Sensors (Input Devices)

NOTE: Most sensors and other output devices will indicate whether their termination is NPN or PNP.  Some will give you a choice of which type of output that you prefer for your circuit. 

Switch Transistor NPN Basic switching logic component for semiconductors.  Consists of three terminations called Base, Emitter, and Collector.  When a voltage is applied to the Base, a current can now flow from the Collector to the Emitter.  It is “ON”. Remove the voltage and it is “OFF” Used in sensors to control when a signal is passed on from one device to another, or to a detector. 
Switch Transistor PNP Basic switching logic component for semiconductors. Also consists of three terminations called Base, Emitter, and Collector. However, when a voltage is applied to the base, then no current can flow from the collector to the emitter.  It is “OFF”. Remove the voltage (usually connect it to ground) and it is “ON” Used in sensors to control when a signal is passed on from one device to another, or to a detector. 

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Sensor type Subtype/Principle How it works Use Case 
Proximity Reed Switch Reed switches for proximity detection consist of two parallel strips of ferromagnetic material. As a magnet nears the sensor, the two strips magnetically attract, closing a circuit. They are often encased in a sealed glass housing. Rotation counting, cycle counting, magnetic field detection. 
Proximity Hall Effect The Hall effect is a property of semiconductors, where a current is flowing in the material.  Magnetic proximity causes a voltage potential perpendicular to the current flow. Current sensing, magnetic sensing 
Presence Photoelectric Through-Beam Photoelectric sensors of the through-beam type have a transmitter and receiver separated by a gap. If an object interrupts the gap the receiver reports loss of signal. Many types of transmitters and receivers are used at varying frequencies depending on the needs. Presence or absence of an object.  Object counting on a conveyor 
Presence Retro – Reflective A retro-reflective sensor has the transmitter and receiver in the same housing. Transmitted light reflects off a target and back toward the receiver. Interruption of the light beam by an object shows as loss of signal by the receiver Presence or absence of an object.  Object counting on a conveyor 
Proximity Diffuse Light  Diffuse light sensors have transmitter and receiver in a single housing but separated. The light path reflects off the object and continues to the detector. It is a V-shaped light path, so closer objects have a narrower spacing between the transmitter and receiver and vice versa. Position of a reflective object. 
Proximity Triangulation A triangulation sensor typically uses a collimated light source like a laser.  As a detectable object moves in front of the sensor, light from the transmitter is reflected to the receiver.  Depending on where the reflected light is brightest determines the distance to the object. Position of a reflective object. 
Proximity Time of Flight Often referred to as TOF sensors, these shoot out a pattern of laser pulses and measure the time to receive those signals back. Counting the return time, since light has a fixed speed, results in a “point cloud” that define the size, shape and distance of objects. Collision avoidance, mapping, speed measurement 
Color Light Color sensors consist of a “pure white” source and a detector which looks for a reflection in the Red, Green and Blue spectra.  The percentage contribution of each of the three colors indicates the color of the object. Color detection, color matching 
Temperature Thermocouple A thermocouple consists of a junction of two different metals which creates a voltage differential that changes with temperature. Wide variety of metal combinations available. -270˚ C to  + 2320˚ C 
Temperature RTD (Resistance Temperature Detector) RTDs are often pure metals whose resistance is positively correlated to temperature. Platinum is the high performance standard -200˚C to +850˚C 
Temperature Thermistor (Thermal Resistor) Thermistors are usually metal ceramic hybrids whose resistance is non-linear with respect to temperature.  It can have a positive or a negative temperature coefficient Temperature “sweet spot” is around 50˚ C and range is limited 
Proximity Ultrasonic Ultrasonic sensors are used to measure the distance to an object.  A piezoelectric source generates a signal that travels at the speed of sound. It bounces off the object and the time to arrive at the detector is measured and used to calculate distance. Detect objects that are transparent 
Rotation rate Gyroscope Gyroscopes are usually MEMs devices. They contain a pair of oscillating “fingers”, whose angular momentum changes based on rotational speed. Amplitude changes are proportional to the rate of rotation. Measuring/Stabilizing rotational movements. Navigation 
Acceleration Accelerometer Accelerometers are usually a MEMs device.  They have a cantilever as the detector with an electrostatic feedback on the end. Deflection of the cantilever is proportional to acceleration.  The feedback acts like a servo to counter the cantilever deflection.  The servo charge required is proportional to the acceleration. Measuring/stabilizing linear movements. Navigation 
Rain Windshield Rain Detector To detect rain on a windshield requires a light sensor.  It is a small squarish sensor directly behind the rear-view mirror and it measures the transmitted ambient light. As raindrops accumulate it reduces the amount of light entering the sensor causing it to trigger the wipers. Detecting rain on windshield 
Rain Irrigation Rain Sensor An irrigation rain sensor uses either a cup or an absorbent sponge to capture water.  In either case, if the sensor is “full” it will make the irrigation system inactive until the indicator is dry. Controlling irrigation 
Moisture Soil moisture There are many types of soil moisture technologies.  In general, a probe pair or antenna is embedded in the soil. Changes to some electrical property in the soil due to moisture are measured: i.e., capacitance, impedance, speed of a travelling wave, etc. Measure soil moisture over a wide area, especially for golf courses, sports arenas and farming. 
