W1016-Annex-1

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Relay

Prerequisites[edit]

Background[edit]

Logic gates may be implemented using a variety of technologies. One such technology is the "relay", which is an electrically operated switch. Other technologies include vacuum tubes, transistors, and integrated circuits. The first relay included in a patent was by Samuel Morse in 1840 for use in his telegraph.

Introduction[edit]

Relays are useful for two primary reasons:

  • They enable us to amplify a signal, such that a low power signal can control a high power signal
  • They enable us to control several circuits through the use of a single signal

Construction[edit]

Solenoid

A basic relay has a coil of wire wrapped around a soft iron core, which functions as a solenoid. The solenoid generates a magnetic field which pulls an iron armature. The armature is connected to a series of contacts, such that the contacts move when the solenoid is energized. When de-energized the armature is returned to its resting position by a spring.

Common Types of Relays[edit]

There are several common types of relays classified by normally open or normally closed, pole, and throw.

Normally Open or Closed[edit]

Normally Open Switch
Normally Closed Switch

Normally open refers to a switch (a relay is a type of switch) in which the contacts are separated by an insulating air gap which prevents current from flowing at normal voltages when the switch is not activated. Normally closed refers to a switch in which the contacts are touching one another enabling current to flow when the switch is not activated. Activating the switch causes the contacts to move to the opposite state.

Pole[edit]

Single Pole Switch
Double Pole Switch

The number of poles in a switch refers to the number of electrically isolated switches, all controlled by the same mechanism. (Multiple poles enable us to control several circuits through the use of a single signal.) A two-pole switch would have two sets of contacts, completely isolated from one another, yet move in unison.

Throw[edit]

A Double Pole, Double Throw Switch
A Double Pole, Six Throw Switch

The number of throws in a switch refers to the number of circuit pathways available for a connected switch. For example, a double-throw switch has two available pathways for each circuit.

Relay Symbols[edit]

Relay symbols

Relays, being switches, share the same terminology and symbols, but include the solenoid.

  • SPST: Single-pole, single-throw
  • SPDT: Single-pole, double-throw
  • DPST: Double-pole, single-throw
  • DPDT: Double-pole, double-throw

Relays as Logic Gate Components[edit]

Relays, as electrically controlled switches, can be used to build logic gates. For each of these exercises, we'll use only:

  • Single-pole, single-throw switches (one or more)
  • Single-pole, double-throw relays (one or more)
  • A power supply
  • A lamp for output
  • Wire

Buffer[edit]

Relay As Buffer

A buffer can be implemented using a single relay. When switch A is turned on, the lamp is turned on. Click on the image to the right and ensure that you understand the flow.

Exercises[edit]

HintIcon.png
Helpful Hint

From within Falstad's editor:

  • A switch may be added to your diagram using the character "s" or from the menu via Draw | Passive Components | Add Switch
  • A relay may be added to your diagram using the character "R" or from the menu via Draw | Passive Components | Add Relay
  • A power supply may be added to your diagram using:
    • Ground: Using the character "g" or from the menu via Draw | Inputs and Sources | Add Ground
    • Voltage Source: Using the character "V" or from the menu via Draw | Inputs and Sources | Add Voltage Source (1-terminal)
  • A lamp may be added from the menu via Draw | Outputs and Labels | Add Lamp
  • Wire may be added using the character "w" or from the menu via Draw | Add Wire


ExercisesIcon.png
Exercises
  • For ALL of these exercises, you may use ONLY:
    • Single-pole, single-throw switches (one or more)
    • Single-pole, double-throw relays (one or more)
    • A power supply
    • A lamp for output
    • Wire
  • Each switch must control exactly one relay and each relay must be an integral part of the functioning circuit
  • Construct your work using Falstad's Editor
  • Begin each exercises with a Blank Circuit
  • Label each circuit diagram (using Text) with:
    • Your name
    • The date
    • The name of the logic gate (e.g. "NOT")
  • Export each circuit using the File | Export As Text... option from the menu bar
  • The text contains all of your work for the exercise. Select the entire text, copy it, and paste it into your journal using the correct exercise number.

  1. Construct a NOT gate
  2. Construct an OR gate
  3. Construct an AND gate
  4. Construct an XOR gate

Bonus:

  1. Construct a NAND gate
  2. Construct a NOR gate

References[edit]


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