Half Adder & Full Adder.......

Half Adder

With the help of half adder, we can design circuits that are capable of performing simple addition with the help of logic gates.

Let us first take a look at the addition of single bits.

0+0 = 0

0+1 = 1

1+0 = 1

1+1 = 10

These are the least possible single-bit combinations. But the result for 1+1 is 10. Though this problem can be solved with the help of an EXOR Gate, if you do care about the output, the sum result must be re-written as a 2-bit output.

Thus the above equations can be written as

0+0 = 00

0+1 = 01

1+0 = 01

1+1 = 10

Here the output ‘1’of ‘10’ becomes the carry-out. The result is shown in a truth-table below. ‘SUM’ is the normal output and ‘CARRY’ is the carry-out.

INPUTS                 OUTPUTS

A             B             SUM      CARRY

0              0              0              0

0              1              1              0

1              0              1              0

1              1              0              1

From the equation it is clear that this 1-bit adder can be easily implemented with the help of EXOR Gate for the output ‘SUM’ and an AND Gate for the carry. Take a look at the implementation below.

                       Half Adder Circuit

For complex addition, there may be cases when you have to add two 8-bit bytes together. This can be done only with the help of full-adder logic.

Full Adder

This type of adder is a little more difficult to implement than a half-adder. The main difference between a half-adder and a full-adder is that the full-adder has three inputs and two outputs. The first two inputs are A and B and the third input is an input carry designated as CIN. When a full adder logic is designed we will be able to string eight of them together to create a byte-wide adder and cascade the carry bit from one adder to the next.

The output carry is designated as COUT and the normal output is designated as S. Take a look at the truth-table.

INPUTS                 OUTPUTS

A             B             CIN         COUT    S

0              0              0              0              0

0              0              1              0              1

0              1              0              0              1

0              1              1              1              0

1              0              0              0              1

1              0              1              1              0

1              1              0              1              0

1              1              1              1              1

From the above truth-table, the full adder logic can be implemented. We can see that the output S is an EXOR between the input A and the half-adder SUM output with B and CIN inputs. We must also note that the COUT will only be true if any of the two inputs out of the three are HIGH.

Thus, we can implement a full adder circuit with the help of two half adder circuits. The first will half adder will be used to add A and B to produce a partial Sum. The second half adder logic can be used to add CIN to the Sum produced by the first half adder to get the final S output. If any of the half adder logic produces a carry, there will be an output carry. Thus, COUT will be an OR function of the half-adder Carry outputs. Take a look at the implementation of the full adder circuit shown below.

Full Adder Circuit

Though the implementation of larger logic diagrams is possible with the above full adder logic a simpler symbol is mostly used to represent the operation. Given below is a simpler schematic representation of a one-bit full adder.

Single-bit Full Adder

With this type of symbol, we can add two bits together taking a carry from the next lower order of magnitude, and sending a carry to the next higher order of magnitude. In a computer, for a multi-bit operation, each bit must be represented by a full adder and must be added simultaneously. Thus, to add two 8-bit numbers, you will need 8 full adders which can be formed by cascading two of the 4-bit blocks. The addition of two 4-bit numbers is shown below.

Multi-Bit Addition using Full Adder

স্মৃতিশক্তি বাড়ানোর কৌশল !!

June 5, 2015
মনে থাকে না শব্দটা ছা্ত্রজীবনে বেশ পরিচিত । পড়তে পড়তে ক্লান্ত হয়ে পড়লেও পরীক্ষার হলে ডুকলেই যেন সব ভুলে যাওয়াই কমন! আমরা সব কিছু চাইলেও বাস্তবে সেটা সম্ভব নয়। কিন্তু মনে রাখার পরিমাণ বাড়ানো সম্ভব। এজন্য আপনাকে কিছু নিয়ম কানুন মানতে হবে।

 

 

