Project Report

February 25, 2018 | Author: vkkhoti | Category: Field Effect Transistor, Capacitor, Diode, Transistor, Resistor
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minor project report...




Radio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. Information is carried by systematically changing (modulating) some property of the radiated waves, such as amplitude, frequency, or phase. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information. FM broadcast radio sends music and voice with higher fidelity than AM radio. In frequency modulation, amplitude variation at the microphone causes the transmitter frequency to fluctuate. Because the audio signal modulates the frequency and not the amplitude, an FM signal is not subject to static and interference in the same way as AM signals. Due to its need for a wider bandwidth, FM is transmitted in the Very High Frequency (VHF, 30 MHz to 300 MHz) radio spectrum. VHF radio waves act more like light, traveling in straight lines; hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in Long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.



WORKING Because this is a super regenerative design, component layout can be very important. The tuning capacitor, C3, has three leads. Only the outer two leads are used; the middle lead of C3 is not connected. Arrange L1 fairly close to C3, but keep it away from where your hand will be. If your hand is too close to L1 while you tune the radio, it will make tuning very difficult. L1 sets the frequency of the radio, acts as the antenna, and is the primary adjustment for super-regeneration. Although it has many important jobs, it is easy to construct. Get any cylindrical object that is just under 1/2 inch (13 mm) in diameter. I used a thick pencil from my son's grade school class, but a magic marker or large drill bit work just fine. #20 bare solid wire works the best, but any wire that holds its shape will do. Wind 6 turns tightly, side-by-side, on the cylinder, then slip the wire off. Spread the windings apart from each other so the whole coil is just under an inch (2.5 cm) long. Find the midpoint and solder a small wire for C2 there. Mount the ends of the wire on your circuit board keeping some clearance between the coil and the circuit board. C3 does not come with a knob and I have not found a source. A knob is important to keep your hand away from the capacitor and coil when you tune in stations. The solution is to use a #4 nylon screw. Twist the nylon screw into the threads of the C3 tuning handle. The #4 screw is the wrong thread pitch and will jam (bind) in the threads. This is what you want to happen. Tighten the screw just enough so it stays put as you tune the capacitor. The resulting arrangement works quite well. If the radio is wired correctly, there are three possible things you can hear when you turn it on: 1) a radio station, 2) a rushing noise, 3) a squeal, and 4) nothing. If you got a radio station, you are in good shape. Use another FM radio to see where you are on the FM band. You can change the tuning range of C3 by squeezing L1 or change C1. If you hear a rushing noise, you will probably be able to tune in a station. Try the tuning control and see what you get. If you hear a squeal or hear nothing, then the circuit is oscillating too little or too much. Try spreading or compressing L1. Double check your connections. If you don't make any progress, then you need to change R4. Replace R4 with a 20K or larger potentiometer (up to 50K). A trimmer potentiometer is best. Adjust R4 until you can reliably tune in stations. Once the circuit is working, you can remove the potentiometer, measure its value, and replace it with a fixed resistor. Some people might want to build the set from the start with a trimmer potentiometer in place (e.g., Mouser 569-72PM25K).



Many of the parts are fairly common and might already be in your junk box. Only certain component values are critical. The RF choke should be in the range of 20 to 30 uh, although values from15 to 40 uh might work. The tuning capacitor value is not critical, but if you use values below 50 pf you should reduce or remove C1. The circuit is designed for the high impedance type earphone. Normal earphones can be used, but the battery drain is much greater and the circuit must be changed. To use normal earphones, change R3 to 180 ohms. Q1 can be replace with any high-frequency N-channel JFET transistor, but only the 2N4416, 2N4416A, and J310 have been tested. A MPF102 probably will work. C2 is not too critical; any value from 18 to 27 pf will work. C7 is fairly critical. You can use a .005 or .0047 uf, but don't change it much more than that.

