Home > Circuit Bricks > Logic Gate Monstable
UPDATED:.21:09 21 October 2013

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


  • This monostable is built around 2 LOGIC gates.
  • It produces an output pulse, either high or low by using
    NOR or NAND gates.
  • The time of the pulse is set by 2 components.
  • Useful for producing brief pulses for latches and clocks, from 1us to 1 second, for longer durations use a
    555 monostable.
CIRCUIT DETAILS
Input

NOR GATE MONOSTABLE - Circuit Diagram
  • Trigger
    This is activated by a low to high input signal. A high to low transistion has no effect.
Output

When triggered, the output will pulse high for the pre-determined time. The output can only sink/source about 2-3mA, just enough for an LED (but this will reduce the output voltage).

It should really only be used to trigger other logic devices - but you can add a driver for higher current.


Input

NAND GATE MONOSTABLE - Circuit Diagram
  • Trigger
    This is activated by a high to low input signal. The opposite to the NOR monostable.
Output

When triggered, the output will pulse low for the pre-determined time.

Again the output is only 2-3mA and should be connected to another logic device.

CALCULATING THE TIME...
The duration of the pulse is calculated as follows:

Time (secs) = 0.7 x C1 x R1

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 time you require:

Less than 1 ms. = 1nF,
Up to 100 ms. = 47nF,
Up to 1 sec. = 470nF

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

R1 = Time / (0.7 x C1)

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


Try using a preset or a potentiometer for R1. Then you can adjust the time 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
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