Mono 1.1 Monostable Multivibrators 1 Technician Series ©Paul Godin Updated 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 inputQ OutputtWtW Monostable Basics t W =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 QRQR Q NR Q R = Retriggerable Q NR = 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 C To RC network

Mono 1.8 Triggering ◊Unused triggering and reset elements must not be ignored. Monostable Triggers Q Reset R/C C To 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 t w =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. tWtW 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

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.

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