Transcript Monostables
Monostable Multivibrators 1 Technician Series ©Paul Godin Updated March 2015 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. Describe RC circuits in a timing application 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 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 1.13 End prgodin @ gmail.com Mono 1.14