VDC would be the abbreviation for Volt Direct Current which translated would volts direct current. The idea of ??this type is unidirectional current to the load. Direct current is normally produced by batteries, thermocouples, solar cells and a dynamo electric machine type. The direct current can flow in conductors such as wires, but may also flow through semiconductors, insulators ... In the image: 1 - current per pulse 2 - Direct Current (DC) 3 - Alternating Current (AC) 4 - Variable Current

These are two types of contacts widely used in electrical installations and circuits. Distinguishing the difference of these two types is very simple.

• A NO (normally open) type contact is one that does not let the current flow while the same or the device is in standby or standby mode. The most common type NO contact is eg house switches, which remain open (not closedCircuit) until the switch is pressed.
• The NC contacts (normally closed) are the opposite of the NO contact, they let the current pass while they are in standby or at rest to be actuated. An example would be a thermal protection relay of a motor, which remains closed while there is voltage.

The photoelectric sensors are those sensors that act by closing or opening the circuit depending on the light intensity they receive.
The system consists of an emitter system in charge of emitting light and a receiver system, in charge of managing its function according to the light received.
We can find several types of photoelectric sensors although we can divide them into several more generic groups, barrier photocell (A), Self-reflective (B) and reflective with reflector (C).

The barrier sensor is possibly one of the most used sensors due to its simplicity of use, easy installation and reliable operation.
Its operation is simple, the transmitter is placed in line with the receiver being one in front of the other. Thanks to this distribution external aspects such as dirt, dust, humidity affect to a lesser extent its operation.
This type of barrier sensors have a range ofAnd much greater detection to the rest, it is possible to say that being an emitter and a receiver this type of sensors are not able to detect color in objects as other sensors can make and it is necessary to take into account that these sensors emit a light that is captured By the emitter, if when crossing the barrier we do with a transparent or translucent object will not be activated, as the beam of light will get through the object, therefore alone andS valid for objects with a high degree of opacity.
These types of sensors are widely used in security and automation systems, for example opening or closing a barrier or door, activating an alarm, etc.
When faced with, the emitter emits a beam of light to the receiver, if the receiver is receiving the light beam it will be in standby mode to act. When an object passes between the sender and the receiver, cutting the view directlyAnd leaving the receiver without receiving the light this will activate the function for which it is programmed, for example in a security door, the door remains closed while the light beam is received by the receiver, if a person passes between the two transmitters The beam and light will be interrupted by the body of the person, so that the sensor will act by opening the door.

This is one of the most economical models of detectors by photocell, but against is one of the least recommended in situations with high humidity or places with a lot of dust or dirt. The reception distance is also shortened as opposed to the barrier, so they do not have as much detection range. This factor is directly linked to the color of the object, since the light emitted must bounce on it, it will not haveThe same index of reflection an object of black color (that absorbs the light) to a white object (that bounces the light), therefore, objects of light colors will be better detected with a self-reflecting photocell.
This type of sensors unlike those of barrier, its emitter and receiver are in a single device. The emitter emits light on the object, when light reaches the object it bounces and is received by the receiver, qYou will be in charge of executing the action, like stopping a tape, adding a unit, triggering a mechanism, etc.
As it is a single device its installation is minimized, since it is only necessary to feed a point, unlike the barrier that are two points. They are ideal for installations where it is complicated or there is no space to place the receiving device.
This type of photocells can be divided into 3 groups,Standard or self-reflecting photocells, background suppression and autoreflective foreground photocells (FGS, Foreground suppression)

• Self-reflective or non-suppressed photocells (the standards) are those whose detection distance is affected by the color of the detected object. As a result of this small inconvenience,Have improved these sensors by creating two variants, with background suppression and foreground suppression.
• Self-reflective or suppressed photocells work in the same way as the previous ones, but also use several triangulations of the light to calculate the exact position in which the object is. Thanks to these triangulations the sensor is able to ignore any object that may be behind and pUeda interfere in the detection, in such a way that they are not so affected by lighter colors, shapes or brightness of the object.
• Self-reflective photocells with foreground suppression are those that work just like background suppression but focus on the background surface, adjusting the detection zone at that distance. Basically, what the sensor does is focus on the background and will detect any object thatIs placed on said background, for example, is the system used for the detection of objects on a conveyor belt or other surface.

two.

VAC would be the abbreviation of Volt Altern Current which would be translated Volts of alternating current. The difference between VDC is that its sinusoidal oscillation achieving a more efficient energy transmission.

The alternating current is the energy that we receive in our homes, it is generated in the power plants by means of alternators.

The intensity of this type of current varies with time, and changes direction 50 times throughsecond (50hz). The generated voltage changes in the form of sine wave so it is not constant.

In the image:

1. Current by pulsations.
2. Direct current (DC).
3. Alternating current (AC).
4. Variable current.