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 Determine Which Wire is “Hot” in Household Wiring

Your home repair project involves installing a light fixture and an electrical outlet. You flipped the breakers and disconnected the existing fixture, but now you’re uncertain which wire is the “hot” one. What now?

In the United States, household wiring code offers us a clue as to which wire SHOULD correspond to what. Extreme caution should be used, however, when dealing with electricity, even if you feel certain that you know which wire is which. The possibility exists that a misguided handy-man who came before you made the wrong choice connecting the wires. You can never be too careful when your life is at risk.

According to code, the black wire is always the "hot" or "live" wire, meaning, it is connected to a source supplying 110 VAC. If you are connecting this wire to an electrical outlet, this wire should connect to the narrow spade, or brass colored terminal. If you have not switched off the breakers like you're supposed to and you grab the copper conductor of this wire, you will probably feel an unmistakable tingling sensation quickly travel up your arm. If you do feel it, let go of the wire because this sensation is not good for you. If this happens to you often, be sure to have someone around who knows CPR and first aid when you do home repairs.

The green wire is your friend. This is the ground wire. Its sole purpose for existing is to save your life by providing electricity an alternate path to ground. In the case of a short circuit, the current passes through the ground wire and overloads the breakers, causing them to trip. Sometimes this wire will also have a yellow stripe. Be sure to connect the ground wire properly when performing an electrical repair. If the copper conductor of the "hot" wire that we just discussed were to maneuver it's way over to the metal casing of, for example your ceiling fan, and make contact with it without the ground wire attached, you would be in for a nasty surprise when you touched the metal casing of the fan.

The white wire is known as the "neutral" wire. In any circuit, whether AC or DC, there has to be a complete path in order for the electrical current to flow. The white wire alone should not have a charge and should not hurt you if you touch it. If installing an electrical outlet, this wire connects to the wide spade or silver colored terminal.  Sometimes, you will find a red wire living side-by-side with the black, white, and green wires. This wire is known as a "traveler" wire and is used to connect power between 3-way light switches.

Always use caution when working with household wiring. Turn off the breaker that corresponds to the circuit that you are working on. If you are not certain which one is which, switch off the main breaker. Even then, it is a good idea to test the wires with a multimeter to be absolutely certain that the wires are not "hot". It's a good idea to test between the black wire and the white wire and then between the black wire and the green wire. It is important to note that when reading AC voltage with a multimeter, if you connect your probes between two wires that are both "hot" (110VAC) the reading will be 0 volts. (A lesson that I learned the hard way!) Always know what you are doing when dealing with electricity. Small mistakes could prove to be fatal.



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