Monostable Multivibrators 1 ©Paul Godin Updated November 2007

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”.

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)

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).

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.

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

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

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.

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.

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.

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

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

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?

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.

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

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

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

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