Fritzing

It has been a great struggle to to do hobby electronics projects when I was studying engineering as there was no single good solution for designing schematics, PCB designing and prototyping. I had to switch between different tools to draw schematics and design PCB. But none seemed to be a good fit for a newbie to learn designing and prototyping. I tried various tools like eagle, procad and other PCB designing tools but they were too complex to learn and for a newbie to get acquainted with EDA tools. Recently I stumbled upon a this nice initiative called Fritzing.

Fritzing is an open-source open-source initiative to support designers, artists, researchers and hobbyists to work creatively with interactive electronics. Using tool you can draw schematics, designing PCBs, teach electronics, prototyping, and share your hobby projects with others. Fritzing is being developed by researchers in the Interaction Design Lab at the University of Applied Sciences Potsdam, Germany.

The important feature of Fritzing is that you can switch between the three views: breadboard view, Schematic and PCB view. editing in view will automatically reflect the changes in other view as well. Fritzing will simply the whole process of simple electronic projects development. It is good tool for newbies who want to learn Electronic Design Automation Tools (EDA).

To download Fritzing visit http://fritzing.org/welcome/

Nokia launches bicycle-powered charger

NOKIA, The world's largest mobile phone maker, launched four low-priced handsets and a recharger that can be connected to a bicycle's dynamo which charges when the wheels turn.The bike kit has a charger, dynamo and a holder to secure the phone to the bicycle.

The bicycle charger kit and handsets, come with a standby battery time of up to six weeks, FM radio and flashlights are aimed at users with limited access to electricity.
 

The price of the charger kit, which also includes a holder for securing the phone to the bicycle, will vary according to market, but in countries like Kenya,  it would be around 15 euros (18.43 dollars)
 

The dynamo, a small electrical generator, uses the movement of the wheels to charge the handset through a standard 2 mm charging jack used in most Nokia handsets.

To begin charging, a cyclist needs to travel around six kilometres per hour (four miles per hour), and while charging times will vary depending on battery model, a 10 minute journey at 10 kmh produces around 28 minutes of talk time or 37 hours of standby time, the spokesman said. The faster you ride, the more battery life you generate.
The charger is set to become available through selected retailers and the company's online store before year-end, Nokia said.

How to make an LED work

An LED (Light Emitting Diode) is a semiconductor that emits light energy when a current flows through it. Current will only flow one direction, just like a regular diode. There are a few things you need to know about an LED before you use one. First, and most importantly, is that an LED has very low internal resistance. This means that left to itself, an LED will pass so much current that it will burn up. They require an external resistor to limit the current.

Most LED's have a current rating, which determines the size of the resistor you will need. The current rating tells you what the maximum allowable current for the part is. In general, the higher the current, the brighter the LED.

Most LED's seem to handle at least 15mA. If you are using a 5 volt circuit, then Ohms law tells you what resistor value to use. R = V / I, so R = 5v / .015A = 333 ohms.

BJT configurations

BJT Configurations

There are plenty of texts around on basic electronics, so this is a very brief look at the three basic ways in which a bipolar junction transistor (BJT) can be used. In each case, one terminal is common to both the input and output signal. All the circuits shown here are without bias circuits and power supplies for clarity.

Common Emitter Configuration

Here the emitter terminal is common to both the input and output signal. The arrangement is the same for a PNP transistor. Used in this way the transistor has the advantages of a medium input impedance, medium output impedance, high voltage gain and high current gain.

Common Base Configuration

Here the base is the common terminal. Used frequently for RF applications, this stage has the following properties. Low input impedance, high output impedance, unity (or less) current gain and high voltage gain.

Common Collector Configuration

This last configuration is also more commonly  known as the emitter follower. This is because the input signal applied at the base is "followed" quite closely at the emitter with a voltage gain close to unity. The properties are a high input impedance, a very low output impedance, a unity (or less) voltage gain and a high current gain. This circuit is also used extensively as a "buffer" converting impedances or for feeding or driving long cables or low impedance loads.

