Home > Circuit Bricks > Gated Logic Gate Astable
UPDATED:.20:51 21 October 2013

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


  • This astable is based upon 2 LOGIC gates wired as inverters.
  • It is similar in function to the NOT gate astable but can be gated, ie can be switched on or off with a logic signal.
  • The speed of this switching can be controlled by 2 components.
  • Useful for producing sound and high speed clocks.
CIRCUIT DETAILS

  • The circuit is basically a LOGIC gate astable with an enable input.
  • 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, the NAND works woith a high logic signal and the NOR with a low:

Input
  • ENABLE
    A
    high input will enable the astable while a low input will stop it functioning.
    The 100K resistor is only needed if the enable is connected to a switch. If it is connected to another logic gate, then it can be removed.

Output

The output will generate a square wave at the set frequency when the ENABLE input is high.

When the ENABLE is low, the output will stay low.

GATED NAND ASTABLE
Circuit Diagram



Input
  • ENABLE
    A
    low input will enable the astable while a high input will stop it functioning.
    The 100K resistor is only needed if the enable is connected to a switch. If it is connected to another logic gate, then it can be removed.

Output

The output will generate a square wave at the set frequency when the ENABLE input is high.

When the ENABLE is low, the output will stay low.

GATED NOR ASTABLE
Circuit Diagram


In both examples:
Frequency (Hz) = 1 / (2.2 x R1 x C1)
CALCULATING THE FREQUENCY...
The frequency of the output can be calculated from the following formula:

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

It can be tricky to choose values to start with, so try the following:


1
Choose one of the following values for the capacitor, depending on the frequency you require:

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

2
Now rearrange the formula to work out the required resistor value R1

R1 = 1 / (2.2 x Frequency x C1)

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

3 Now make:

RB = R1 x 10

or as close as you can get. No higher than 4.7M


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

PACKAGE DETAILS
Both the NOR and NAND 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.

4001B
2ip NOR GATE

4011B
2ip NAND 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 - 2010
www.edutek.ltd.uk - Working Electronics For Students & Teachers