One hertz is one cycle per second, meaning repeating cycle as an event. For example, hertz is applied physics measuring the number of times for a second wave (either acoustic or electromagnetic) is repeated or can be applied, among other uses, to ocean waves that reach the Beach vibrations per second or a solid. The quantity that measures the frequency hertz is called,in this regard, the inverse of the period. One hertz is an oscillation frequency of suffering a particle over a period of one second.

The lux (symbol lx) is the derivative unit of the International System of Units for the illuminance or lighting level. Equal to one lumen/m². It is used in photometry as a measure of luminance, taking into account the different wavelengths by the luminosity function, a standard model of the light sensitivity of the human eye.

Proximity or range sensors are used in many installations requiring an action according to the distance (D) of an object.
They are also used in robotics to control the movement according to the environment, in motorized or domotic installations, the applications can be many and very different.
There are a variety of proximity sensors, including the most common are position switches, Capacitive sensors, inductive sensors, photoelectric sensors where an emitter (E) and a receiver (R), ultrasonic sensors and magnetic sensors are required.
Each of these sensors have the same function, but each of them uses different systems for the detection, either by contact, sound, etc.
Its detection capacity varies according to the type of sensor, and can vary from millimeters to meters. We can vEr these sensors in automatic floor cleaning systems, safety barriers, rear parking sensors for cars, mobile phones, etc.

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.