Types of Control Devices and Sensors in Electrical Systems

The article discusses types of control devices and their functions, including voltage and current control, as well as various control mechanisms such as switches, sensors, and variable resistors. Additionally, it explores the role of sensors, actuators, and transducers in electrical systems, providing examples and applications for each type of device.

Control devices allow for control of some functions on the circuit. Although they are often mentioned alongside circuit protection, it is important to understand that control and protection are two distinct aspects of circuit design and circuit components. Confusion often arises on this point as the actions of the two aspects seem similar—both provide some form of manual and automatic disconnection. Protection devices respond to overcurrent, overloading, overvoltage, or earth leakage and automatically disconnect via blowing a fuse or tripping a circuit-breaker or residual current device. Control devices respond to nearly any other circumstance or input. Although switches or sensors are the most common types of control devices, variable resistors and resistor banks can also be used as control devices.

Types of Control

The two general types of control are control that limit or affect the:

  • voltage
  • current.

Basic electrical theory states the relationship between voltage, current, and resistance and how either the voltage or current can be varied by varying the resistance. Some types of resistor devices can be manually adjusted or switched into circuits using timing and contactor control. Two common types of variable-resistor control devices are the potentiometer and the rheostat.

This use of timing and contactor control is applied as a method for both reduced-voltage starters and speed control. By increasing the resistance, the voltage drop across the resistor is increased, reducing the amount of voltage available for the remainder of the circuit. The concept also allows voltage control for the motor starting by the use of a variable resistor or resistor load banks. Types of three-phase induction motor reduced-line voltage starters include:

Secondary resistance starters with a wound rotor use resistors or resistor load banks as a method for speed regulation: the reduction in voltage causes a reduction in speed.

Current control is the more common type of control and is often simpler to understand. Since there is only one current path in a series circuit, it should be clear that open-circuiting the series path means that the current will no longer flow, and the circuit will stop operating. This is usually done via switches or the use of contacts and relays.

Types of Control Devices

Control devices can be manually operated switches such as pushbuttons or micro or limit switches that are operated by product position or machine sequence. There are also non-contact switches and sensors that operate automatically. Common types of control devices are listed in Table 1.

Manually operated (contact) switches Mechanically operated switches Relays Sensors (contact and non-contact)
  • Pushbutton switches (latching and momentary)
  • Toggle switches
  • Selector switches
  • Thumbwheel switches
  • DIP switches
  • Limit switches
  • Proximity switches
  • Pressure switches
  • Temperature control
  • Float and flow switches
  • General-purpose relays
  • Solid state
  • Controllers (timer, counter, and temperature)
  • Photoelectric (fiber optic and laser)
  • Capacitive
  • Inductive
  • Temperature
  • Ultrasonic
  • Pressure/flow
  • Code readers/OCR

Table 1. Common Control Devices

Control devices include:

  • Switches that respond to an input by changing state.
  • Bimetallic switches used in thermostats or overcurrent devices that respond to heat, causing the metal to bend.
  • Contactors and relays responding to the current flowing in their coil, causing them to operate.
  • Float switches like those used in bore water pumps to stop the pump from pumping dry, responding to water level by movement of a ball operating contacts or a magnetic reed switch.
  • Light sensors used for street lighting operate by detecting light falling on either phototransistors or photoresistors. The amount of light falling on the sensor (or light level) during daytime will vary the amount of resistance, limiting current.
  • Limit switches and proximity switches that operate based on the position of objects. Limit switches are operated by an object moving to a position where it stops, for example, by an automatic door opening. The limit switch is actuated by the door reaching its limit of travel.
  • Proximity switches that operate by producing either electromagnetic or infrared radiation as either a beam or electromagnetic field and detecting the return signal. Proximity switches can be seen in use detecting items on supermarket conveyer belts.

The problem with manual control switches is that their contacts inevitably wear out and cause failure. Control devices that use resistors while not requiring manual input are more susceptible to over-power situations, which cause them to burn out. Limit and proximity switches require correct placement and alignment; if they are misaligned, they may detect the wrong object or not detect an object at all. The advantage that manual control devices have over automatic devices is that they are robust and comparatively cheap (depending on the application). Some devices do not need manual input as they receive an automatic input (such as light) rather than human intervention.

Switches operate in two ways: by interrupting the flow of current when opening the circuit or by allowing current flow by closing the circuit. As simple and limited as these may seem, it is possible to create complex responses to inputs. An example of this is the float switch that will not allow a pump to turn on if the water level is too low, even if it is set via a timer to do so. This is an example of what is called a ‘logic circuit’. Logic circuits are the foundation for automation and are commonly used in large plants and factories.