A note about Phase Shifts

In both the the common base and emitter follower configurations, the input and output signals are in phase, but with the common emitter configuration only, the input and output signals are phase inverted, a positive input resulting in a negative output and vice versa. This is also known as  phase displacement.bbjt

Estimating Transmitter Distance

Here is a much simplified equation for analysing low power radio transmitters, for line of sight. It does not take into account probagation conditions or other limiting factors, but does include a variable for the losses in the antenna and tank circuit of a transmitter. It may be applied to low power transmitter circuits such as the circuits on this site. In deriving this equation, I have had to estimate two unknowns, the loss and inefficiency of a telescopic whip antenna, and the small signal high frequency collector current of the transistor in this circuit. Here is an example circuit:



The general equation for estimating transmitter field strength is calculated from the equation below:



Where d is distance in meters, E is the field strength in V/m and Pt is the total power from the transmitter. By finding your radio receivers field strength,(usually in the manual) then the equation can be transposed to solve distance:



The next step is to work out the power from the transmitter. The 2 stage circuit above works from a 9 volt battery, its output frequency was measured to be 107.2MHz. The final common emitter stage of this circuit, develops power in the tank circuit, which is transferred to the antenna, in this case a 30cm telescopic whip. Most of the power is developed in the coil, there are three ways to calculate this:



At resonance the voltage and current in the oscillator tank circuit will be in phase. Therefore all that is needed is to find the impedance of the tank circuit and either the voltage across it or the current flowing through it. The problem in measuring the ac voltage across the tank circuit is that most meters will not give accurate results at high frequencies. This is the same for high frequency currents. To estimate the ac collector current in the tank circuit, I have worked out the dc collector current. The two values will be slightly different, but as this is only an approximation, the error will not be significant. To find the dc collector current, measure the dc voltage across the emitter resistor and use ohm's law. In my circuit, this measured 2.99V across the 470 ohm emitter resistor, the dc collector current is therefore :

2.99 / 470 = 6.362mA

This value will be substituted for the ac collector current. The impedance of the tank circuit at resonance is given by the following equation: =100K

The R is the dc resistance of the coil in the tank circuit. At VHF, this is small as coils have only a few turns. In this circuit the dc resistance was measured at 0.1 ohm.

Small signal Analysis:
The equivalent output circuit for the transmitter is now worked out and drawn as below. The impedance of the tank circuit (100K) is in parallel with the output impedance of the transistor. This value, around 40k can generally be ignored, but in this case it is in parallel with the output circuit and makes an appreciable difference. Also the 3.3pf capacitor is in series with the 470 ohm resistor. This is also considered at short circuit as the power supply is decoupled with a capacitor. The capacitive reactance of the 22nF capacitor is a short circuit. The effective load or impedance of the output will be as below:



The overall output circuit is the parallel combination of these components. The 3.3p capacitor has an impedance of around 450 ohms at 107.2 MHz. The combined impedance is therefore:-
40k // 100k // (450+470) = 891.3 ohms

Having now found the impedance, the approximate power in the tank circuit can be calculated:-



Having now found a value for total transmitter power,Pt and using a radio receiver with a known sensitivity of 20uV/meter the distance the signal would be received is worked out:



This equation assumes that all the power in the tank circuit, 36mW is transferred without loss to the antenna and that the antenna has a gain of unity. The result also assumes there are no losses incurred from transmitter to receiver due to probagation effects as well. Using a 30 cm length of telescopic antenna , I have modified the equation to compensate for losses in the antenna and coupling circuit . I have assisgned a variable called A_L into the equation and estimated its value at 1%. The modified equation is then:



The new result calculates effective distance from transmitter to a radio receiver with 20uV/m sensitivity. This is clearly a vast reduction in distance from the first result. To test this result, i went to a large field. Holding the transmitter at roughly 1 meter high from the ground, i walked away carrying the receiver. The signal was clearly audible 300 meters from the transmitter giving a strong reading on the signal strength meter of the receiver. This was about the length of the field. I must stress again that the above calculations are ONLY approximate, but if anyone repeats this experiment, i would like to hear from you.

 

CAPACITORS

Capacitor Circuit

In an electronic circuit, a capacitor is shown like this:

When you connect a capacitor to a battery, here's what happens:

• The plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing.
• The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery.

Once it's charged, the capacitor has the same voltage as the battery (1.5 volts on the battery means 1.5 volts on the capacitor). For a small capacitor, the capacity is small. But large capacitors can hold quite a bit of charge. You can find capacitors as big as soda cans that hold enough charge to light a flashlight bulb for a minute or more.

Even nature shows the capacitor at work in the form of lightning. One plate is the cloud, the other plate is the ground and the lightning is the charge releasing between these two "plates." Obviously, in a capacitor that large, you can hold a huge amount of charge!

Let's say you hook up a capacitor like this:

Here you have a battery, a light bulb and a capacitor. If the capacitor is pretty big, what you will notice is that, when you connect the battery, the light bulb will light up as current flows from the battery to the capacitor to charge it up. The bulb will get progressively dimmer and finally go out once the capacitor reaches its capacity. If you then remove the battery and replace it with a wire, current will flow from one plate of the capacitor to the other. The bulb will light initially and then dim as the capacitor discharges, until it is completely out.