Light Phototransistor A phototransistor is very much like a regular transistor where the base charge is supplied by a base material that converts photons to charge. Simple light detection, light beam interrupter 
Flow Liquid or gas flow Flow meters measure the volume of liquid or gasses flowing through the sensor.  This can be done through physical means (pressure drop, paddlewheels) or through some property of the measured substance (doppler, magnetic resonance) Many varied uses: Water treatment plants, oil & gas refineries, chemical plants 
Heart Beat Infrared Reflection To measure heartbeat an infrared source illuminates the skin and the reflected signal is captured by a photodiode. Each beat changes the profile of the reflected signal triggering the detector. Diagnosis of heart irregularities, measurement of heart rate during exercise 
Alcohol Infrared Transmission Alcohol testers or breathalyzers measure the absorption of ethanol as the sample passes between a sender/receiver pair. The IR signal is absorbed by ethanol and the degree of absorption is a measure of the percentage content of ethanol Breathalyzers 
PIR Passive Infrared PIR sensors consist of a pair of Infrared detector chips side-by-side.  Built-in circuitry looks at the differential output from this chip pair and if there is a difference it means that in infrared source (usually a person) has moved into range. Motion detection /camera trigger /light switch 
Gas Presence Various depending on gas Common gas detectors include CO2, CO, O2, CH4, H2S and others. Types of detectors are targeted to either Toxic or Combustible gases. Toxics use Metal Oxide detectors or electrochemical sensors and Combustibles use infrared or catalytic sensors. Safety, Agriculture, Explosive potential, Oil & Gas operations 
Smoke Sensor Ionizing Ionizing smoke detectors cause the air between two conductive plates to conduct a slight current.  As particles are generated by smoke, they reduce the current.  When the current gets low enough, it triggers an alarm. Fire detection.  Best with lots of dark particles (fast burning fires) 
Smoke Sensor Photoelectric Photoelectric smoke detectors sample the air with a light source. Reflectivity of the air increases as the number and size of particles increases. Above a threshold an alarm signals. Fire detection. Best with small white particles, smoldering fires. 
Temperature LM35/LM35C Semiconductor This semiconductor device is packaged in a TO-92 and is pre-calibrated in degrees centigrade.  Its standard range is -55 to 150˚C. Also available as LM35C designation rated -40 to 110˚ C. Mfg: TI General purpose temperature measurement. Also available in 8 pin SM and a TO-220. 
Light LDR (Light dependent resistors) Metal alloy has a high resistance. When exposed to a strong light source, the resistance drops dramatically.  The change in resistance is non-linear and dependent on the wavelength of light. These can have significant latency. General purpose light detection. Street lamps, light meters 
Humidity Various Humidity sensors measure some changing electrical property of the air related to the % of water vapor present.  The three types of sensors are capacitive, resistive and thermal Capacitive: An open metal oxide capacitor is exposed to air and its capacitance varies due to moisture content. Resistive: Two metals strips are encased in a salt, the ions in the salt react to moisture changing the resistance. Thermal: One thermal sensor is encased in dry nitrogen and the other is exposed to air. The difference in current flow is due to difference in humidity. HVAC, food processing, pharmaceuticals, meteorology, agriculture. 