১. কোনো কিছুকে গুরুত্ব দিয়ে না শুনলে কিংবা না দেখলে তা আপনার মনে থাকবে না। সুতরাং নিজেকে বিষয়টি সম্পর্কে সজাগ এবং সচেতন রেখে গুরুত্ব সহকারে মনোযোগ রাখেন। এই মাত্র যার সঙ্গে আপনার পরিচয় হলো তার নামটি যদি আপনি মনে রাখতে চান তাহলে কয়েকবার নিজে নিজে বলুন । ডাইরিতে লিখে  নিলে তা মনে থাকে ভাল।
২. স্বভাবের দিক দিয়ে গোছানো হলে অনেক কিছু সহজে মনে থাকে। নিত্যপ্রয়োজনীয় জিনিসগুলো  নির্দিষ্ট জায়গায় রাখলে প্রয়োজনের সময় হাত বাড়াতে পাবেন সহজে।এতে করে নিজের প্রতি নিজের বিরক্তি আসবে না,যেটি আপনার স্মৃতি বৃদ্ধিতে সহায়ক।কেননা নিজের প্রতি বিরক্তি আসলে অনেক সহজ জিনিস ভুলে যাবেন অতি সহজেই।
৩. দিন তারিখ মনে রাখার জন্য আমরা ক্যালেন্ডার, ঘড়ি ব্যবহার করছি। ঘুম থেকে ওঠার জন্য এলার্ম ঘড়ি ব্যবহার করছি। আজকাল মোবাইল ফোন এবং অন্যান্য ইলেক্ট্রনিক যন্ত্রে রিমাইন্ডার নামে একটা ব্যবস্থা আছে। যা আপনাকে কাজের কথাটা মনে করিয়ে দেবে। যে জিনিসটা নিয়ে বাইরে যেতে হবে তা দরজার মুখে রাখুন, ওষুধটা বেসিনের উপরে তাকে রাখুন। প্রয়োজনের জিনিস গুলো আগে গুছিয়ে রাখুন, কোন জিনিস ফেলে যাওয়ার সম্ভাবনা থাকবে না।
৪. শরীর সুস্থ থাকলে মন সুস্থ থাকে। তাই নিয়মিত ব্যায়াম করুন, পরিমান মত আহার করুন, সিগারেট থেকে বিরত থাকুন। কানে শুনতে, চোখে দেখতে অসুবিধা থাকলে ডাক্তার দেখিয়ে প্রয়োজনীয় ব্যবস্থা নিবেন, এতে অন্য জন কি বলছে কি করছে আপনি আর মিস করবেন না। ঘুমের বড়ি না খাওয়া ভাল। আপনি আরো বেশি এ্যালার্ট থাকবেন তাহলে।
৫. নিয়মিত স্বাস্থ্য পরীক্ষা করানোর ব্যাপারটা আমাদের অভ্যাসের বাইরে। ফলে রোগ গভীর না হওয়া পর্যন্ত আমরা তার উপস্থিতি টের পাই না। প্রত্যেকের উচিত প্রতি বছর ডাক্তারকে দিয়ে স্বাস্থ্য পরীক্ষা করানো। এতে উচ্চ রক্তচাপ, ডায়াবেটিস, চোখের এবং কানের সমস্যা সহজে ধরা পরবে।
৬. শারীরিক কাজ না করলে শরীর দুর্বল হয়ে যায়। কেউ যদি কয়েক সপ্তাহ বিছানায় শুধু শুয়ে থাকে তার পা সরু হয়ে যাবে, মাংস পেশীতে টান পড়বে, হাঁটতে অসুবিধা হবে। তেমনি কার্য ক্ষমতা কমে যায়। সুশিক্ষিত বুদ্ধিমান ব্যক্তি যারা নিয়মিত বুদ্ধির চর্চা করেন বয়সের সঙ্গে সঙ্গে তাঁদের স্মৃতি হ্রাস তুলনা মূলক ভাবে কম হয়। তাই ভাল স্মৃতিশক্তি পেতে চাইলে নিয়মিত পড়ুন, শিখুন ও সমস্যা সমাধানে অংশগ্রহণ করুন। অলস ভাবে বসে থাকলে স্মৃতিশক্তি কখনও বাড়বে না।
৭. মনে রাখার কিছু চমৎকার কৌশল আছে। ইংরেজিতে লেফটেন্যান্ট বানানটা মনে রাখা আপনার জন্য খুব কষ্টকর হলে মনে রাখুন। মিথ্যা তুমি দশ পিঁপড়া শব্দ গুলোর ইংরেজি লিখে নেন। বানান পেয়ে গেলেন, মনে রাখার জন্য কল্পনা শক্তির ব্যবহার খুব জরুরি। যে কল্পনা শক্তিকে যত চমৎকার ভাবে ব্যবহার করতে পারবে তার মনে থাকবে তত বেশি।অথবা আপনার কাছে যে জিনিস গুলো মনে রাখা কষ্টকর হয়ে দাঁড়ায় সেগুলো আপনার প্রিয় জিনিসের সাথে তুলনা করে মনে রাখার চেষ্টা করুন।
৮.অধিকাংশ মানুষ যারা নিজেদের স্মরণশক্তি নিয়ে দুশ্চিন্তা গ্রস্ত নন তারা সব কিছু মনে রাখতে পারে তা নয়। যুবকদের মধ্যে যারা ভুলে যায় তারা হয়ত অজুহাত দেখায়। প্রেমে পরেছি মন অন্য দিকে নেই। পড়াশোনা নিয়ে খুব ব্যাস্ত ইত্যাদি ইত্যাদি। বয়স্করা ভাবেন আমার কি স্মৃতিভ্রংশ দেখা দিল? আসলে এরা সবাই স্বাভাবিক। মানুষ একটু আধটু ভুলবে। যতক্ষণ পর্যন্ত এটা সাধারণ কাজকর্মে ব্যাঘাত না ঘটাচ্ছে ততক্ষণ দুশ্চিন্তার কিছু নেই।সত্যি যদি মনে ভুলে যাওয়া ব্যাপারটা আপনাকে বেশ ভোগায় তাহলে ডাক্তারের পরামর্শ নিন। সে ক্ষেত্রে যত দ্রুত চিকিৎসা শুরু করবেন তত ভাল।