 

Like a Water Tower

One way to visualize the action of a capacitor is to imagine it as a water tower hooked to a pipe. A water tower "stores" water pressure -- when the water system pumps produce more water than a town needs, the excess is stored in the water tower. Then, at times of high demand, the excess water flows out of the tower to keep the pressure up. A capacitor stores electrons in the same way and can then release them later

 

Farad

A capacitor's storage potential, or capacitance, is measured in units called farads. A 1-farad capacitor can store one coulomb (coo-lomb) of charge at 1 volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons. One amp represents a rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 amp-second of electrons at 1 volt.

A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda bottle, depending on the voltage it can handle. For this reason, capacitors are typically measured in microfarads (millionths of a farad).

To get some perspective on how big a farad is, think about this:

• A standard alkaline AA battery holds about 2.8 amp-hours.
• That means that a AA battery can produce 2.8 amps for an hour at 1.5 volts (about 4.2 watt-hours -- a AA battery can light a 4-watt bulb for a little more than an hour).
• Let's call it 1 volt to make the math easier. To store one AA battery's energy in a capacitor, you would need 3,600 * 2.8 = 10,080 farads to hold it, because an amp-hour is 3,600 amp-seconds.

If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! Obviously, it's impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.

Applications
The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge. That's why the electronic flash on a camera uses a capacitor -- the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. This can make a large, charged capacitor extremely dangerous -- flash units and TVs have warnings about opening them up for this reason. They contain big capacitors that can, potentially, kill you with the charge they contain.

Capacitors are used in several different ways in electronic circuits:

• Sometimes, capacitors are used to store charge for high-speed use. That's what a flash does. Big lasers use this technique as well to get very bright, instantaneous flashes.
• Capacitors can also eliminate ripples. If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the valleys.
• A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges. However, any alternating current (AC) signal flows through a capacitor unimpeded. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing.

 

A family of capacitors

Capacitive Touch Screens

One of the more futuristic applications of capacitors is the capacitive touch screen. These are glass screens that have a very thin, transparent metallic coating. A built-in electrode pattern charges the screen so when touched, a current is drawn to the finger and creates a voltage drop. This exact location of the voltage drop is picked up by a controller and transmitted to a computer. These touch screens are commonly found in interactive building directories and more recently in Apple's iPhone.

RESISTORS and RESISTANCE

What do resistors do?

Resistors limit current. In a typical application, a resistor is connected in series with an LED:

Enough current flows to make the LED light up, but not so much that the LED is damaged. Later in this Chapter, you will find out how to calculate a suitable value for this resistor. (LEDs are described in detail in Chapter 5.)

The 'box' symbol for a fixed resistor is popular in the UK and Europe. A 'zig-zag' symbol is used in America and Japan:

Resistors are used with transducers to make sensor subsystems. Transducers are electronic components which convert energy from one form into another, where one of the forms of energy is electrical. A light dependent resistor, or LDR, is an example of an input transducer. Changes in the brightness of the light shining onto the surface of the LDR result in changes in its resistance. As will be explained later, an input transducer is most often connected along with a resistor to to make a circuit called a potential divider. In this case, the output of the potential divider will be a voltage signal which reflects changes in illumination.

Microphones and switches are input transducers. Output transducers include loudspeakers, filament lamps and LEDs. Can you think of other examples of transducers of each type?

In other circuits, resistors are used to direct current flow to particular parts of the circuit, or may be used to determine the voltage gain of an amplifier. Resistors are used with capacitors (Chapter 4) to introduce time delays.

Most electronic circuits require resistors to make them work properly and it is obviously important to find out something about the different types of resistor available, and to be able to choose the correct resistor value, in , , or M, for a particular application.

 

 

Fixed value resistors

The diagram shows the construction of a carbon film resistor:

During manufacture, a thin film of carbon is deposited onto a small ceramic rod. The resistive coating is spiralled away in an automatic machine until the resistance between the two ends of the rod is as close as possible to the correct value. Metal leads and end caps are added, the resistor is covered with an insulating coating and finally painted with coloured bands to indicate the resistor value.

Carbon film resistors are cheap and easily available, with values within ±10% or ±5% of their marked, or 'nominal' value. Metal film and metal oxide resistors are made in a similar way, but can be made more accurately to within ±2% or ±1% of their nominal value. There are some differences in performance between these resistor types, but none which affect their use in simple circuits.

