Monthly Archives: September 2013

How do photovoltaic cells work?

Your calculator probably has a darkish colored panel just above the display. The panel is made up of solar cells that power up your calculator if there is enough light. You may also have seen some solar panels, which people use for charging up their cell phones. Earlier, these solar cells or photovoltaic cells were exclusively used to power the electrical systems of satellites. However, they are now commonly used in less exotic ways as well.

How do photovoltaic cells convert light to electricity? For this, you must understand the way these cells are constructed. A photovoltaic cell has two silicon plates bonded together. Pure silicon is an insulating material and is unable to conduct electricity. This is because of the atomic structure of silicon, which has place for eight electrons in the outermost shell of its atoms. However, there are only four electrons present.

Therefore, when silicon atoms come together, they share their electrons. Each atom shares one electron with its neighbor and they become a pair. That means at any time, four atoms surround each silicon atom, bringing up its catch of electrons to eight on the outer shell. Since all the electrons are now bound up, there is none left free to move about and carry electric charge.

To make the silicon plates able to carry electric charge, one of the two plates must have some free electrons and the other plate must have some holes or lack of electrons. This is done by the process of doping. While making the plates, one of them is given a few phosphorus atoms as impurities. Since phosphorus has five atoms in the outermost shell of its atom, when combining with the silicon atoms, one of its electrons remains unpaired. This makes the silicon plate with the phosphorus impurity have excess electrons and this is called the n-type silicon.

Likewise, the other plate is doped with boron, which has only three electrons in the outermost shell of its atoms. This leaves the combination of silicon and boron atoms with a deficit of electrons and this is called the p-type silicon. This is like a hole, which will readily grab a wandering electron to fill up its vacant space.

Light is essentially a barrage of energetic particles called photons. Photons impart their energy to the surface where they land, which is why you feel warm when you stand in sunlight. If light or photons are allowed to fall on the n-type silicon plate that has extra electrons, they receive the excess energy from the photons. The extra energy allows them to dislodge themselves from their original positions and wander off until they come to the other plate with the holes, where they are eagerly absorbed.

However, the n-type silicon plate that supplied the electrons now has a deficiency of electrons that it must fill up. For electrons to flow, the circuit must be externally completed. This is usually done by connecting a load to the solar cell through external wires. The plate makes up its deficiency of electrons by borrowing them from the connecting wire. In essence, photons drive the electrons through the entire circuit, and that makes the current flow through the solar cell and the load connected to it.

As soon as light falling on the solar cell is removed, the running electrons lose their drive, and the flow of current stops. Although the output from each cell is usually very tiny, by combining them in series and parallel, an impressive amount of power can be generated.

Transistors: What Is The Difference Between BJT, FET And MOSFET?

BJTs, FETs and MOSFETs are all active semiconductor devices, also known as transistors. BJT is the acronym for Bipolar Junction Transistor, FET stands for Field Effect Transistor and MOSFET is Metal Oxide Semiconductor Field Effect Transistor. All three have several subtypes, and unlike passive semiconductor devices such as diodes, active semiconductor devices allow a greater degree of control over their functioning.

Depending on their subtypes, operating frequency, current, voltage and power ratings, all the three types of transistors come in a large variety of packages, and all of them are susceptible to ESD or Electro Static Discharge. That means when you handle these devices, you must take adequate precaution against static charges destroying them.

he basic construction of a BJT is two PN junctions producing three terminals. Depending on the type of junctions, the BJT can be a PNP type or an NPN type. The three terminals are identified as the Emitter or E, the Base or B and the Collector or C. BJTs usually function as current controlling switches. The three terminals can be connected in three types of connections within an electronic circuit – Common Base configuration, Common Emitter configuration and Common Collector configurations. All the three connections have their own functions, merits and demerits. The BJT is Bipolar because the transistor operates with both types of charge carriers, Holes and Electrons.

The FET construction does not have a PN junction in its main current carrying path, which can be made from an N-type or a P-type semiconductor material with high resistivity. A PN junction is formed on the main current carrying path, also called the channel, and this can be made of either a P-type or an N-type material. The three leads of a FET are the Source (S), Drain (D) and Gate (G), with Source and Drain forming the ends of the channel and the Gate controlling the channel conductivity. Unlike the BJT, the FET is a unipolar device since it functions with the conduction of electrons alone for the N-channel type or on holes alone for a P-channel type.

The input impedance at the gate of an FET is very high, unlike the BJT, which comparatively has much lower impedance. Additionally, the conductivity of the channel depends on the voltage applied to the Gate, essentially making it a voltage-controlled device, unlike the BJT, which is current-controlled. The voltage applied to the Gate controls the width of the channel, allowing the FET to carry current between the Drain and Source pins. The Gate voltage that cuts off the current flow between Drain and Source is called the pinch off voltage and is an important parameter.

The MOSFET is a special type of FET whose Gate is insulated from the main current carrying channel. It is also called the IGFET or the Insulated Gate Field Effect Transistor. A very thin layer of silicon dioxide or similar separates the Gate electrode and this can be thought of as a capacitor. The insulation makes the input impedance of the MOSFET even higher than that of a FET. The working of the MOSFET is very similar to the FET.