Digital Bangladesh and ICT development

Key ICT Progresses in Last Five Years

ICT is the backbone of any digital initiative. ICT covers the vast area of information technology, communication technology and the telecommunication technology. ICT is also a combination of physical backbone and intellect. Computer systems, network machineries, software, wire and wireless connectivity systems, broadcast hardware and many other hardware and accessories are the physical backbone. The trained human behind the backbone are the intellect. Digital Bangladesh is an Idea that includes the IT use for management, administration and governance to ensure transparency, accountability and answerability at all levels of society and state. To materialize the idea of digital Bangladesh, development of countrywide backbone and expected number of human recourses are the basic needs.

Despite having 50 years of history the government has only from 1997 started the process of developing a national ICT strategy. In 2002 Bangladesh identified ICT as a "thrust sector" as it represents potential for quick wins in reforms, job creation, industry growth, improving governance and facilitating inclusion, and it has high spillover effects to other sectors. Today, in Bangladesh, the overall IT sector (excluding telecoms) is small, valued at $300 million, with IT/ITES claiming 39% ($117 million) of that value. The overall IT/ITES industry has enjoyed a high growth rate of 40% over the last five (5) years and this trend is expected to continue.

 

• ORGANIZED BY

• 
• 

• 

• CONTACT

  Digital World Secretariat
  BDBL Bhaban (5th Floor - West),
  12 Kawran Bazar, Dhaka -1215
  Phone: +880 096-1234-DW15, Fax: +880 2 8151197
  Email: secretariat@digitalworld.org.bd

Digital Bangladesh as an engine for economic growth

Can tech powerhouses emerge from Bangladesh?

