mono 1.1 monostable multivibrators 1 technician series ©paul godin updated march 2015
TRANSCRIPT
Mono 1.1
Monostable Multivibrators 1
Technician Series
©Paul GodinUpdated March 2015
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.
Describe RC circuits in a timing application
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 Discussion
◊ Monostables are used as debouncers. Explain.
◊ Monostables are used to cause a delay in an action. For instance, they can delay a switch so that it will turn on a light after a 2 second delay for 4 seconds. Demonstrate.
◊ Monostables can be used to control a sequence of timed events, or even create an oscillator. Demonstrate.
Mono 1.14
End
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