You can read more about transistors in depth here.

Automate Your Home HVAC System from the Internet Using the Raspberry Pi

The HVAC devices in your home, typically the air-conditioner, thermostats, heating and ventilation, use one or more remote handheld devices working on Infrared (IR) technology. As the HVAC devices are from different manufacturers, you will most likely own a multitude of remote devices, making it difficult to handle and set each of them independently.

However, with the Raspberry Pi or RBPi, a small board called the IR Remote Shield and a wireless interface, you can control all the HVAC devices and that too from the Internet. Imagine setting up the environment in your home just as you are leaving office, so that you have a cozy atmosphere to relax at home.

There are two steps in this project. The first step involves teaching the Raspberry Pi and IR Remote Shield combination the codes that the remote handheld devices utilize to control the various functions of each of the HVAC devices. The second step is to connect the RBPi to the Internet through any one of the wireless interfaces such as Wi-Fi, 3G, GPRS, Bluetooth, and ZigBee or 802.15.4. These interfaces are available from Cooking Hacks, and you can choose one.

After you connect your RBPi to the Internet and feed in the IR codes used by your HVAC components, you can use a webserver, a laptop or even your Smartphone to control all your home HVAC appliances from anywhere in the world. But, a few words about Infrared technology first.

Started in 1993, IrDA or Infrared Data Association is the technology popularly used for controlling devices such as air-conditioners, TVs, radios, audio systems and many others. It is based on light rays in the infrared spectrum and invisible to the human eye. Using infrared transmitters and receivers, communication between two devices can be established in direct line of vision. The infra-red transmitters use special types of Light Emitting Diodes and the receiver uses a photocell sensitive only to the infra-red light.

Infra-red communication or control uses serial data transfer by emitting pulses of light, which is coded in binary, a language micro-processors are capable of deciphering. Therefore, for deciphering the binary code protocol that the remote is sending, you must hold the remote in front of the receiver on the IR Remote Shield mounted on your Raspberry Pi.

To decode and copy an IR code, press the “Receive” button on the IR Remote Shield. This will allow RBPi to capture the code the remote button is sending. In the software, you will have to tag each code with its individual function, for example, a certain code may be for raising the temperature and another for lowering it.

Once all codes from all the remotes are in the RBPi, it is a simple matter to map the codes and their functions on a web application. As the RBPi is connected to the Internet, any browser on the Internet can call up the web application, and the specific settings for the HVAC units altered. This allows the software program running on the RBPi to send the altered binary code to the specific HVAC unit via its IR link and change its status.

How do battery powered pico-projectors work?

Once upon a time, very long ago, the projector world was ruled by the intense light of arcs. As they were rather unwieldy, xenon lamps took their place. With the unrelenting march of innovation, the era of OHPs or overhead projectors that could project images of transparencies, came into existence. These soon became obsolete as computers evolved and could be directly connected to projectors with LCD screens. The latest in line is the Pico-projector, which uses tiny batteries and the light from LEDs to project large displays.

Although Pico-projectors are small – as small as mobile phones, and sometimes even smaller – they can project large displays, sometimes up to 100 inches. Even though their brightness and resolution is not up to the mark of their bigger brethren, Pico-projectors are relatively new in the innovation chain, and as the market expands, they are expected to develop further.

Several companies have developed their own methods of producing battery-powered Pico-projectors. Of them, the three major technologies are DLP or Digital Light Processing, LCoS or Liquid Crystal on Silicon and LBS or Light Beam Steering. DLP and LCoS use a white light source and a system of filtering techniques to create different color and brightness of each pixel. On the other hand, LBS uses a small liquid crystal display to control the amount of light going to each pixel.

Digital Light Processing or DLP is pioneered by Texas Instruments (TI). Their idea is to use tiny mirrors on a chip to direct the light. Each mirror controls how much light goes onto each pixel of the display. The mirror can be turned on or turned off on command many times a second, and the on to off time ratio defines the brightness of the pixel. For color, there is a color wheel in front of the light source, splitting the beam into red, green and blue. Each mirror controls all the three light beams.

Liquid Crystal on Silicon or LCoS, as the name suggests, uses an LCD to control the amount of light reaching the pixel of the display. For color, two techniques are used. One is the Color Filter where three sub pixels are used, and they each have their own color, Red, Green and Blue. The other is the FSC or Field Sequential Color that requires a fast LCD and a color filter to split the image into RGB, the three main colors sequentially. The LCD is refreshed three times, once for each color. For LCoS, the light source could be an LED or a diffused Laser.

Laser Beam Steering or LBS creates the image one pixel at a time. The technique uses three directed laser beams, red, green and blue. The three beams are combined using optics and are guided using mirrors. So that the eye does not notice the pixel-by-pixel design, the image is scanned at over 60Hz.

LBS has some advantages over the other two techniques. The size is small and power consumption lowest, as the darker pixels require less energy, while the black pixel does not require any energy at all. The image from an LBS system is always focused, even on curved surfaces. On the other hand, lasers are expensive, cause random intensity patterns and are a concern for eye safety.