The selection of control devices for a circuit depends on the following factors:

  • the power the device is expected to handle
  • the type and amount of voltage seen across the device
  • the type and amount of current flowing in the device, and how long it will do so
  • the type and number of circuits to be controlled
  • how the circuit is to be affected (meaning whether it is to be opened or closed).

These factors are often called ‘duty ratings’ and must be considered when selecting all manner of components—from pushbuttons and pilot lamps to relays and timers. The selection of devices and components may have been specified in a bill of materials or specification from the client or engineer. Equally, though, the installer may be required to source components themselves from the manufacturer’s catalog or website.

When installing the control components into a circuit, the general rule is to place them at the start but after the circuit protection. The multiple devices that perform control allow for parallel configurations, but care should be taken when using limit and proximity switches: while they are control devices, they perform functions not dissimilar to protection devices and, therefore, should be installed in series with emergency stops and other control devices.

Sensors, Actuators and Transducers

While sensors generally provide an input to a control circuit, the control outcome is an action or response. These types of devices that result in some form of action or movement are referred to as ‘actuators’. A third classification of devices is the transducer. Depending on their operation, transducers can be classified as either sensors or actuators.

Sensor Types

Control sensors are, in a way, very similar to our own senses of sight, hearing, taste, smell, and touch. Human biological sensors detect sensory signals such as light and sound, and these are converted into electrical signals as messages to the brain, which then outputs a response.

Sensors perform an ‘input’ function to the control process. They usually detect either a physical change or some type of energy such as heat, light, chemical, or motion and then convert these signals into either an analog or digital representation of the input signal. Sensors are commonly used to detect levels, presence, proximity, temperature, and pressure. They can be categorized as follows.

Temperature Sensors

  • Thermocouple—a thermoelectromotive force resulting from two different types of metal joined together at one end. When the joined end is heated, a potential difference occurs between the wires at the open end. This is known as the ‘Seebeck effect’.
  • Thermistor—produces a decrease in resistance when heated.
  • Resistance temperature detector—when they are heated, material resistance increases.
  • Semiconductor temperature sensor—silicon (positive temperature coefficient) and germanium (negative temperature coefficient) can both be used for temperature sensing.

Manometer and Temperature Sensor on Pipeline

Figure 1. Manometer and Temperature Sensor on Pipeline

Pressure/Flow Sensors

These devices are equipped with a pressure-sensitive element. This measures the pressure of a gas or liquid against a diaphragm which converts the measured value into an electrical output signal. Diaphragms are typically stainless steel or silicon. Different sensors are used for liquids, gases, flammable substances, and corrosive substances.

Pressure Sensors

  • Strain gauge transducer—measures strain or tension. They are accurate and reliable.
  • Piezo-resistive transducer—used to sense ranges from 10 to 5000 psi (the engineering unit relating to pressure). They are used in lubrication, pneumatic, and hydraulic systems.

Code Readers/Optical Character Recognition (OCR)

These devices read barcodes. A barcode is a display of information in the form of bars (black portions) and spaces (areas between the bars) of varying widths. Barcodes are used in a variety of different industries and applications, notably in distribution and logistics. Outwards remittance (billing forms and statements) uses code-reader technology for scanning, collating and mail-out to customers.

Photoelectric Sensors

Two common types of photoelectric sensors are those that operate by line of sight (also referred to as ‘through beam’ sensors) and those that use a reflector. Both systems detect presence. They comprise a transmitted light source known as the ‘emitter’ and a receiver to detect the light. Both systems detect objects when a light beam is broken. The line-of-sight type has an emitter and a receiver lined up directly opposite each other. The reflector type has an emitter and receiver housed together in one unit. Light from the emitter is directed towards a reflector, which reflects it back to the receiver.

Reflectors can be in various sizes and can be round or rectangular or can be reflective tape. These systems can be affected by dust, dirt, smoke, moisture, airborne contaminants, direct and reflected sunlight. They require regular emitter, receiver, and reflector wiping and cleaning to operate effectively.

Fibre Optic and Laser Sensors

The use of optical fiber is another method of transmitting light. It can be ideal for detecting small objects or focusing on very small sensing areas at close range. Lasers are also sometimes used as high-intensity visible light sources for the detection of extremely small objects at a distance. Industrial, commercial, and control applications for photoelectric, fiber optic and laser sensors include:

  • People, object, and product detection
  • product counting—bottles, cans, cartons, boxes, and packages.

Measurement and Displacement Sensors

These are non-contact devices for presence detection, proximity, displacement, and measurement. They function on optical, inductive, capacitive, and ultrasonic technology and principles. Displacement sensors are devices that measure the distance between the sensor and an object and any changes (displacement) in position or some form of movement by part of the object. A measurement sensor measures the position and dimensions and the height, width, and thickness of an object.