 

Technology also contributes to growth by increasing competition. It is estimated that in the US, online goods are on average 10% cheaper than those sold by traditional bricks and mortar stores. This is partly because of increased price transparency and the ability of consumers to use online price comparison tools to find the cheapest products. Secondly the Internet e-commerce platforms, most famously Amazon, offer massive economies of scale.

Ebay auction sites were also a disruptive technology in retailing space by making the sale of used products and new goods much easier, again putting downwards pressure on prices.

So Internet markets are argued to be more efficient due to the ease of information transmission, which leads to lower transaction costs. Different pricing and selling mechanisms, such as online auctions, also contribute to increased efficiency.

We have already seen a rapid growth of e-commerce in Bangladesh with the Digital Bangladesh initiative continuing to encourage the expansion of this sector. Recent initiatives by Bangladesh Bank to allow e-payment facilities online, using credit cards such as by enabling Paypal in Bangladesh, will help the growth of the sector even further. In a country like Bangladesh, where supply chains are not that efficient, ICT will reduce the scope for price distortions and manipulation by unscrupulous middlemen to ensure that consumers receive fair prices.

To the extent that we accept that the effective utilisation of technology can have a transformative impact on the economy, it is important to determine what steps policymakers should take in Bangladesh. Firstly, they should ensure world class digital infrastructure and networks at competitive prices. In this area, the Bangladesh government has made major progress in recent years with the dramatic fall in broadband prices, the rapid rollout of 3G services, and the prospects for fast introduction in the near future of 4G and LTE.

Secondly, the government can encourage the development of a strong local IT ecosystem. Thirdly, expanding the IT skills base is critical with more vocational colleges to increase the supply of programmers and IT management and marketing professionals. Effective use of ICT can also play a key role in female empowerment by bringing more women into the workplace and allowing them greater flexibility in working from home/working hours that enables greater balance with family commitments.

It is also important that the government ensures a stable and supportive regulatory framework both in terms of avoiding regulatory volatility and inconsistent or negative taxation policies for the IT sector.

The government has committed to establishing 12 new technology parks in Bangladesh, with the first being the 238 acre park in Kaliakoir near Dhaka. But there are some other encouraging initiatives from the private sector in the pipeline. Sajeeb Wazeb Joy, ICT Advisor to the Prime Minister, noted in his speech at the Digital World 2015 that: “One of the problems is financing. In America, new innovations don’t get support from the government. They get their funding from the private sector.”

In this context, the announcement in November that Fenox, a Silicon Valley venture capital firm, was raising a $200m fund for Bangladesh IT, is especially encouraging. Kyle King from Fenox noted:

“Bangladesh has a large, young population, an outstanding Internet and mobile growth with an unexplored entrepreneurial system which all make Bangladesh a place for innovation, discovery, change, disruption, creation, and investment … For Fenox, Bangladesh qualifies as the right country to be a part of in terms of developing the world’s most influential startups.”

Sajeeb Wazed Joy also stated: “I want companies like Google and Facebook to emerge from Bangladesh.”

On the face that may sound like wishful thinking. But in fact technology is levelling the competitive playing field so fast that in our generation it is definitely conceivable, perhaps even likely, that a boy from a Bangladeshi village has the potential to create a company with a disruptive technology as powerful as Google or Facebook.

To paraphrase NY Times columnist Thomas Friedman. Technology is the primary driver of such “flattening” of the global economic landscape.

In conclusion, we believe that ICT is every bit as important as energy and infrastructure investment in raising the trend growth rate. Effective implementation of Digital Bangladesh may yet prove to be the single most important weapon in poverty alleviation allowing Bangladesh to achieve its potential as the fastest growing economy in Asia

Digital World 2015, the recently concluded four-day conference, was the fourth largest ICT event in the world, with 120 private companies and 100 governmental organisations from 25 countries. It also featured representatives from leading technology companies, most notably Google, which sent a broad range of executives, as well as Facebook and Microsoft, among others.
While the primary focus of the event was to highlight the advances of the “Digital Bangladesh” initiative of the current government as well as potential growth opportunities in the ICT sector, in this long form we want to discuss some of the broader potentials of technology as a catalyst for economic growth
- See more at: http://www.dhakatribune.com/long-form/2015/feb/25/digital-bangladesh-engine-economic-growth#sthash.1EyFe3OG.dpuf

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Can tech powerhouses emerge from Bangladesh?

