## What is an electric circuit?

An electric circuit is a path in which electrons form a voltage or current source flow. The point where those electrons enter an electrical circuit is called the “source” of electrons.

### The Source

The point where those electrons enter an electrical circuit is called the “source” of electrons. The point where the electrons leave an electrical circuit is called the “return” or “earth ground”. The exit point is called the “return” because electrons always end up at the source when they complete the path of an electrical circuit.

The part of an electrical circuit that is between the electrons’ starting point and the point where they return to the source is called an electrical circuit’s “load”. The load of an electrical circuit may be as simple as those that power electrical appliances like refrigerators, televisions, or lamps or more complicated, such as the load on the output of a hydroelectric power generating station.

### Electrical power

Circuits use two forms of electrical power: alternating current (AC) and direct current (DC). AC often powers large appliances and motors and is generated by power stations. DC powers battery operated vehicles and other machines and electronics. Converters can change AC to DC and vice versa. High-voltage direct current transmission uses very big converters.

## What is voltage?

### Voltage

Voltage is a force that makes electricity move through a wire. It is measured in volts. Voltage is also called electric tension or electromotive force (EMF). It was named after Alessandro Volta.

Technically, the voltage is the difference in electric potential between two points. Voltage is always measured between two points, for example between the positive and negative ends of a battery, or between a wire and ground.

As seen in volt#Hydraulic analogy, voltage can be seen as the pressure on the electrons to move out of the source. It is directly proportional to the pressure exerted on the electrons. In other words, the higher the voltage, the higher the pressure. For example, a battery of 3 volts will exert pressure on the electrons twice as hard as a battery of 1.5 volts.

The voltage can push the electrons into a component, like a resistor, creating a current. Usually, the voltage and the current are related by a formula (see impedance).

Note that there must be both voltage and current to transfer power (energy). For example, a wire can have a high voltage on it, but unless it is connected, nothing will happen. Birds can land on high voltage lines such as 12kV and 16kV without dying because the current does not flow through the bird.

There are two types of voltage, DC voltage, and AC voltage. The DC voltage (direct current voltage) always has the same polarity (positive or negative), such as in a battery. The AC voltage (alternating current voltage) alternates between positive and negative. For example, the voltage from the wall socket changes polarity 60 times per second (in America). The DC is typically used for electronics and the AC for motors.

## What is a resistor?

A resistor limits the electrical current that flows through a circuit. Resistance is the restriction of current. In a resistor the energy of the electrons that pass through the resistor are changed to heat and/or light. For example, in a light bulb there is a resistor made of tungsten which converts the electrons into light.

### Series and parallel

Resistors can be linked in various combinations to help make a circuit:

1. Series – Where the resistors are linked one after another.
2. Parallel – Where the resistors are linked over one another.

There are many different types of resistors. Resistors have different ratings to tell electricians how much power they can handle before they break and how accurately they can slow the flow of electricity. Connecting two resistors in series results in a higher resistance than when you connect the same two resistors in parallel. To prevent the resistor from reaching its capacity, place the resistors in parallel to keep the total resistance lower. Nowadays the electrical industry in many cases uses so called surface-mount technology based resistors which can be very small.

## What is an inductor?

An inductor is an electrical device used in electrical circuits because of magnetic charge.

An inductor is usually made from a coil of conducting material, like copper wire, that is then wrapped around a core made from either air or a magnetic metal. If you use a more magnetic material as the core, you can get the magnetic field around the inductor to be pushed in towards the inductor, giving it better inductance. Small inductors can also be put onto integrated circuits using the same ways that are used to make transistors. Aluminum is usually used as the conducting material in this case.

## How inductors are used

Inductors are used often in analog circuits. Two or more inductors that have coupled magnetic flux make a transformer. Transformers are used in every power grid around the world.

Inductors are also used in electrical transmission systems, where they are used to lower the amount of voltage an electrical device gives off or lower the fault current. Because inductors are heavier than other electrical components, people have been using them in electrical equipment less often.

Inductors with an iron core are used for audio equipment, power conditioning, inverter systems, rapid transit and industrial power supplies.

## What is a sensor?

A sensor is a device that measures a physical quantity and converts it into a ‘signal’ which can be read by an observer or by an instrument. For example, a mercury thermometer converts the measured temperature into the expansion and contraction of a liquid which can be read on a calibrated glass tube. Video cameras and a digital cameras have an image sensor.

In the broadest definition, a sensor is an object whose purpose is to detect events or changes in its environment and sends the information to the computer which then tells the actuator (output devices) to provide the corresponding output. A sensor is a device that converts real world data (Analog) into data that a computer can understand using ADC (Analog to Digital converter).

Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps which dim or brighten by touching the base, besides innumerable applications of which most people are never aware. With advances in micromachinery and easy-to-use micro controller platforms, the uses of sensors have expanded beyond the most traditional fields of temperature, pressure or flow measurement, for example into MARG sensors. Moreover, analog sensors such as potentiometers and force-sensing resistors are still widely used. Applications include manufacturing and machinery, airplanes and aerospace, cars, medicine, robotics and many other aspects of our day-to-day life.

A sensor’s sensitivity indicates how much the sensor’s output changes when the input quantity being measured changes. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is 1 cm/°C (it is basically the slope Dy/Dx assuming a linear characteristic). Some sensors can also affect what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. Sensors need to be designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages. Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology. In most cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches.

### Types of Sensors

• Photo electric sensors (Diffuse, retro-reflective, emitter-receiver, fibre-optic)
• Ultrasonic sensors (Digital, analog, near and far)
• Temperature sensors (T-gauge sensor with Teach facility)
• Register mark sensors (Bulletproof R58 Expert, fibre-optic)
• Vision inspection systems (iVU Optical Recognition sensor with Teach facility, no PC required)
• Wireless communication systems (Dataradios, DX70, DX80, digital or analog signals)
• LED work lights and indicators (Tower lights, multicolour, audio-alarms, working area strip lights)
• Machine safety solutions (Safety curtains, controllers, relays, door and hinge-switches)

## Chemical sensor

A chemical sensor is a self-contained analytical device that can provide information about the chemical composition of its environment, that is, a liquid or a gas phase.

The chemical sensor provides information in the form of a measurable physical signal that is correlated with the concentration of a certain chemical species (termed as analyte). Two main steps are involved in the functioning of a chemical sensor, namely, recognition and transduction. In the recognition step, analyte molecules interact selectively with receptor molecules or sites included in the structure of the recognition element of the sensor. Consequently, a characteristic physical parameter varies and this variation is reported by means of an integrated transducer that generates the output signal. A chemical sensor based on recognition material of biological nature is a biosensor. However, as synthetic biomimetic materials are going to substitute to some extent recognition biomaterials, a sharp distinction between a biosensor and a standard chemical sensor is superfluous. Typical biomimetic materials used in sensor development are molecularly imprinted polymers and aptamers.