Flex Resistive A resistive element is sandwiched between two flexible strips.  As the “flex strips” bend, the resistance changes in proportion to the amount of bending. Dataglove as an input to a VR system, measuring bending in human joints for medical diagnostics. Security in door by measuring movement at the hinge 
Touch or Tactile Capacitive Glass film with embedded electronics around the edge can measure and locate when a finger touches the screen.  Allows for multi-touch operation. Mobile phones and computer screens. One-touch faucets, automotive controls. Robotics, CMMs 
Touch or Tactile Resistive Sandwich with two conducting films separated by an insulating membrane. Pressure on the screen changes the resistance between the two conductive layers. Can only indicate on/off Touchpads, musical instruments, office equipment 
Light (Solar) Semiconductor Usually, a phototransistor Detect the presence or absence of light 
Vibration MEMS, SW420 Semiconductor This is a vibration sensitive switch with a single NC contact. Once it senses vibration the switch opens. The increase in resistance is sensed and an LED is illuminated as an alarm. Machinery vibration, anti-theft device 
Rotation rate Optical Encoder Using an internal IR light source, detector and a code wheel, an optical encoder can precisely indicate position or speed when attached to a rotating shaft. The output is digital, either serial or parallel and can detect with a resolution of 10 arcsec. Motor speed control: both servo motors and synchronous motors driven by VFD. High accuracy environments.  
Rotation rate Magnetic Encoder Similar in principle to an optical encoder but more compact.  It relies on a series of radial “spokes” of alternating N-S poles with a magnetic detector to produce multiple sine waves during a single revolution.  These sign waves can be interpolated to produce high output counts with a typical accuracy of 1 arc minute. Motor speed control: both servo motors and synchronous motors driven by VFD. High shock environments. 
Angular position Absolute Optical Using an internal IR light source, detector and a Gray-coded code wheel, an absolute optical encoder can precisely indicate angular position when attached to a rotation shaft. The output is digital, either SSI or parallel and can detect a resolution of 10 arcsec very repeatably. Position indication of aiming devices, telescopes, antennas or measuring depth of holes in Oil/Gas industries, by tracking drill pipe lengths. 
Linear position Optical These are linear analogs of the rotary optical encoders where the code wheel is replaced with a code track. Resolutions in the 10’s of microns are readily available Used mostly in the machining market for tool or workpiece positioning. 
Noise (clapping) Electrostatic, or Piezoelectric Otherwise known as microphones, these devices consist of a flat disc that changes its electrostatic potential when activated, or in the case of piezoelectric it generates a voltage proportional to the energy in the sound wave. Electrostatic: most widely used as a microphone due to flat frequency response over a wide range and long term stability. Piezoelectric: Low frequency sound-level meters. 
Switch Momentary Usually consist of a pushbutton with a single electrical contact that is spring loaded either as NC or NO. Pressing on the button will change the state of the contact as long as pressure is applied.  When released it reverts back to its normal state. Testing a part of a circuit, initiating a sequence of steps, lighting an LED indicator (signaling) 
Switch On/Off Common versions of On/Off switches are pushbutton, rocker, and toggle.  Their functions are the same, to open or close a contact, but the activation varies.  A pushbutton has an internal latching mechanism, so that the contact condition reverses with each subsequent push of the button: i.e. open/closed/open etc. A rocker is a bi-stable switch. Pushing on one end or the other changes state from open to closed. The state of the switch is evident by seeing which end has been depressed.  A toggle operates in the same way as a rocker; however, it has a “stem” that protrudes from the switching mechanism to indicate the state, open or closed. Activate part of a circuit, supply power to a device, positive indicator for status. 
Switch Limit This is a special type of momentary contact switch.  It usually has a spring loaded NC contact that is intended to be activated by a moveable extension that is aligned with the switch.  Activation is intended to stop the machine motion by opening the contact, which cuts power off. Often it is designed to require a manual reset to restore power. Limit the motion of a piece of equipment to prevent damage or harm. 

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