How to Test Whether a Sensor has a PNP or NPN Type Output Using a Multi meter

If you are replacing a 3 wire sensor which operates on DC voltage, it is important to know whether the sensor's output is a PNP or NPN type. Often this information is printed on the sensor itself, but it is not uncommon for it to be scratched off over a long period of time.

Set the multi meter to DC voltage. This is indicated by either the letters "VDC" or "DCV" or by a symbol which looks like 3 dashed lines over a solid line. There are usually several levels within the DC voltage setting. Choose the "600" level.

The power will need to be ON to perform this test, so use caution when attempting the following. Connect two of the sensor wires to the power supply. If the color combination of the wires is blue, black, and brown, then normally, the blue wire connects to 0v and the brown wire connects to positive volts. Touch the black meter probe to the 0V wire of the sensor. Connect the red meter probe to the signal output wire of the sensor. This wire is normally black. The meter should read "0."

Force the sensor to output. If it is a photoelectric sensor, block the photoelectric beam. If it is an inductive proximity switch, introduce a small piece of metal in front of the sensor. For an ultrasonic sensor or a capacitive sensor, you can just use your hand to make the sensor output. Be sure that the sensor is detecting the object. Many sensors have a small LED that illuminates when the sensor detects it's target.

Watch the meter display as you force the sensor to output. If the readout changes to a number between 10 and 30, then the sensor output is a PNP type, also known as "sourcing." If the meter display remains at "0", then the sensor output is an NPN type, also known as "sinking."

If you believe that the sensor is NPN, there is an additional test that may be done to confirm. Remove the meter probes from the wires. Now place the red meter probe on the positive voltage sensor wire, normally a brown wire. Touch the black meter probe to the signal output wire of the sensor, normally black. When the sensor does not detect it's target, the meter display should read between 10 and 30. When the sensor senses an object, the display should drop to "0." This will confirm that the sensor has an NPN type output.

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How to Measure Level of Material in a Large Bin or Hopper

Let's say you have a large bin or hopper and you regularly fill it with wood chips or powder or even liquid. How can you accurately measure the level in the bin?  In applications where it is necessary to measure the level of material in a large collection bin, there is not always an obvious way. This article will describe one method, using an ultrasonic sensor.

Purchase an industrial ultrasonic sensor (uses sound waves) with a range long enough to bounce from the top of the bin down to the lowest possible level of the material, or at least the lowest level that you care to measure. Be sure to select an ultrasonic sensor with an analog output.  Be aware that all ultrasonic sensors have a "blind zone" beginning at the face of the sensor and extending to various distances away. For this reason, the material to be measured should never be allowed to enter this "blind zone" because accurate measurement cannot take place in this range.  Also note that ultrasonic sensors are affected by drastic changes in temperature and may need to be recalibrated occasionally.

Install the ultrasonic sensor in the top of the bin pointing downward so that it faces the material in the bin. The analog output from the sensor will need to interface with the PLC or computer so that the reading may be interpreted and any necessary functions may take place when the level reaches certain points.

Empty the bin and take note of the output from the sensor. Then, fill the bin to the maximum desired level and take another reading from the sensor. Now you have your high and low margins. You can program certain things to happen when the material hits a certain level, for instance, when the level of water in a bin reaches the top, the pump that supplies the water shuts off until the water is lowered again to another predetermined level.



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