Super regenerative detectors The super regenerative detector is an amazing circuit invented by Armstrong that uses a super regenerative amplifier. Understanding a super regenerative amplifier is a bit involved. There are two basic types, self-quenched and externally-quenched. Don't worry about the terminology; I simply want to state that this discussion is for the self-quenched type. Here is how you make a super regenerative circuit. Choose a regenerative amplifier circuit configuration that requires more current during oscillations than when not oscillating. Adjust the regenerative amplifier to oscillate. Add a small circuit that uses the current of the amplifier to charge a capacitor while oscillations are taking place. Once the capacitor is charged, the voltage on the capacitor is used to kill the oscillations of the circuit. (Technically, the capacitor voltage shifts the operating point of the amplifier to reduce its gain and stop the oscillations.) When the oscillations stop, the capacitor discharges (through a resistor). Once the capacitor is discharged, the oscillations begin again. A super regenerative amplifier thus oscillates at two frequency. It oscillates at the tuned frequency of the amplifier and it has this secondary stop-start-stopstart oscillation. If the primary oscillation is at 100 MHz, the secondary should be at about 30 KHz for optimal performance.. (Please email me if you have detailed technical information on this frequency relationship.) Note that the 30 KHz is above audio frequencies, so the listener does not hear any noise associated with either oscillation. The super regenerative technique does not work well at lower radio frequencies because the optimal frequency of the secondary oscillation ends up in the audio frequency range.



Frequency modulation In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequencyshift keying. FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrow band form is used for voice communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space.

An example of frequency modulation. This diagram shows the modulating, or message, signal, xm(t), superimposed on the carrier wave, xc(t)

The modulated signal, y(t), produced from frequency-modulating xc(t) with xm(t).



COMPONENTS C1a,C1b C2 C3 C4 C5,C8 C6 C7 C9 D1 EPH1 L2 Q1 R1 R2, R3 R4 R5 R6 S1 screws for C3 nylon screw battery connector

10 pf, 50 v, ceramic disc capacitor 22 pf, 50 v, ceramic disc capacitor RF tuning capacitor 330 pf, 50 v, ceramic disc capacitor 0.001 uf, 50 v, ceramic disc capacitor 0.22 uf, 50 v, film capacitor 0.0047 uf, 50 v, ceramic disc capacitor 22 uf, 16 v, electrolytic capacitor TL431AIZ voltage control Zener (shunt regulator) High impedance earphone 22 uh RF choke 2N4416A JFET transistor 470K, 1/4 w, resistor 1K, 1/4 w, resistor 10K, 1/4 w, resistor 1M, 1/4 w, resistor 100 ohm, 1/4 w, resistor Small SPST switch screws for mounting C3 (2 needed) #4 nylon screw used for tuning C3 mini battery snap






INTRODUCTION OF VOX The VS1 is a universal voice-operated-switch (VOX) module which may be used in any application where VOX operation is desired. It may be used with virtually any type of microphone. The circuit itself draws only 10 mA at 9 volts DC and will directly switch low voltage loads up to 100 mA. Numerous small and inexpensive relays are available to permit switching of higher voltage and current. The VS- 1 VOX may be used to control ham radio transmitters, CB transceivers, and similar Equipment for other radio services. In addition, it can be used to control tape recorders or any other device for which you envision voice-operated switching.

WORKING The dual op amps of the LM358 IC amplify the microphone signal. R2, R3 and C2 configure the LM358 for operation from a single voltage supply. The output from the op amp is rectified to DC by the diodes D1 and D2. The VOX delay (length of time that the transistor Q3 is switched on) is Determined by C4 and R7. The 220K ohm value for R7 produces a VOX delay quite useful for most applications. A 500K or 1 mega ohm trimmer in place of R7 permits a wide range of VOX delay settings. Transistors Q1 and Q2 provide enough drive for efficient switching by Q3.


