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Frequently Asked Questions - General Sensor FAQs

FAQs - General Sensor FAQs

Four of the most common sensor outputs are: NPN current sinking, PNP current sourcing, Two-wire DC/AC and Relay.

NPN Current Sinking

NPN Current Syncing

One simple way to understand NPN wiring is to look at the following simple electrical circuit.

NPN Simple

The positive side of the battery is attached to the load. The negative side of the load is switched.

PNP Current Sourcing


One simple way to understand PNP wiring is to look at the following simple electrical circuit.

PNP Simple

The negative lead of the battery is attached to the load. The positive side of the load is switched.

Two-Wire Sensor

5 two wire sensor


One good question is to ask is whether or not the common of the PLC is connected to + voltage or 0 voltage. 
  • If the DC voltage PLC has a positive voltage common, an NPN output sensor is required.
  • If the DC voltage PLC has a 0v common, a PNP output sensor is required.


In order to calculate response time of a rotating target we have to convert rotational speed to linear speed.

Target Length / Linear Speed = Required Response Time

For example: The rotating part has a 15mm long piece of reflective tape attached to 50mm diameter shaft. The maximum speed of the shaft is 6000 revolutions per minute = 100 revolutions per second.

Rotating Target Calculation

  • Converting rotating speed to linear velocity: Circumference of shaft PI (π) * diameter= π * 50mm= 157mm
  • Linear velocity of the shaft circumference = 100revolutions per Second * 157mm/revolution = 15,700 mm/second.
  • Target length/linear speed = 15mm/15,700 mm/sec = .000955seconds = .955milliseconds required response time.

This is assuming that the sensor's effective beam is small. The larger the effective beam the faster the response time needs to be.

Response time of a sensor is the maximum amount of time it takes for the sensor to detect a target object and change the state of the output signal.

Calculating response time is an important component in specifying a sensor especially when the application is very high speed or when detecting very small objects.

The following information is required in order to determine response time:
  1. Size of the target object
  2. Distance or space between target objects
  3. Speed 

Response Time Calculation

Response Time Calculation

  1. 7mm wide registration mark
  2. 160mm spacing between marks
  3. 300 ft/minute 

300ft/minute = 5ft/second
5ft/second = 60"/second
60"/second x 25.4 = 1524mm/second

Response Time Calculation

  1. 12,000 parts per minute
  2. 20mm wide target object
  3. Parts equally spaced 5mm apart 
12,000 parts/min. = 200 parts/second
Each part is 22mm wide + 3mm spacing = 25mm of linear travel
Speed of the parts = 25mm/part x 200 parts/second = 5000mm/second

IP69K is a rating of German standard DIN 40050-9 extending the IEC 60529 rating system for high-pressure, high-temperature wash down applications.


Test Condition:
  • A sensor on the turn table that rotates 5 times per minutes
  • Pressure: 80~100 bar
  • Flow Rate: 14~16 liter/minute
  • Temperature: +80°C / -5°C
  • Distance from Nozzle: 100~150mm
  • Nozzle Angle: 0°, 30°, 60°, 90°
  • Testing Period: 30sec per angle 
IP69K does not guarantee the sensor's working condition under the test condition example given above. The sensor will not work properly when the lens is wet and the light is refracted.

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