Wiring Connection for a Three Wire Solid State DC Proximity Sensor Without PLC

The previous article illustrated the concept of the wiring connection of a 3 wire DC proximity sensor to a PLC (Programmable Logic Controller). Alternatively, in this article, I will explain another approach on the wiring connection of the same proximity sensor but without using a PLC.

The connection of a proximity sensor to an electrical circuit is not limited to only one possibility. Instead of using a PLC to construct a working electrical circuit for a certain application, making use of a traditional wiring method is also a mutually exclusive choice which is another workable option when a PLC is not available.

Bear in mind that a three wire proximity sensor has two basic types though, the NPN and the PNP, so the same procedure should be taken into consideration and applied to the target circuit when doing a conventional wiring connection for the proximity sensors which should be projected in the same manner as when connecting them to a PLC.

The following diagrams clearly illustrates the traditional way of properly establishing a working electrical circuit for both PNP and NPN three wire proximity sensor. Take special attention on the difference of the terminal connection between these two methods of wiring, one with a PNP and another with an NPN proximity sensor.

Take note also that a three wire proximity sensor would always follow a standard color coding for its terminal connection cables. The brown wire is for connection to the positive volts (e.g. +24V) of the power supply, then the blue wire is for connection to the reference voltage 0V of the power supply, and finally the black wire serves as the output terminal of the proximity sensor.

Figure 1: Traditional Wiring Method of PNP Proximity Sensor Without Using PLC to Construct a Working Electrical Circuit

The electrical diagram shown in Figure 1 above provides a clear explanation on the analogy of the wiring configuration of a PNP proximity sensor, which is so connected to conform to the electrical polarity of the PNP transistor. A step-down transformer is included to step the 220 volts AC supply down to 24 volts AC, which is further converted to 24 volts DC required for the DC circuit of the PNP proximity sensor and the relay, which as shown on the example circuit in Figure 1 are both rated to work with a 24 volts DC circuit.

When the proximity sensor is activated, its internal PNP transistor is switched ON, thereby permitting the +24V supply voltage to flow from the emitter down to the collector of the transistor, this will complete the electrical path going to the relay's coil to energize the relay. When the relay is energized, its isolated contact points changes state, the normally-open contact switches from open to close which in this case will cause the external 220 AC voltage to provide power to the lamp even when the push button switch is not pushed ON.

Figure 2: Traditional Wiring Method of an NPN Proximity Sensor Without Using PLC to Construct a Working Electrical Circuit

On the other hand, the resulting interchange route in a reverse polarity condition is shown in the above electrical diagram of Figure 2 which illustrates the required wiring connection for a working electrical circuit using an NPN proximity sensor to energize a relay. The relay is switched ON once the proximity sensor is activated which causes the internal NPN transistor to permit the 0V reference voltage to flow across the collector and emitter terminals which completes the electrical flow path to the coil of the relay. 

The internal isolated contact points of the relay will then switch condition, its normally-open contact connected across the push button switch will change to a close contact which supplies complete AC 220 volts to the lamp, this will turn ON the lamp even without manually pushing ON the push button switch.

Another circuit distinctly different from what was featured in this article is the nontraditional advanced method, which is an alternative modern approach outside the scope of this article discussed previously in my blog post How to connect a 3 wire DC solid state proximity sensor to a PLC.

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