 4 either HIGH or LOW. In the circuit shown below, pushing the button make Pin 4 HIGH and the Astable oscillate so that the LED flashes. When the button is not pushed, the pull down resistor (4k7Ω) keeps Pin 4 LOW and the LED does not flash.

Alternatively the Astable can be controlled by a previous circuit. The system diagram below shows a Monostable with a 10 second time period (Trigger not shown) connected to an Astable with a time period of 0.5 seconds. The output of the Monostable (Pin 3) is connected to Pin 4 on the Astable. When the Monostable is triggered, the LED will flash at 2Hz for 10 seconds. When the output from the Monostable goes LOW, the output of the Astable is reset and also goes LOW and the LED stops flashing.

Relaxation Oscillator

A very simple astable with a mark - space ratio of 1:1 is built around an inverting Schmitt trigger. The capacitor charges through the resistor from the output of the Schmitt trigger. To ensure reliable operation a high value resistor should be used (>10kΩ) and the output should be buffered so that the current from the output of the Schmitt trigger is able to charge the capacitor as expected.

For CMOS based Schmitt trigger chips, the time period T is given by

T = 0.5 R C

The equation for the time period is just an approximation and depends on the thresholds of the Schmitt trigger used.

In operation, Q will be permanently HIGH when the Enable is held LOW. When the Enable is HIGH, or simply not connected, the astable will oscillate.

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 ratio = 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.