• • • • • • • • •

4 4.7 or 10uF electrolytic capacitors (C1, C2, C3, C4) 4 1K ohm resistors [brown-black-red] (R4, R5, R10, R11) 5 10K ohm resistors [brown-black-orange] (R2, R3, R6, R8, R9) 1 220K ohm resistor [red-red-yellow] (R7) 1 1 Mega ohm resistor [brown-black-green] (R1) 2 NPN transistors [marked 2N3904] (Q1, Q2) 1 PNP transistor [marked 221334] (Q3) 1 LM358 8-pin DIP Dual Operational Amp IC (U1) 2 1N4148 diode (D1, D2)





COMPONENT DESCRIPTION 1. RESISTORS A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current through it in accordance with Ohm's law: V = IR Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistively alloy, such as nickel/chrome). The primary characteristics of a resistor are the resistance, the tolerance and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance depends upon the materials constituting the resistor as well as its physical dimensions; it's determined by design. Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals) are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power. The value of a resistor can be measured with an ohmmeter, which may be one function of a multimeter. Usually, probes on the ends of test leads connect to the resistor. .

Resistor marking Most axial resistors use a pattern of colored stripes to indicate resistance. Surfacemount resistors are marked numerically, if they are big enough to permit marking; more-recent small sizes are impractical to mark. Cases are usually tan, brown, blue, or green, though other colors are occasionally found such as dark red or dark gray. For example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description can be as followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end, creating 560,000Ω at ±2% tolerance accuracy. 560,000Ω changes to 560 kΩ ±2% (as a kilo- is 103).


FM RADIO WITH VOICE ACTIVATED SWITCH Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in the chart below.

Color 1st band 2nd band

3rd band (multiplier)

4th band (tolerance)

Temp. Coefficient

Black 0



Brown 1



±1% (F)

100 ppm





±2% (G)

50 ppm

Orange 3



15 ppm

Yellow 4



25 ppm

Green 5



±0.5% (D)





±0.25% (C)

Violet 7



±0.1% (B)





±0.05% (A)

White 9





±5% (J)



±10% (K)


±20% (M)



2. CAPACITORS A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the plates. The effect is greatest between wide, flat, parallel, narrowly separated conductors. An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage. The properties of capacitors in a circuit may determine the resonant frequency and quality factor of a resonant circuit, power dissipation and operating frequency in a digital logic circuit, energy capacity in a high-power system, and many other important system characteristics.

TYPES Practical capacitors are available commercially in many different forms. The type of internal dielectric, the structure of the plates and the device packaging all strongly affect the characteristics of the capacitor, and its applications.

Dielectric materials

Capacitor materials. From left: multilayer ceramic, ceramic disc, multilayer polyester film, tubular ceramic, polystyrene, metalized polyester film, aluminum electrolytic. Major scale divisions are in centimeters. Most types of capacitor include a dielectric spacer, which increases their capacitance. These dielectrics are most often insulators. However, low capacitance devices are available with a vacuum between their plates, which allows extremely high voltage operation and low losses. Variable capacitors with their plates open to the atmosphere were commonly used in radio tuning circuits. Later designs use polymer foil dielectric between the moving and stationary plates, with no significant air space between them.


