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