Brewing beer with Raspberry Pi

Ever since man first tasted naturally made beer, there has been no looking back. Not only man, animals also find beer irresistible. Beer brewers have always been looking for improving on the natural method to make beer tastier. Their work has become somewhat simpler and more high-tech with the introduction of Raspberry Pi or RBPi.

By using open source software and ultra-cheap computer hardware such as the RBPi and the Arduino, people are interconnecting all types of existing devices making them interact with each other. An ardent home brewer and Dutch electrical engineering student, Elco Jacobs has turned his refrigerator into a home beer brewing system. He calls it his BrewPi system and plans to sell kits. He has turned over his instructions and source code on-line for free, since he thinks they might be useful to others even if they do not brew beer.

Elco Jacobs has essentially beefed up his refrigerator with sensors, which send their data to an Arduino board. The Arduino adjusts the controls on the refrigerator for temperature and displays the results on an OLED display. The RBPi has a web server loaded and it provides the web-based interface for viewing. A Python script running on the RBPi allows it to communicate with the Arduino.

By using Jacob’s code, anyone can build a web interface to control an Arduino. The code makes it easy to use an Arduino to control an OLED display and present data on it after filtering the sensor data. When Jacob first started brewing beer at home, he was still a university student. He became interested after learning that to start brewing beer would cost him about 60 euros. He quickly learnt that temperature control was the main thing required when brewing to determine the fermentation rate of the beer and subsequently its flavor.

Hefeweizen, the favorite style of beer for Jacob, is particularly sensitive to temperature fluctuations. The taste of this beer alters radically if the heat to the beer buckets was not under finely tuned temperature controls. However, commercial temperature controls being outside the affordability of a university student, Jacobs wanted something cheap that he could control from the web.

That is how he hit upon the combination of the Arduino (for temperature control) and the RBPi for the web interface). Jacob’s goal is to sell a kit, which will require no soldering. The kit will not have the RBPi and the refrigerator.

By controlling the temperature of the fridge that holds the carboy, BrewPi is able to control the temperature of the fermenting beer accurately. Two zones of temperature are controlled separately, the fridge temperature and the beer temperature. This allow the beer temperature to be held far more steady than if a single thermostat were to be used. However, you can set BrewPi to operate in three modes: constant fridge temperature, constant beer temperature or allow it follow a temperature profile for your beer.

Highlights of BrewPi: four outputs for actuators, single wire bus for all sensors, a OneWire distribution board and lots of pluggable terminals.

What Are Inductors and How Do They Work

An inductor or an induction coil is a tightly woven coil of wire. Now, you would not expect an ordinary piece of wire to show any special property on passage of current through it. A coil with several loops or turns however, exhibits a remarkable property when current passes through it. The current through the coil creates a magnetic field in the immediate space surrounding the coil. The field stores electrical energy during the passage of current and for a very short while, even if you cut off the current.

Another amazing fact of an inductor coil is that if you place the coil in a varying magnetic field, a current starts to flow through it. The amount of current depends upon the rate at which you change the field.

Bulb and Coil Experiment

You can make out this amazing property of an inductor coil from a simple experiment. Consider a simple circuit with a battery, bulb and a switch. The bulb glows when you close the switch while it stops glowing the moment you open or release the switch.

If you now include a coil of wire wound around an iron bar across the bulb, the bulb will light up as you close the switch. However, instead of glowing at a constant brightness, the intensity of the light changes from bright to dim. If you now open the switch, the bulb does not turn off immediately as you would expect. Instead, the brightness gradually decreases before turning off completely.

Explaining the Observations

You can attribute this curious behaviour to the inductor coil placed across the bulb. When you close the switch, current flows from the battery through the bulb, causing it to glow. At the same time, current flows through the inductor coil too. This generates a magnetic field in the space surrounding the coil. The magnetic field varies in the short time the current builds up. The changing magnetic field induces a current to flow through the coil. However, according to the rules of electricity, this current is opposite to the original current sent by the battery. Hence, the effective current through the coil increases with time, while decreasing that passing through the bulb. This causes the bulb to reduce its glow from bright to dim.

When you open the switch, the magnetic field falls. During the fall of the field, the induced current causes the voltage across the inductor to rise for a moment. This causes the bulb to brighten up briefly. When the current reduces to zero, the bulb turns off.

Inductance

The physical quantity associated with this property is called inductance. The value of this quantity is measured in Henrys. Inductance depends upon four features, which include the number of turns in the coil, the degree of overlap, area of the cross section of the wire and the material of the core inside the coil.

You can increase the inductance by increasing the number of turns and the cross section area of the coil. You may also increase the value by increasing the degree of overlap i.e. by using a tightly wound coil.

Uses of Inductors

You must have wondered how traffic signalling works. Traffic light sensors make use of inductors, which form filter circuits along with capacitors. Inductors are essential components in electronic circuits and devices like receivers, transmitters, oscillators and voltage regulators, as well.