Wirewound resistors are made by winding thin wire onto a ceramic rod. They can be made extremely accurately for use in multimeters, oscilloscopes and other measuring equipment. Some types of wirewound resistors can pass large currents wihtout overheating and are used in power supplies and other high current circuits.

 

 

Current limiting

You are now ready to calculate a value for the resistor used in series with an LED. Look at the circuit diagram:

A typical LED requires a current of 10 mA and has a voltage of 2 V across it when it is working. The power supply for the circuit is 9 V. What is the voltage across resistor R1? The answer is 9-2=7 V. (The voltages across components in series must add up to the power supply voltage.)

You now have two bits of information about R1: the current flowing is 10 mA, and the voltage across R1 is 7 V. To calculate the resistance value, use the formula:

Substitute values for V and I:

Look out! The formula works with the fundamental units of resistance, voltage and current, that is, ohms, volts and amps. In this case, 10 mA had to be converted into amps, 0.01 A, before substitution.

If a value for current in mA is substituted, the resistance value is given in :

The calculated value for R1 is 700 . What are the nearest E12/E24 values? Resistors of 680 , 750 and 820 are available. 680 is the obvious choice. This would allow a current slightly greater than 10 mA to flow. Most LEDs are undamaged by currents of up to 20 mA, so this is fine. What is the colour code for a 680 resistor?

 

Power rating

When current flows through a resistance, electrical energy is converted into heat. This is obvious in an electric torch where the lamp filament heats up and glows white hot. Although the result may be less evident or imperceptible, exactly the same process of energy conversion goes on when current flows through any electronic component.

The power output of a lamp, resistor, or other component, is defined as the rate of change of electrical energy to heat, light, or some other form of energy. Power is measured in watts, W, or milliwatts, mW, and can be calculated from:

where P is power.

What is the power output of a resistor when the voltage across it is 6 V, and the current flowing through it is 100 mA?

0.6 W of heat are generated in this resistor. To prevent overheating, it must be possible for heat to be lost, or dissipated, to the surroundings at the same rate.

A resistor's ability to lose heat depends to a large extent upon its surface area. A small resistor with a limited surface area cannot dissipate (=lose) heat quickly and is likely to overheat if large currents are passed. Larger resistors dissipate heat more effectively.

Look at the diagram below which shows resistors of different sizes:

The standard size of carbon film resistor used in most circuits has a power rating of 0.5 W. This means that a resistor of this size can lose heat at a maximum rate of 0.5 W. In the example above, the calculated rate of heat loss was 0.6 W, so that a resistor with a higher power rating, 1 W or 2 W, would be needed. Some resistors are designed to pass very large currents and are cased in aluminium with fins to increase surface area and promote heat loss.

Input and signal processing subsystems in electronic circuits rarely involve large currents, but power rating should be considered when circuits drive output transducers, such as lamps, LEDs, and loudspeakers.

 

 

Resistors in series and parallel

In a series circuit, the current flowing is the same at all points. The circuit diagram shows two resistors connected in series with a 6 V battery:

Resistors in series

It doesn't matter where in the circuit the current is measured, the result will be the same. The total resistance is given by:

In this circuit, R_total=1+1=2 . What will be the current flowing? The formula is:

Substituting:

Notice that the current value is in mA when the resistor value is substituted in .

The same current, 3 mA, flows through each of the two resistors. What is the voltage across R1? The formula is:

Substituting:

What will be the voltage across R2? This will also be 3 V. It is important to point out that the sum of the voltages across the two resistors is equal to the power supply voltage.

The next circuit shows two resistors connected in parallel to a 6 V battery:

Resistors in parallel

Parallel circuits always provide alternative pathways for current flow. The total resistance is calculated from:

This is called the product over sum formula and works for any two resistors in parallel. An alternative formula is:

This formula can be extended to work for more than two resistors in parallel, but lends itself less easily to mental arithmetic. Both formulae are correct.

What is the total resistance in this circuit?

The current can be calculated from:

How does this current compare with the current for the series circuit? It's more. This is sensible. Connecting resistors in parallel provides alternative pathways and makes it easier for current to flow. How much current flows through each resistor? Because they have equal values, the current divides, with 6 mA flowing through R1, and 6 mA through R2.

To complete the picture, the voltage across R1 can be calculated as:

This is the same as the power supply voltage. The top end of R1 is connected to the positive terminal of the battery, while the bottom end of R1 is connected to the negative terminal of the battery. With no other components in the way, it follows that the voltage across R1 must be 6 V. What is the voltage across R2? By the same reasoning, this is also 6 V.