Digital Bangladesh as an engine for economic growth - See more at: http://www.dhakatribune.com/long-form/2015/feb/25/digital-bangladesh-engine-economic-growth#sthash.1EyFe3OG.dpuf

Digital Bangladesh as an engine for economic growth - See more at: http://www.dhakatribune.com/long-form/2015/feb/25/digital-bangladesh-engine-economic-growth#sthash.1EyFe3OG.dpuf

Brushless Outrunner Motor

Brushless Outrunner Motor:::

 ................................ Max thrust: 1650gm ...............................
Turnigy D3548/6 790KV Brushless Outrunner Motor
BDT ৳ 3,000.00
Battery: 3~5 Cell /11.1~18.5V
RPM: 790kv
Max current: 40A
No load current: 1.8A
Max power: 715W
Internal resistance: 0.040 ohm
Weight: 159g (including connectors)
Diameter of shaft: 5mm
Dimensions: 35x48m
Prop size: 11.1V/12×12 18.5V/11×7
Max thrust: 1650gm

Transformer -- power supply

PARTS AND MATERIALS

• Power transformer, 120VAC step-down to 12VAC, with center-tapped secondary winding (Radio Shack catalog # 273-1365, 273-1352, or 273-1511).
• Terminal strip with at least three terminals.
• Household wall-socket power plug and cord.
• Line cord switch.
• Box (optional).
• Fuse and fuse holder (optional).

Power transformers may be obtained from old radios, which can usually be obtained from a thrift store for a few dollars (or less!). The radio would also provide the power cord and plug necessary for this project. Line cord switches may be obtained from a hardware store. If you want to be absolutely sure what kind of transformer you're getting, though, you should purchase one from an electronics supply store.
If you decide to equip your power supply with a fuse, be sure to get a slow-acting, or slow-blow fuse. Transformers may draw high "surge" currents when initially connected to an AC source, and these transient currents will blow a fast-acting fuse. Determine the proper current rating of the fuse by dividing the transformer's "VA" rating by 120 volts: in other words, calculate the full allowable primary winding current and size the fuse accordingly.


CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 1: "Basic AC Theory"
Lessons In Electric Circuits, Volume 2, chapter 9: "Transformers"


LEARNING OBJECTIVES

• Transformer voltage step-down behavior.
• Purpose of tapped windings.
• Safe wiring techniques for power cords.

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS
 
Warning! This project involves the use of dangerous voltages. You must make sure all high-voltage (120 volt household power) conductors are safely insulated from accidental contact. No bare wires should be seen anywhere on the "primary" side of the transformer circuit. Be sure to solder all wire connections so that they're secure, and use real electrical tape (not duct tape, scotch tape, packing tape, or any other kind!) to insulate your soldered connections.
If you wish to enclose the transformer inside of a box, you may use an electrical "junction" box, obtained from a hardware store or electrical supply house. If the enclosure used is metal rather than plastic, a three-prong plug should be used, with the "ground" prong (the longest one on the plug) connected directly to the metal case for maximum safety.
Before plugging the plug into a wall socket, do a safety check with an ohmmeter. With the line switch in the "on" position, measure resistance between either plug prong and the transformer case. There should be infinite (maximum) resistance. If the meter registers continuity (some resistance value less than infinity), then you have a "short" between one of the power conductors and the case, which is dangerous!
Next, check the transformer windings themselves for continuity. With the line switch in the "on" position, there should be a small amount of resistance between the two plug prongs. When the switch is turned "off," the resistance indication should increase to infinity (open circuit -- no continuity). Measure resistance between pairs of wires on the secondary side. These secondary windings should register much lower resistances than the primary. Why is this?
Plug the cord into a wall socket and turn the switch on. You should be able to measure AC voltage at the secondary side of the transformer, between pairs of terminals. Between two of these terminals, you should measure about 12 volts. Between either of these two terminals and the third terminal, you should measure half that. This third wire is the "center-tap" wire of the secondary winding.
It would be advisable to keep this project assembled for use in powering other experiments shown in this book. From here on, I will designate this "low-voltage AC power supply" using this illustration:

