1.2 SENSOR WIRING
• There are a variety of basic methods for connecting sensors to devices. These include,
- Plain Switches - normally open or closed to allow current to flow.
- TTL (Transistor Transistor Logic) - for low voltage logic using 0V and 5V.
- Sinking/Sourcing - DC current is drawn through the sensor.
- Solid State Relays - These are used for switching AC outputs.
1.2.2 Transistor Transistor Logic (TTL)
• This method switches between two logical DC voltages.
• Typically the low voltage is 0V and it indicates a false condition.
• The true state is normally indicated with a high voltage. 5V is quite common, but other voltages can be used for certain sensors.
• The voltage levels have certain tolerances built in. For example 0V indicates false, but a voltage up to 1.2V might still be considered false. This also means there is an ambiguous zone where the voltage will be judged neither true or false. This zone can be eliminated using schmitt triggers.
• These scheme is used for a variety of reasons.
- they can be made to appear like normal switches.
- they can be used to switch low current devices and hence replace controllers for simple applications.
• The two types indicate which way the current and voltage are switched to the output. The methods also refer to transistor types because of the similar behavior (Note: in fact they do use transistors inside).
• The two main differentiation is,
- Sinking (NPN) - In this case, when actuated, the sensor will connect to ground, or pull the input low. If this is the case you need to use an output that normally stays high, true, floats high, etc. The figure below shows an approximate representation.
- Sourcing (PNP) - In this case, when actuated, the sensor will connect to V+, or pull the input high. If this is the case you need to use an output that normally stays low, flase, floats low, etc. The figure below shows an approximate representation.
• To directly connect devices to these sensors we can use the arrangement below.
• It is worth stating the obvious - The output of sensors will be the inputs for other devices, such as PLCs - this will lead to confusion when specifying PLC input devices. Some manufacturers indicate what the input type is, others specify what it is for. This basic result of this is that you must look at the electrical connections of the card, and not just the designation.
• Note, if using these sensors with PLCs, care is required to select the appropriate cards and connections. The figure below is for NPN sensors.
• The figure below is for PNP sensors. These are generally more common combinations.
• It is quite common for manufacturers to offer PLC output cards that will handle PNP and NPN sensors. In this case the card will require both V+ and a common connection, and the each output must be set for either NPN or PNP.
• Two wire sensors are also common because they reduce the wiring.