FM RADIO WITH VOICE ACTIVATED SWITCH Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic materials. Paper was used extensively in older devices and offers relatively high voltage performance. However, it is susceptible to water absorption, and has been largely replaced by plastic film capacitors. Plastics offer better stability and aging performance, which makes them useful in timer circuits, although they may be limited to low operating temperatures and frequencies. Ceramic capacitors are generally small, cheap and useful for high frequency applications, although their capacitance varies strongly with voltage, and they age poorly. They are broadly categorized as class 1 dielectrics, which have predictable variation of capacitance with temperature or class 2 dielectrics, which can operate at higher voltage. Glass and mica capacitors are extremely reliable, stable and tolerant to high temperatures and voltages, but are too expensive for most mainstream applications. Electrolytic capacitors and super capacitors are used to store small and larger amounts of energy, respectively, ceramic capacitors are often used in resonators, and parasitic capacitance occurs in circuits wherever the simple conductor-insulator-conductor sequence is formed unintentionally. Electrolytic capacitors use an aluminum or tantalum plate with an oxide dielectric layer. The second electrode is a liquid electrolyte, connected to the circuit by another foil plate. Electrolytic capacitors offer very high capacitance but suffer from poor tolerances, high instability, gradual loss of capacitance especially when subjected to heat, and high leakage current. The conductivity of the electrolyte drops at low temperatures, which increases equivalent series resistance. While widely used for power-supply conditioning, poor high-frequency characteristics make them unsuitable for many applications. Tantalum capacitors offer better frequency and temperature characteristics than aluminum, but higher dielectric absorption and leakage. OS-CON (or OC-CON) capacitors are polymerized organic semiconductor solid-electrolyte types that offer longer life at higher cost than standard electrolytic capacitors. Several other types of capacitor are available for specialist applications. super capacitors store large amounts of energy. Super capacitors made from carbon aero gel, carbon nanotubes, or highly porous electrode materials offer extremely high capacitance (as much as 2,500 farads) and can be used in some applications instead of rechargeable batteries. Alternating current capacitors are specifically designed to work on line (mains) voltage AC power circuits. They are commonly used in electric motor circuits and are often designed to handle large currents, so they tend to be physically large. They are usually ruggedly packaged, often in metal cases that can be easily grounded/earthed. They also are designed with direct current breakdown voltages of at least five times the maximum AC voltage.



Capacitor packages: SMD ceramic at top left; SMD tantalum at bottom left; throughhole tantalum at top right; through-hole electrolytic at bottom right. Major scale divisions are cm. The arrangement of plates and dielectric has many variations depending on the desired ratings of the capacitor. For small values of capacitance (microfarads and less), ceramic disks use metallic coatings, with wire leads bonded to the coating. Larger values can be made by multiple stacks of plates and disks. Larger value capacitors usually use a metal foil or metal film layer deposited on the surface of a dielectric film to make the plates, and a dielectric film of impregnated paper or plastic - these are rolled up to save space. To reduce the series resistance and inductance for long plates, the plates and dielectric are staggered so that connection is made at the common edge of the rolled-up plates, not at the ends of the foil or metalized film strips that comprise the plates. The assembly is encased to prevent moisture entering the dielectric - early radio equipment used a cardboard tube sealed with wax. Modern paper or film dielectric capacitors are dipped in a hard thermoplastic. Large capacitors for high-voltage use may have the roll form compressed to fit into a rectangular metal case, with bolted terminals and bushings for connections. The dielectric in larger capacitors is often impregnated with a liquid to improve its properties. A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefore they are sometimes called tuning capacitors), or as a variable reactance, e.g. for impedance matching in antenna tuners

This product is used for fm audio systems. Made of PVC but the inner strips r made of copper or iron. Comes in three models 065 079 088



3. TRANSISTORS In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much larger than the controlling (input) power, the transistor provides amplification of a signal. The transistor is the fundamental building block of modern electronic devices, and is used in radio, telephone, computer and other electronic systems. Some transistors are packaged individually but most are found in integrated circuits.

Field-effect transistor The field-effect transistor (FET), sometimes called a unipolar transistor, uses either electrons (in N-channel FET) or holes (in P-channel FET) for conduction. The four terminals of the FET are named source, gate, drain, and body (substrate). On most FETs, the body is connected to the source inside the package, and this will be assumed for the following description. In FETs, the drain-to-source current flows via a conducting channel that connects the source region to the drain region. The conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals; hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate–source voltage (Vgs) is increased, the drain– source current (Ids) increases exponentially for Vgs below threshold, and then at a roughly quadratic rate ( ) (where VT is the threshold voltage at which drain current begins) in the "space-charge-limited" region above threshold. A quadratic behavior is not observed in modern devices, for example, at the 65 nm technology node. For low noise at narrow bandwidth the higher input resistance of the FET is advantageous. FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as metal–oxide–semiconductor FET (MOSFET), from their original construction as a layer of metal (the gate), a layer of oxide (the insulation), and a layer of semiconductor. Unlike IGFETs, the JFET gate forms a PN diode with the channel which lies between the source and drain. Functionally, this makes the N-channel JFET the solid state equivalent of the vacuum tube triode which, similarly, forms a diode between its grid and cathode. Also, both devices operate in the depletion mode, they both have a high input impedance, and they both conduct current under the control of an input voltage. Metal–semiconductor FETs (MESFETs) are JFETs in which the reverse biased PN junction is replaced by a metal–semiconductor Schottky-junction.


FM RADIO WITH VOICE ACTIVATED SWITCH These, and the HEMTs (high electron mobility transistors, or HFETs), in which a two-dimensional electron gas with very high carrier mobility is used for charge transport, are especially suitable for use at very high frequencies (microwave frequencies; several GHz). Unlike bipolar transistors, FETs do not inherently amplify a photocurrent. Nevertheless, there are ways to use them, especially JFETs, as light-sensitive devices, by exploiting the photocurrents in channel–gate or channel–body junctions. FETs are further divided into depletion-mode and enhancement-mode types, depending on whether the channel is turned on or off with zero gate-to-source voltage. For enhancement mode, the channel is off at zero bias, and a gate potential can "enhance" the conduction. For depletion mode, the channel is on at zero bias, and a gate potential (of the opposite polarity) can "deplete" the channel, reducing conduction. For either mode, a more positive gate voltage corresponds to a higher current for N-channel devices and a lower current for P-channel devices. Nearly all JFETs are depletion-mode as the diode junctions would forward bias and conduct if they were enhancement mode devices; most IGFETs are enhancement-mode types.

4. DIODES In electronics, a diode is a two-terminal device (thermionic diodes may also have one or two ancillary terminals for a heater). Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property. The varicap diode is used as an electrically adjustable capacitor. The unidirectional most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve. Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.

Types of semiconductor diode



Figure 7: Typical diode packages in same alignment as diode symbol. Thin bar depicts the cathode.

Figure: Several types of diodes. The scale is centimeters. There are several types of junction diodes, which either emphasize a different physical aspect of a diode often by geometric scaling, doping level, choosing the right electrodes, are just an application of a diode in a special circuit, or are really different devices like the Gunn and laser diode and the MOSFET: Normal (p-n) diodes, which operate as described above, are usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop (typically 1.4–1.7 V per “cell”, with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode’s metal substrate), much larger than a silicon diode of the same current ratings would require. The vast majority of all diodes are the p-n diodes found in CMOS integrated circuits, which include two diodes per pin and many other internal diodes.

Esaki or tunnel diodes These have a region of operation showing negative resistance caused by quantum tunneling, thus allowing amplification of signals and very simple bistable circuits. These diodes are also the type most resistant to nuclear radiation.

Light-emitting diodes (LEDs) In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be ELECTRONICS & COMMUNICATION

FM RADIO WITH VOICE ACTIVATED SWITCH produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to violet. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs produce incoherent, narrow-spectrum light; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.

Laser diodes When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication.

Photodiodes All semiconductors are subject to optical charge carrier generation. This is typically an undesired effect, so most semiconductors are packaged in light blocking material. Photodiodes are intended to sense light(photodetector), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light). A photodiode can be used in solar cells, in photometry, or in optical communications. Multiple photodiodes may be packaged in a single device, either as a linear array or as a two-dimensional array. These arrays should not be confused with charge-coupled devices.

Schottky diodes Schottky diodes are constructed from a metal to semiconductor contact. They have a lower forward voltage drop than p-n junction diodes. Their forward voltage drop at forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation. They can also be used as low loss rectifiers although their reverse leakage current is generally higher than that of other diodes. Schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down many other diodes — so they have a faster “reverse recovery” than p-n junction diodes. They also tend to have much lower junction capacitance than p-n diodes which provides for high switching speeds and their use in high-speed circuitry and RF devices such as switched-mode power supply, mixers and detectors.