Inductive Proximity Sensors

Inductive sensors are non-contact proximity sensors that produce an electromagnetic field to detect ferrous targets. They consist of four major components: a ferrite core with coils, an oscillator, a Schmitt trigger, and an output amplifier. When a ferrous target enters this magnetic field, eddy currents are induced on the metal’s surface, changing the reluctance of the magnetic circuit.

Inductive Proximity Sensor

Figure 2. Inductive Proximity Sensor

Eddy current sensors are different from proximity and displacement sensors in that they use an air-core coil instead of a ferromagnetic core. Due to the air core, the sensor measurement distance needs to be closer to the product, so there is a narrower air gap.

Capacitive Sensors

Capacitive proximity sensors produce an electrostatic field. They are passive transducers that require an external force for operation, converting changes in capacitance into an electrical signal. They can detect through some containers, sense metals, and non-metallic materials such as paper, glass, cloth, and liquids.

When an object nears the sensing surface, the electrostatic field of the electrodes changes the capacitance in the oscillator circuit. Changes in capacitance will result in changes in the oscillator’s amplitude. Product sensing and detection causes an increase in amplitude, triggering an output signal.

Ultrasonic Sensors

Ultrasonic proximity sensors are transducer devices that are capable of presence detection, displacement and product dimension measurement of all materials. They can transmit and receive high-frequency sound signals as sound waves to detect a target or object. When these sound waves reflect off an object, an echo is created. This echo is the reflected sound wave bouncing from the detected object back to the sensor. The time taken for this echo is directly proportional to the distance between the object and the sensor. This feature makes ultrasonic sensors useful for:

  • people, object, and product detection
  • level measurement in small containers
  • height and stack height sensing
  • contour recognition
  • product counting.


An actuator is a specific type of transducer. Actuators perform an ‘output’ function, usually to carry out a physical process or task, or to control an external device such as a motor. An actuator converts energy into motion.

A bimetallic strip is an example of a thermal actuator device, directly converting thermal energy into motion as thermal expansion. When heat is applied to two dissimilar strips of metal joined along their entire lengths, the bimetallic strip bends in the direction of the metal with the smaller coefficient of thermal expansion.

Multi-Turn Electric Actuator for the Oil Industry

Figure 3. Multi-Turn Electric Actuator for the Oil Industry

Relays, contactors, and solenoids are examples of actuators. Although these are considered as ‘outputs’, relays and their contacts form part of the internal logic control for programmable relays and programmable logic controllers. In those applications, only the contactors, solenoids, and indicating lamps are considered as outputs. An electric motor acts as both a transducer and an actuator. Motors convert electrical energy to magnetic energy and then to mechanical energy or motion.


By definition, a transducer is a device where any variation in energy magnitude of any form is able to reproduce that variation in another measurable form—generally these days as an electrical voltage, even though it may only be in millivolts.

Rotary encoders are sensors that are sometimes referred to as ‘shaft encoders. They are electromechanical transducers that can provide an output signal or pulse due to shaft rotation, shaft position, and speed. The output pulse from multiple encoders can be used for speed synchronization and control of multiple conveyors as part of an industrial production or processing environment.

A thermocouple is a type of transducer where two dissimilar metals produce a voltage on being heated at their junction. For example, a copper/constantan thermocouple with the cold end kept at a constant temperature produces 4.3 mV at 100°C and 14.8 mV at 300°C. A graph of these values is approximately linear over a restricted range and can be used to indicate the temperature on a voltmeter that has been suitably calibrated to read temperature. Alternatively, the thermocouple can be connected to a dedicated controller, programmable relay or an input for a PLC controlling a process.

Measurement and displacement sensors are examples of transducers. Table 2 gives further examples of transducers and their application.

Transducer type Application(s)
Electroacoustic transducer Loudspeaker—converts electrical signals into sound
Microphone—converts sound waves into analog electrical signals
Electromagnetic transducer Generator—converts motion in a magnetic field into electrical energy
Electromechanical transducer Strain gauge—converts the deformation (strain) of an object into electrical resistance
Galvanometer—converts the electric current of a coil in a magnetic field into movement
Generator—converts mechanical energy (motion) into electrical energy
Motor—converts electrical energy into mechanical energy
Electrochemical transducer Battery—converts chemical energy directly into electrical energy
Thermoelectric transducer Thermocouple—converts heat energy into electrical energy
Temperature-sensitive resistor (a thermistor)—changes heat energy to electrical energy

Table 2. Examples of Transducers and their Applications

Key Takeaways

Understanding and implementing control devices, sensors, actuators, and transducers are vital in electrical systems for ensuring efficient operation, safety, and automation. These components play crucial roles in controlling voltage and current, detecting changes in physical parameters, and converting energy into mechanical motion or vice versa. Their applications range from simple circuit control to complex industrial automation processes, highlighting their importance in various sectors, including manufacturing, transportation, and infrastructure.