Transcript Monostables

Monostable Multivibrators 1
©Paul Godin
Updated November 2007
Mono 1.1
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 1.2
Monostable Basics
A Monostable produces a single pulse of fixed length.
An input edge is required to trigger the event.
Triggering edge
Trigger input
Q Output
wait / steady state
tW
triggered state
tW=pulse width (time)
Mono 1.3
Common Monostable Applications
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
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 1.4
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 1.5
Triggering
Accepts
new
Trigger
Trigger
QR
QNR
QR = Retriggerable
QNR = Non-retriggerable
Ignores
new
Trigger
Mono 1.6
Common Monostable
◊ Monostables generally have positive and negative triggers,
and an asynchronous reset.
◊ By default, Monostables are non-retriggerable.
Q
Triggers
R/C
C
To RC network
Reset
Common Monostable
Mono 1.7
Triggering
◊ Unused triggering and reset elements must not be ignored.
Monostable
Q
Triggers
R/C
C
To RC network
Reset
Describe the configuration requirement for a
positive edge input, and for a negative edge input.
Mono 1.8
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 1.9
Calculating the Timing Element
◊ The time of the triggered pulse is usually regulated by a time-based
circuit such as an RC network.
◊ Generally the formula used is:
tw=k  RC, where k = a numerical factor provided by the manufacturer.
◊ The value of “k” varies by device, by manufacturer and by capacitor
range. You MUST check the specification sheet.
Select a value for C, then calculate the value for R.
Mono 1.10
Values for R and C
◊ To help determine the approximate range of values of R and C,
many specification sheets include a chart.
R
This chart will help you
select a value for C
tW
C
Some monostables have an internal resistor
Mono 1.11
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
74123
74221
4528
Mono 1.12
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 1.13
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 1.14
In-Class Exercise 3
◊ Using EWB, design a monostable that will
produce a 4 second pulse. Use a 10μF
Capacitor.
Mono 1.15
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.
Detector
MONO
Light
Variable time
Mono 1.16
In-Class Exercise 5
◊
Design a switch that will:
1. Turn on a light after a 2 second delay for 4 seconds
2. Turn on a light after a 2 second delay (and leave it on)
Similar to a final Exam question
Mono 1.17
End
prgodin @ gmail.com
Mono 1.18