COMPUTER SIMULATION
Schematic with SPICE node numbers:

Netlist (make a text file containing the following text, verbatim):
transformer with center-tap secondary
v1 1 0 ac 120 sin      
rbogus1 1 2 1e-3
l1 2 0 10
l2 5 4 0.025
l3 4 3 0.025
k1 l1 l2 0.999
k2 l2 l3 0.999
k3 l1 l3 0.999
rbogus2 3 0 1e12
rload1 5 4 1k
rload2 4 3 1k

* Sets up AC analysis at 60 Hz:
.ac lin 1 60 60

* Prints primary voltage between nodes 2 and 0:
.print ac v(2,0)

* Prints (top) secondary voltage between nodes 5 and 4:
.print ac v(5,4)

* Prints (bottom) secondary voltage between nodes 4 and 3:
.print ac v(4,3)

* Prints (total) secondary voltage between nodes 5 and 3:
.print ac v(5,3)
.end

 

 

Sensitive audio detector

PARTS AND MATERIALS

• High-quality "closed-cup" audio headphones
• Headphone jack: female receptacle for headphone plug (Radio Shack catalog # 274-312)
• Small step-down power transformer (Radio Shack catalog # 273-1365 or equivalent, using the 6-volt secondary winding tap)
• Two 1N4001 rectifying diodes (Radio Shack catalog # 276-1101)
• 1 kΩ resistor
• 100 kΩ potentiometer (Radio Shack catalog # 271-092)
• Two "banana" jack style binding posts, or other terminal hardware, for connection to potentiometer circuit (Radio Shack catalog # 274-662 or equivalent)
• Plastic or metal mounting box

Regarding the headphones, the higher the "sensitivity" rating in decibels (dB), the better, but listening is believing: if you're serious about building a detector with maximum sensitivity for small electrical signals, you should try a few different headphone models at a high-quality audio store and "listen" for which ones produce an audible sound for the lowest volume setting on a radio or CD player. Beware, as you could spend hundreds of dollars on a pair of headphones to get the absolute best sensitivity! Take heart, though: I've used an old pair of Radio Shack "Realistic" brand headphones with perfectly adequate results, so you don't need to buy the best.
Normally, the transformer used in this type of application (audio speaker impedance matching) is called an "audio transformer," with its primary and secondary windings represented by impedance values (1000 Ω : 8 Ω) instead of voltages. An audio transformer will work, but I've found small step-down power transformers of 120/6 volt ratio to be perfectly adequate for the task, cheaper (especially when taken from an old thrift-store alarm clock radio), and far more rugged.
The tolerance (precision) rating for the 1 kΩ resistor is irrelevant. The 100 kΩ potentiometer is a recommended option for incorporation into this project, as it gives the user control over the loudness for any given signal. Even though an audio-taper potentiometer would be appropriate for this application, it is not necessary. A linear-taper potentiometer works quite well.


CROSS-REFERENCES
Lessons In Electric Circuits, Volume 1, chapter 8: "DC Metering Circuits"
Lessons In Electric Circuits, Volume 2, chapter 9: "Transformers"
Lessons In Electric Circuits, Volume 2, chapter 12: "AC Metering Circuits"


LEARNING OBJECTIVES

• Soldering practice
• Use of a transformer for impedance matching
• Detection of extremely small electrical signals
• Using diodes to "clip" voltage at some maximum level

