Some photoelectric sensors, ultrasonic sensors, and rotary encoders generate a digital dc output called a push-pull signal. The external environment activates the sensor, which, in turn, closes a circuit on either a positive supply voltage or a negative ground voltage to trigger an output. Push-pull sourcing and sinking, as it’s called, happens in two transistors:
- Sinking happens in npns, sometimes called open collectors. Npns are bipolar transistors with two negatively (n) doped semiconductor layers that donate electrons to the circuit and sandwich a positively (p) doped base. When on, npns make a positive potential between the base and upstream n layer (called the collector). The base amplifies current through the downstream n layer (called the emitter) for sensor-triggered output.
- Sourcing happens in pnps, sometimes called line drivers. Pnps are bipolar transistors in which p-doped semiconductor material sandwiches an n-doped base. When on, the pnp makes a negative potential between the base and upstream emitter. A downstream collector amplifies current from the base for sensor-triggered output.
The ability to sink or source is useful because installers can network push-pull sensors to machines without knowing in advance which signals the controls use when a machine has a PLC and it’s unclear what operation function it accepts, for example.
Push-pull outputs are simple to wire, protect against short circuits and reversed polarity, and resist electromagnetic interference. For example, encoders with push-pull outputs handle long cable runs and supply better square waves (of motion or position information to a controller) because they’re constantly driven. In other words, such encoders supply a path to ground when off and positive voltage when on. In contrast, an open collector alone generates floating (nonreferenced) voltage when on, and a line driver alone does the same when off, making them both intermittently vulnerable to noise.
Some push-pull-based sensors generate four different outputs, thanks to “4-in-1” circuitry. A user adjusts the sensor’s potentiometer to the mode that suits the application. Circuits in the sensor detect which output the controls need. Such sensors connect loads in light-on or dark-on for one wire, and V+ or V– for the other. That way, users only have to stock one sensor for a given application (instead of four models to cover the same outputs).
For example, 4-in-1 photoelectric sensors can operate light-on or dark-on plus automatically detect output load and sink or source output accordingly, to operate in pnp-light-on, pnp-dark-on, npn-light-on, or npn-dark-on. In light-on mode they trigger output when a target is present (low to high), and in dark-on mode they trigger output when no target is present — and drop out once objects are detected (high to low).
One caveat: Push-pull outputs don’t let users connect multiple sensors together, because if one sensor pushes and the other pulls, the transistor can be damaged. So users must wire these sensors individually.
Zach Steck at Pepperl+Fuchs provided information for this column.