Snap-off or Step recovery diodes The term ‘step recovery’ relates to the form of the reverse recovery characteristic of these devices. After a forward current has been passing in an SRD and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers.

Varicap or varactor diodes These are used as voltage-controlled capacitors. These are important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock. A PLL is faster than an FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband signal). They also enabled tunable oscillators in early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a voltage-controlled oscillator.

Zener diodes Diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see below). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or Transorb, a registered trademark). The Zener diode is named for Dr. Clarence Melvin Zener of Southern Illinois University, inventor of the device.

5. RF CHOKE Choke coils are inductances that isolate AC frequency currents from certain areas of a radio circuit. Chokes depend upon the property of self-inductance for their operation. They are used to block alternating current while passing direct current (contrast with capacitor). Common-mode choke coils are useful in a wide range of prevention of


FM RADIO WITH VOICE ACTIVATED SWITCH electromagnetic interference (EMI) and radio frequency interference (RFI) from power supply lines and for prevention of malfunctioning of electronic equipment. Chokes used in radio circuits are divided into two classes – those designed to be used with audio frequencies, and the others to be used with radio frequencies. Audio frequency coils, usually called A.F. chokes, can have ferromagnetic iron cores to increase their inductance. Chokes for higher frequencies (ferrite chokes or choke baluns) have ferrite cores. Chokes for even higher frequencies have air cores. Radio frequency coils, (R.F. chokes), usually don't have iron cores. In high power service so much heat would be produced in making and destroying the field in the core that the coil would burn up.




FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrow band form is used for voice communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space. FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis, was popularized by early digital synthesizers and became a standard feature for several generations of personal computer sound cards. As the name implies, wideband FM (W-FM) requires a wider signal bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio" switch a device such as a computer or appliance or lamp on and off by voice command or other sound. controlling a tape recorder motor




1. 2. 3. 4. 5. 6.

Electronics Devices and Circuits By: Robert L Boylestad Applied Electronics By: R S Sedha Op-Amp and Linear integrated Circuits By: R A Gayakwad




Minor Project Submitted towards partial fulfillment of the degree of Bachelor of Engineering Year 2008-2009 Department of Electronic & Communication Engineering

Guided By: Mr. Atul Vyas KHOTI(1115) Mr. Rahul Maheshwari





This is to certify that Mr. Vineet khoti and Mr. Sunny Manghani working in a group have satisfactorily completed their minor project towards the partial fulfillment of degree of Bachelor of Engineering (ELECTRONICS & COMMUNICATION ENGINEERING) awarded by Rajeev Gandhi Technical University, Bhopal(M.P).

Project guide:

Head of the Department

(Mr. Atul Vyas)

(Mr. Kamlesh Gupta)

(Mr. Rahul Maheshwari)






Every endeavor we understand takes an indomitable urge, perseverance, and proper guidance especially when it is most needed. Internally motivated to undertake some appreciable work as our degree project, unsure though, but with a hope we took this project work to be completed. Initially we had hardly ever thought of, the kind of work we were going to do. We express our heartfelt thanks to our revered project guide Mr. Atul Vyas and Mr. Rahul Maheshwari for their encouragement and benevolent guidance. They are inspired stints and their experience and knowledge provided us with educative support. Here we think from the core, everybody who has helped us at work. In true sense, besides our guides we are indebted to many other members at SVITS for their constant help and invaluable contribution on all occasions when they were most needed. Again, we find the opportunity to acknowledge our regards and thanks to our family members and friends who have been true support behind us.



1. FM RADIO………………………….. 1.1 Introduction 1.2 Working 1.3 Components 1.4 Circuit diagram 2. VOICE ACTIVATED SWITCH………………. 2.1 Introduction 2.2 Working 2.3 Components 2.4 Circuit diagram 3. COMPONENT DESCRIPTION 3.1 Introduction Register Capacitor Transistor Diode RF choke

3.2 Data sheets 1N4148 diode 2N3904 transistor LM358 dual op-amp IC TL431AIZ voltage control zener



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