Mono 1.1

Monostable Multivibrators 1

©Paul GodinUpdated November 2007

Mono 1.2

Definitions

◊ Monostable◊ A single stable state (mono = one).◊ The stable state is the wait state. ◊ A trigger causes a single output pulse.

◊ Monostable Multivibrators are also known as “One Shots”.

Mono 1.3

A Monostable produces a single pulse of fixed length.An input edge is required to trigger the event.

Trigger input

Q Output tW

Monostable Basics

tW=pulse width (time)wait / steady state triggered state

Triggering edge

Mono 1.4

Examples of monostables in everyday use:

- Thermostat (on/off delay time)- Outdoor Sensor Light- Alarm Clock’s Sleep Timer- Reset Safety Control (Anti-Sleep devices for locomotives)- Automatic Doors- Back-lit Displays that are on for a few seconds

Common Monostable Applications

Monostables are used where a device needs to perform a timed output after the application of an input trigger.Typically, the timed output is short (from μS to a few minutes).

Mono 1.5

Triggering Types

◊ Monostables can be:

◊ Retriggerable, where the timing will begin from t=0 in the unsteady (triggered) state after a trigger edge.

◊ Non-Retriggerable, where the timing will not begin again after a trigger edge until the output is in the steady (wait) state.

Mono 1.6

Triggering

Trigger

QR

QNR

QR = RetriggerableQNR = Non-retriggerable

Accepts new

Trigger

Ignores new

Trigger

Mono 1.7

Common Monostable

◊ Monostables generally have positive and negative triggers, and an asynchronous reset.

◊ By default, Monostables are non-retriggerable.

Common Monostable

Triggers

Q

Reset

R/C

CTo RC network

Mono 1.8

Triggering

◊ Unused triggering and reset elements must not be ignored.

Monostable

Triggers

Q

Reset

R/C

CTo RC network

Describe the configuration requirement for a positive edge input, and for a negative edge input.

Mono 1.9

Timing Element

◊ The timing element for a Monostable is an RC network.

◊ Different Monostables require different RC configurations.

◊ It is important to look up the specification sheet for the monostable you intend to use.

Mono 1.10

◊ The time of the triggered pulse is usually regulated by a time-based circuit such as an RC network.

◊ Generally the formula used is:

◊ The value of “k” varies by device, by manufacturer and by capacitor range. You MUST check the specification sheet.

Calculating the Timing Element

tw=k RC, where k = a numerical factor provided by the manufacturer.

Select a value for C, then calculate the value for R.

Mono 1.11

Values for R and C

◊ To help determine the approximate range of values of R and C, many specification sheets include a chart.

tW

C

R

This chart will help you select a value for C

Some monostables have an internal resistor

Mono 1.12

In-Class Exercise 1

◊ Look up the specification sheets for the following monostable multivibrators and determine:◊ The k-factor for calculating the pulse width◊ If the monostable is retriggerable or non-retriggerable◊ The physical RC configuration◊ The function table

74123742214528

Mono 1.13

In-Class Exercise 2

◊ How could you configure a retriggerable monostable as a non-retriggerable monostable?

◊ Challenge: How could you configure a non-retriggerable monostable as a retriggerable monostable?

◊ How would you cause one monostable to trigger another?

Mono 1.14

The Monostable in EWB

◊ Notes regarding EWB’s monostable:◊ The factor in the help file is wrong. It should read 0.693◊ The device is non-retriggerable.◊ The mono will sometimes corrupt the file. Save under

different filenames.

Mono 1.15

In-Class Exercise 3

◊ Using EWB, design a monostable that will produce a 4 second pulse. Use a 10μF Capacitor.

Mono 1.16

In-Class Exercise 4

◊ Design an outdoor sensor light that will turn on for a variable 5 to 10 seconds. Use a switch to simulate the detector.

MONODetector Light

Variable time

Mono 1.17

In-Class Exercise 5

◊ Design a switch that will:1. Turn on a light after a 2 second delay for 4 seconds2. Turn on a light after a 2 second delay (and leave it on)

Similar to a final Exam question

Mono 1.18

End

prgodin @ gmail.com