1. Monostable Multivibrators 1
©Paul Godin
Updated November 2007

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”.

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

4. Examples of monostables in everyday use:
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).

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.

6. Triggering QR QNR Trigger QR = Retriggerable QNR = Non-retriggerable
Accepts new Trigger
Trigger QR
QNR
Ignores new Trigger
QR = Retriggerable
QNR = Non-retriggerable

7. Common Monostable
Monostables generally have positive and negative triggers, and an asynchronous reset.
By default, Monostables are non-retriggerable.
Triggers Q
Reset
R/C C
To RC network
Common Monostable

8. Triggering Unused triggering and reset elements must not be ignored.
Monostable Q
Triggers
R/C
To RC network C
Reset
Describe the configuration requirement for a positive edge input, and for a negative edge input.

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.

10. tw=k  RC, where k = a numerical factor provided by the manufacturer.
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:
The value of “k” varies by device, by manufacturer and by capacitor range. You MUST check the specification sheet.
tw=k  RC, where k = a numerical factor provided by the manufacturer.
Select a value for C, then calculate the value for R.

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

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
74123
74221
4528

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?

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.

15. In-Class Exercise 3
Using EWB, design a monostable that will produce a 4 second pulse. Use a 10μF Capacitor.

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.
MONO
Light
Detector
Variable time

17. Similar to a final Exam question
In-Class Exercise 5
Design a switch that will:
Turn on a light after a 2 second delay for 4 seconds
Turn on a light after a 2 second delay (and leave it on)
Similar to a final Exam question

18. End
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