Home > Circuit Bricks > Logic Gate Astable
UPDATED:.20:52 21 October 2013

FUNCTION BLOCK
DESCRIPTION
THIS USES LOGIC DEVICES
Logic 1 = High = +V
Logic 0 = Low = 0V


  • This astable is based upon 2 inverter LOGIC gates
  • An astable continually produces output pulses (switches on and off repeatedly).
  • The speed of this switching can be controlled by 2 components.
  • Useful for producing sound and high speed clocks.
CIRCUIT DETAILS

  • The frequency (or speed) at which the pulses occur is measured in Hertz (pulses per second) and can range from about 1Hz up to about 1,000,000Hz (1MHz) or more.
  • The output signal is called a square wave because of the shape it produces when connected to an oscilloscope.
  • It oscillates between high and low logic levels.
  • The output can only sink/source about 2-3mA. If you want to drive a speaker to produce sound, you will need to add a driver

There are two types that are easy to use: (see also gated astables)

NOT GATE ASTABLE
Output

The output will repeatedly change from high to low at a set frequency.

To calculate the values, select C1 first, depending on the frequency you want, by following this guide:

1Hz to 100Hz = 470nF,
100Hz to 10KHz
= 4.7nF,
10KHz to 1MHz
= 100pF

Then rearrange the formula to find R1 as follows

R1 = 1 / (2.2 x Frequency x C1)

Finally make RB = 10 x R1

To ensure that the caluclations are correct, use either Mohms & uF or Ohms & Farads as units

Circuit Diagram

Frequency (Hz) = 1 / (2.2 x R1 x C1)

SCHMITT NOT GATE ASTABLE
Output

This astable produces exactly the same output, but uses a single SCHMITT NOT gate rather than 2 standard NOT gates.

This is also known as a Relaxation Oscillator.

To calculate the values, select C1 first, depending on the frequency you want, by following this guide:

1Hz to 100Hz = 470nF,
100Hz to 10KHz
= 10nF,
10KHz to 1MHz
= 220pF

Then rearrange the formula to find R1 as follows

R1 = 1 / (1.2 x Frequency x C1)

To ensure that the caluclations are correct, use either Mohms & uF or Ohms & Farads as units

Circuit Diagram

Frequency (Hz) = 1 / (1.2 x R1 x C1)

Try using a preset or a potentiometer in series with R1. Then you can adjust the frequency manually.

PACKAGE DETAILS
Both the NOT and the SCMITT NOT gates come in a 14pin DIL package. Any gate can be used for any purpose. CMOS types have been chosen since their supply voltage can range from 3 to 15v.

4069B
NOT GATE

40106B
SCHMITT NOT GATE

When using ICs, it is always best to use an IC socket so the IC can be removed easily if needed.

Unused gates should have their inputs connected to 0v
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers