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A Light Dependent Resistor ( LDR, photoconductor, or photocell) is a device which has a resistance which varies according to the amount of light falling on its surface. They will be having a resistance of 1 MOhm in total darkness, and a resistance of a 1 to 10 of kOhm in bright light. A photoelectric device can be either intrinsic or extrinsic. Applications: An LDR can even be used in a simple remote control circuit using the backlight of a mobile phone to turn on a device - call the mobile from anywhere in the world, it lights up the LDR, and lighting can be turned on remotely! Light Dependent Resistor Circuits: There are two basic circuits using light dependent resistors - the first is activated by darkness, the second is activated by light. In the circuit diagram on the left , the led lights up whenever the LDR is in darkness . The 10K variable resistor is used to fine-tune the level of darkness required before the LED lights up. The 10K standard resistor can be changed as require...

Figure shows the connection diagram ! its a 311 opamp with 4 th terminal gnded R2 is an LDR whose dark resistance is greater than 10kohm A laser torch can be used to power the ckt which is directed at the LDR ! when the light is falling on the LDR its resistance will be less than 10k ohm,then the voltage drop across the resistance will be less ! hence the o/p voltage of the opamp will be 0.When the light is interrupted the resistance of the LDR increases hence droping more voltage ,and the o/p will be approximately supply voltage .The o/p can be connected across a buzzer and can be connected to a relay if the alarm ,we want is of non stoping .ie ,it can be turned of only by another process not by turning the light again. the pin out of the 311 However the sytem provides no other extra security! becoz if someone shows a light on the ldr and intrudes the laser then the system may not be able to realise wat happened !So we must use some extra security tecniques ! Those includes 1)f...

CW_Player click on the image for a better view This freeware program generates the standard Morse caracters, Q codes, abbreviations, random series of characters and text files. It controls your knowledge through a Quiz test and helps you to get a good keying by monitoring your CW signals. Keying off air and verifying your transmission on the screen will let you go on air with less stress and more confidence. It generates Wave sound files (*.wav) of morse characters that can be played by any media software. Local characters can be created by yourself. 20 lessons are proposed to learn the Morse code. You can modify their contents as you like. MIRROR:click on the fig below to download cw_player :

A quantum computer is based on the amazing properties of quantum systems. In these a quantum bit, also known as a qubit, exists in two states at the same time and the information from two qubits is entangled in a way that has no equivalent whatsoever in the normal world. David Deutsch, Oxford University theoretical physicist, is credited with being the father of quantum computing. Deutsch invented the idea of the quantum computer in the 1970s as a way to experimentally test the "Many Universes Theory" of quantum physics -- the idea that when a particle changes, it changes into all possible forms, across multiple universes. In the classical model of a computer the most fundamental building block, the bit, can only exist in one of two distinct states, a '0' or a '1'. In a quantum computer the rules are changed. Not only can a 'quantum bit', usually referred to as a 'qubit', exist in the classical '0' and '1' states, but it can als...

Knife and guns appear visible through the t-ray technology becoz its sharp edges scatter the radiation. And yet, unlike X-rays, T-ray screeners could be used routinely on people, because the radiation is harmless. But some terahertz imagers have another ability, one not even claimed by the comic-book specs: not only can they see hidden objects, but they can tell what those objects are made of. Many explosives, including all the plastic explosives popular with terrorist groups, reflect and transmit a characteristic combination of terahertz waves that make them distinguishable from other materials, even those that might seem identical to the eye and hand. They have “standoff capability,” meaning they can see a few meters away, a very desirable feature in the security business.In just two or three years, versions that can see at a distance of tens of meters should be available, which would be a great safety boon to security personnel. There are lots of uses outside of the security arena f...

Well ,there is a great difference between these two words but may feel same practically! Once my friend Manu thomas was completing his excel project ,perhaps violated some ckt connections .Anyway a "man" call for me had been made .At that time we were completing our project on robotics . Stunned looking at the result! The project included a laser based voice transmitter and receiver . When he had removed the laser somewhat due to some power supply problems related to the laser ,becoz he used the laser torch which is available in market for just 30.00RS at marine drive , and instead of battery ,he used the 3v from the transmitter , connected only for the demonstration .Anyway demonstration completed successfully.The transmission quality was great with the help of laser .When the wire connecting the laser to the transmitter broken it was found that there was a slight change in quality of the sound transmitted,instead of vanishing of received signal.(he used mobile p...

1.Human beings have ears as their sound sensors .Actually they are filters designed for the audio frequencies.Their frequency response is similar to an LC filter with a specific bandwidth. 2.Eyes are filters designed to filter the light frequencies . 3.Skin is a touch sensor . 4.Nose a biological proximity detector. 5.Brain" a biological processor". 6.A great programme running on the brain which teaches it what to do & when.

This device is just another mixer - used for demodulating a signal in a receiver. The term "product" refers to the multiplication of the two input signals - with sum and difference outputs. In the product detector the product signal ie, the dual sideband or single side band signal is again multiplied with the local carrier . NOTE In case of suppressed carrier systems (dont confuse carrier fully suppressed) there is a need to get the carrier at the receiver at same frequency as that of the transmitter and the phase must be same.Hence we need highly stable oscillators free from drift.Such a process called synchronization must be done at the receiver ,which makes the receiver complex.For these reasons a pilot carrier system is preferred ,where the carrier is generated by the LO referring to the pilot carrier. Receiver blockdiagram is available here In the case of AM signals a synchronous demodulator is preferrable because it offers more stability to the s/n ratio.In the synchro...

A mixer circuit normally has two inputs - from two separate signal sources. In the diagram below, the sources are two oscillators. Each oscillator is a generator producing a sinewave output, one at frequency f1 and the other at frequency f2. The mixer multiplies the signals together,while the balanced modulator is a combination of 2 mixers. Remember that the output comprises a complex mixture of separate sinewaves at many different frequencies beco'z of the square law equation of the current . The main point to note is that the output comprises the two separate input frequencies f 1 and f 2 and their sum, (f 1 + f2), and their difference, (f 1 - f2). In practice, there are other component signals too - but we can ignore those. A filter - which can be any one of various sorts - selects the required output from the mixer. In this diagram, a simple parallel tuned circuit is shown. The output will normally be tuned to the SUM, (f 1 + f2), or tuned to the DIFFERENCE, (f 1 - f2), signa...

Amplitude modulator can be classified as 1.Linear modulators a)collector modulator b)base modulator 2.square law modulators a)Square law diode modulator b)square law balanced modulator or simply balanced modulator 1. Linear modulators a) collector modulator collector modulator is a modulator in which the AF signal is applied at the collector of the BJT &RF signal is applied at the base .(high frequency BJT). b) base modulators In base modulator the RF & AF signal is applied at the base of the BJT . 2. square law modulators a) square law diode modulators In sauare law diode modulators the modulated current follows the square law I p =a 1 e s +a 2 e s 2 ........ e s =e m +e c where I p is the current through the diode e s the sum of the modulating and carrier voltages b) balanced modulator In balanced modulator the product of the input frequencies is produced along with other frequency components .Simply a balanced modulator is a combination of 2 mixers DIFFERENT MODULATION ...

*TIME DOMAIN REPRESENTATION It is the representation with signal amplitude on the vertical axis and with time on the horizontal axis - (an oscilloscope diagram). This is sometimes referred to as the "time domain representation". *FREQUENCY DOMAIN REPRESENTATION Its the representation with signal amplitude on the vertical axis but with frequency on horizontal axis - a Spectrum Diagram. This is sometimes referred to as the "frequency domain".

The technique to impress information (voice, music, picture, or data) on a radio-frequency carrier wave by varying one or more characteristics of the wave in accordance with the intelligence signal is called modulation. There are various forms of modulation, each designed to alter a particular characteristic of the carrier wave. For our study purposes, the most commonly altered characteristics are amplitude and frequency. There are mainly 3 modulation schemes 1.Amplitude modulation 1.Amplitude modulation block 2.frequency modulation 2.frequency modulation block 3.phase modulation

Measurement of the capacity of a communications signal. For digital signals, the bandwidth is the data speed or rate, measured in bits per second (bps). For analog signals, it is the difference between the highest and lowest frequency components, measured in hertz (cycles per second). for better results click here Brainwidth is analagous to bandwidth but used strictly for human capacity to process information and especially to multitask.