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS
This experiment is identical in construction to the "Sensitive Voltage Detector" described in the DC experiments chapter. If you've already built this detector, you may skip this experiment.
The headphones, most likely being stereo units (separate left and right speakers) will have a three-contact plug. You will be connecting to only two of those three contact points. If you only have a "mono" headphone set with a two-contact plug, just connect to those two contact points. You may either connect the two stereo speakers in series or in parallel. I've found the series connection to work best, that is, to produce the most sound from a small signal:

Solder all wire connections well. This detector system is extremely sensitive, and any loose wire connections in the circuit will add unwanted noise to the sounds produced by the measured voltage signal. The two diodes connected in parallel with the transformer's primary winding, along with the series-connected 1 kΩ resistor, work together to "clip" the input voltage to a maximum of about 0.7 volts. This does one thing and one thing only: limit the amount of sound the headphones can produce. The system will work without the diodes and resistor in place, but there will be no limit to sound volume in the circuit, and the resulting sound caused by accidentally connecting the test leads across a substantial voltage source (like a battery) can be deafening!
Binding posts provide points of connection for a pair of test probes with banana-style plugs, once the detector components are mounted inside a box. You may use ordinary multimeter probes, or make your own probes with alligator clips at the ends for secure connection to a circuit.
Detectors are intended to be used for balancing bridge measurement circuits, potentiometric (null-balance) voltmeter circuits, and detect extremely low-amplitude AC ("alternating current") signals in the audio frequency range. It is a valuable piece of test equipment, especially for the low-budget experimenter without an oscilloscope. It is also valuable in that it allows you to use a different bodily sense in interpreting the behavior of a circuit.
For connection across any non-trivial source of voltage (1 volt and greater), the detector's extremely high sensitivity should be attenuated. This may be accomplished by connecting a voltage divider to the "front" of the circuit:
SCHEMATIC DIAGRAM

ILLUSTRATION

Adjust the 100 kΩ voltage divider potentiometer to about mid-range when initially sensing a voltage signal of unknown magnitude. If the sound is too loud, turn the potentiometer down and try again. If too soft, turn it up and try again. This detector even senses DC and radio-frequency signals (frequencies below and above the audio range, respectively), a "click" being heard whenever the test leads make or break contact with the source under test. With my cheap headphones, I've been able to detect currents of less than 1/10 of a microamp (< 0.1 µA) DC, and similarly low-magnitude RF signals up to 2 MHz.
A good demonstration of the detector's sensitivity is to touch both test leads to the end of your tongue, with the sensitivity adjustment set to maximum. The voltage produced by metal-to-electrolyte contact (called galvanic voltage) is very small, but enough to produce soft "clicking" sounds every time the leads make and break contact on the wet skin of your tongue.
Try unplugging the headphone plug from the jack (receptacle) and similarly touching it to the end of your tongue. You should still hear soft clicking sounds, but they will be much smaller in amplitude. Headphone speakers are "low impedance" devices: they require low voltage and "high" current to deliver substantial sound power. Impedance is a measure of opposition to any and all forms of electric current, including alternating current (AC). Resistance, by comparison, is a strictly measure of opposition to direct current (DC). Like resistance, impedance is measured in the unit of the Ohm (Ω), but it is symbolized in equations by the capital letter "Z" rather than the capital letter "R". We use the term "impedance" to describe the headphone's opposition to current because it is primarily AC signals that headphones are normally subjected to, not DC.
Most small signal sources have high internal impedances, some much higher than the nominal 8 Ω of the headphone speakers. This is a technical way of saying that they are incapable of supplying substantial amounts of current. As the Maximum Power Transfer Theorem predicts, maximum sound power will be delivered by the headphone speakers when their impedance is "matched" to the impedance of the voltage source. The transformer does this. The transformer also helps aid the detection of small DC signals by producing inductive "kickback" every time the test lead circuit is broken, thus "amplifying" the signal by magnetically storing up electrical energy and suddenly releasing it to the headphone speakers.
As with the low-voltage AC power supply experiment, I recommend building this detector in a permanent fashion (mounting all components inside of a box, and providing nice test lead wires) so it can be easily used in the future. Constructed as such, it might look something like this: