Editorial Feature

Basics on Sensors: Output and Wiring

Sep 15 2014

Image Credit: schankz/Shutterstock

Sensors are found in a wide variety of products we use on a daily basis including, cars, phones, cameras, laptops etc.

Many sensors carry out switching action through solid-state circuits. The connections between load devices and power supplies are made through two or more wires, and the entire setup is known as the electrical interface circuit. Any logical changes detected by the sensor can be indicated by the signals from programmable logic controller (PLC). PLC is signaled by switching on or off the current or voltage. The sensor output may sometimes be directly applied to switch a load, avoiding the need for PLC.

Sensor Wiring and Outputs

Some of the typical wiring and outputs from sensors are discussed below:

Switches – Switches and relays are the typical examples of sensor outputs. In a sensor having a relay output, the V+ and V- terminals are connected to the power supply. Upon detecting a phenomenon in the sensor, the output of the sensor will become active such that the relays are closed to allow the current flow.
Transistor-transistor logic (TTL) – TTL is operated with respect to two voltage levels, 5 V for true and 0 V for false. Detection of voltages through TTL is subjected to electrical noise. However, TTL outputs are necessary for computers and electronic devices. Simple electronic circuits such as the Schmitt trigger can be used while connecting TTL to other simple circuits.
Field effect transistor (FET)/TRIAC – FET is a solid-state output device that allows rapid switching of alternating and direct current. It does not exhibit any current leakage, and its response time varies from 2 ms to 30 µs. Alternatively, TRIAC suitable for switching only alternating current exhibits higher current leakage when compared to FET.
Sourcing/Sinking – Sourcing sensors enable current from the positive terminal to flow out of the sensor while sinking sensors allow passage of current into the sensor. An NPN transistor is used as the sinking input, and a PNP transistor is used as the sourcing output. The sensor is said to be active if the active line is at 0 V, and it is said to be inactive if the active line is at the V+ value.
Solid-state relays – These relays are employed as outputs in some devices as large loads. They are widely used for changing alternating currents.
Two-wire configuration – Although easy to wire in series or parallel with the load, two-wire sensors have high voltage drop which in turn affects the efficiency of the system. The power for two-wire sensors is obtained from the line where switching action is performed.
Three-wire configuration – Three-wire sensors include three leads, two of which is connected to the power supply while the third lead is connected to the load. In parallel configuration, three-wire sensors with PNP or NPN transistor outputs are connected together. Three-wire NPN output devices connected in series supply negative voltage to the following device thereby providing negative voltage to the load. Three-wire PNP output devices supply power to the following device and to the load.