Wiring Bipolar Transistor-Type Switch Sensors using DTS Hardware

Background

Many optical and induction sensors -- such as proximity sensors -- use internal bipolar transistor (BJT) circuitry to function.  When connecting these types of sensors, it's important to understand the specifics of how each type of transistor works (NPN vs PNP) as well as how much current draw and power is required for the transistor to function.  Moreover, in order to properly measure the 'switching' function of these sensors, you need to connect the sensor to the DAS in a specific way such that the DAS can read the collector voltage changes.  Using these types of sensors requires hardware components external to the DTS DAS for proper functioning -- specifically standard-value resistors.

NOTE:  This article assumes the reader has a basic understanding of BJT 
functioning and electrical engineering principles/theory.

Bipolar Transistor Types

For the purposes of this article, we will be focusing on the commonly-used Common-Emitter Positive-Negative-Positive (PNP) and Negative-Positive-Negative (NPN) transistor types.  It's important to know which type of transistor your sensor is utilizing, as it will affect the wiring for your sensor to the DAS.  You can find basic drawings for these types of transistors below:

It's important to note that PNP-type sensors are extremely rare and that, more often than not, sensor that have BJT technology in them are using NPN-type configurations.  Moreover, if a sensor notes an 'open collector', it is almost certainly an NPN-type.

Positive-Negative-Positive Transistors

PNP transistors work to sink current from the emitter (E) through the base (B).  They use a small base current (iB) and a negative base voltage to control a much larger emitter-to-collector current (iEC).  In other words, the PNP transistor works when the emitter is more positive with respect to the base and also to the collector (C).  These types of BJTs usually output on the emitter connection.

Negative-Positive-Negative Transistors

NPN transistors work to source current from the base (B) through the emitter (E).  They require a positive voltage at the base and a negative voltage at the emitter with an even more positive voltage at the collector (C).  This means that the collector must always be more positive that the base and the emitter.  These transistors output on the collector connection.

Connecting Sensor Output to the DAS

The output terminal on the BJT sensor depends on the transistor type being used.  However, universally, you would connect the output terminal to the DAS -SIG line.  The collector terminal on the transistor should always have a current-limiting resistor between it and it's source/sink voltage so as to protect the circuitry from over-current conditions.

Moreover, it is most common for transistor-switch sensors to have either an 'open collector' or 'open emitter' design -- ie. they do not contain internal pull-up/pull-down circuitry that will hold the output to a specific voltage when the base is 'OFF'.  Since DTS DAS analog input channels also do not have pull-up/down resistors on the input line, we need to create this circuitry external to the DAS and the sensor so that the sensor output on the -SIG line does not 'float' when the sensor is in the 'OFF' condition.    This is accomplished using the following wiring diagrams for each of the BJT types:

When the sensor is in the 'OFF' condition, the -SIG line should be at 2.5V while, in the 'ON' condition, it should be ~1.7V (assuming a voltage drop of 800mV across the transistor).

Recommended Sensor Settings

The recommended sensor settings for a BJT-type switch sensors are the same as those for the basic switch (push-button, toggle, etc.) and are as follows:

Excitation voltage for the sensor will depend on the sensor requirements but is usually either 5V or 10V.

NOTE:  If you are using a DTS DAS product that does not include Half-Bridge 
completion circuitry internal to the analog input channel (eg. SLICE6 and 
related products), then you will need to supply/build the half-bridge 
completion circuitry needed.  You can find an example of using a switch
closure with the SLICE6 in this article on the DTS Help Center.

Powering the Sensor from the DTS DAS

Most BJT-type sensors will be 2-wire or 3-wire configurations, with the wires carrying the following signals:

• Power
• Ground (not present in 2-wire configurations)
• Collector Output (often open-collector)

When connecting these sensors to the DAS, you will use the following wiring:

Important Considerations

One of the most important things to pay attention to when selecting a sensor to work with DTS DAS hardware is voltage and current requirements.  Each analog channel on a DTS DAS has a limited range of excitation voltage (+EX) and current output that it can supply.  Please see the datasheet for your specific DTS DAS to determine what those limitations are.

NOTE:  Selecting a sensor with a higher voltage and/or current requirement 
than your DTS DAS hardware can supply per analog channel will require you 
to provide that voltage and/or current from an alternate or external source.  
This can include an external battery, power supply, etc. or can require you 
to 'chain' multiple analog channels together on the DTS DAS.