Electronics Design

5-BAND GRAPHIC EQUALISER

2009-02-11T05:28:00.007+07:00

By aRie eMDee
This equaliser uses low-cost op-amps. Good-quality opamps powered by a single voltage supply are readily available in the market. The op-amp should have a noise density of less than 24nV/√Hz, slew rate of more than 5V/µs and gainbandwidth product greater than 3MHz. The NE5532 or LM833 used in this circuit meets these requirements. Equaliser circuits typically divide the audio spectrum into separate frequency bands and have independent gain control for each band.

The output of each band is mixed at IC4(A) and then fed to an audio power amplifier.Proper quality factor (Q) needs to be selected to avoid overlap in adjacent bands as this introduces
colouration into the audio signal.

We have used the multiple-feedback bandpass filter topology shown in left-most corner at the bottom of the figure. This is a circuit for single-channel
bandpass filter. If the capacitors are

Download MOdul Equaliser

of the same value, the calculations are fairly simple. For calculating the component values, use the following formulae:
Centre frequency (fo) : 1/2πC√(Ra||Rb)Rc
Bandwidth (B) : 1/πCRc
Quality factor (Q) : fo/B = πfoCRc
Gain (A) : –Rc/2Ra
These can be combined to give the
following formulae:
Ra = Q/2πfoAC
Rb = Q/2πfoC (2Q2–A)
Rc = Q/πfoC
Begin the calculations by choosing a large value of capacitance (~0.1F) and smaller value of resistances. Increasing the capacitance decreases resistances (Ra, Rb and Rc). Care must be taken to avoid overloading on the input buffer op-amp. Note that stray capacitances
on the board reduces the value of ‘C.’ The bandwidth and gain do not depend on Rb. Hence, Rb can be used to modify the mid-frequency without affecting the bandwidth and gain.

For equalisers, there are standard mid-frequencies that are normally used. The exact frequencies depend on the octave division, application and some degree of manufacturers’ preference, but nearly all share the basic octave boundaries that are based on a centre frequency of 1000 Hz.

A balance between the number of filters and bandwidth need to be observed.It is possible to use a wider bandwidth and fewer filters, or narrower bandwidth and more filters. Anything narrower than 1/3 octave is rare, since the complexity of the filters increases for higher values of ‘Q.’ This can get rather expensive and in reality is of limited use for most applications in audio systems.

National Semiconductor lists the following mid-frequencies for a 10-band graphic equaliser: 32,64, 125, 250,500, 1k, 2k, 4k, 8k and 16k. It also recommends a ‘Q’ of 1.7 for equalisers.
The table lists the component values for different centre frequencies of the equaliser. We used ‘Q’ of 1.7 and gain (A) of 4.The circuit for the 5-band equaliser uses IC1 (A) LM833 as the buffer stage for the equaliser. It is a non-inverting amplifier with a gain of ‘2.’ The input signal is divided by ‘2’ by the resistive network comprising R3 and R4. Hence the net gain of this amplifier is unity.

Two 100k resistors (R1 and R2) are used as a voltage divider and the junction voltage is fed to its positive input through R6. This divider has enough power to feed all other op-amps directly. Resistor Ro (R8=R12=R16=R20=R24=R28=R30=100Ω) has the dual function of noise reduction
and resistive isolation of capacitive load. It may be varied between 50 and 150 ohms depending on the noise in the circuit.

The potmeters (VR1 through VR5) are in the signal path and hence should be of the best quality possible. Wrap the body of the pots with bare copper wire and solder the other end of the wire to ground. Since the filters are very sensitive, all resistances should be metal-film type and the capacitors should be polyester type.

Each stage of the op-amp needs to be capacitively coupled to the next stage so that the DC does not get propagated and amplified. For a good low-frequency response, this coupling capacitor should be greater than 1 µF.

A 10µF, 16V capacitor is used in each stage of the circuit here. The circuit is powered by a 12V
DC regulated supply. A well-regulated supply using 7812 is recommended. Ground the Vcc pin of each op-amp with a 0.1µF ceramic disk capacitor to bypass the noise.

A Cmos 4060 Burglar Alarm

2009-01-23T21:14:00.003+07:00

Circuit: Aryanto

Description:
This is a single zone alarm - with automatic exit, entry and siren cut-off timers. It will accommodate all the usual types of normally-closed input devices - such as magnetic-reed contacts - foil tape - PIRs etc.

When the alarm is activated - the siren will sound for a fixed length of time. Then it will switch off - and remain off. The alarm will not re-activate.

I've used a 12-volt supply in the drawing - but the circuit will work at anything from 5 to 15-volts. All you need do is select a siren, buzzer, and relay to suit the voltage you're using.
Schematic Diagram:

Notes:
When you switch the alarm on - the buzzer will sound eight times. This is the exit delay. Before the end of the eighth beep - you can leave the building without activating the alarm.

R6 controls the length and speed of the beeps. It can be adjusted to give an exit delay of anything from about ten seconds - up to about a minute. After the eighth beep - the buzzer should stop sounding. This confirms that the loop has been restored within the time allowed.

If the buzzer does not stop - but changes instead to a continuous beep - the loop is open and the building isn't secure. When this happens - you should switch off the alarm - and check for open doors, windows etc.

When you return and open the door - the buzzer will sound again - and the entry delay will start. The entry delay is the same length as the exit delay. But to distinguish it from the exit delay - the buzzer will sound continuously.

If the buzzer is sounding continuously - the alarm has been activated - and the entry delay has begun. If you don't switch the alarm off before the entry delay expires - the siren will sound.

The siren will sound only once. Just as R6 sets the lengths of the exit and entry delays - it also sets the siren cutoff time. The siren cut-off delay is 30 times the length of the exit delay. With the exit delay set at 30-seconds - the siren will sound for about 15-minutes. Then it will switch off - and remain off.

Of course - you can stop the noise at any time by switching off the alarm.

After the cut-off timer has switched the siren off - the buzzer will continue to sound. So when you return - if the buzzer is sounding - you'll know that the alarm has been activated.

Note that D10 is optional. Its job is to sound the buzzer constantly during the entry delay. It's also responsible for keeping the buzzer going after the siren has stopped.

If you leave out D10 - the buzzer will beep eight times during both the exit and entry delays. And - when the siren cuts-off - the buzzer will cut off also.

Alternative Capacitor:

A regular electrolytic capacitor is polarised. If the charge on its plates is the wrong way round - DC current will flow through the capacitor. If the current is high enough - the capacitor will heat up and explode. The presence of R5 in the circuit means that this is not going to happen. But if you use a polarised capacitor - it may mean that the oscillator won't run - or won't run reliably.

While the oscillator is running - the polarity of the charge on C4 keeps reversing. So C4 needs to be nonpolarised. However - you can simulate a non-polarised 10uF capacitor by connecting two 22uF polarised capacitors back to back - as shown.

How and why this works is explained in the Support Material - which also includes a photograph of the prototype
- a detailed circuit description - a parts list - a step-by-step guide to construction - and more.

Because non-polarised capacitors aren't widely available - the prototype was built using two polarised capacitors.
Veroboard Layout:

To add a normally open loop contact, aryanto +628562501188 on aRie web site

Direct-to-Home TV: All channels without a cable

2009-01-21T21:05:00.002+07:00

DTH services will bring an era of competition for cable TV operators and force them to improve quality. Users will have access to high quality programming on pay channels while persons in far-flung areas where cable operators are yet to reach will be able to use the dish antennas for receiving channels.

After a long pause, the Union Cabinet (government of India) has finally approved the much-awaited DTH (Direct-to-Home) broadcasting service. The service will become a reality in India in coming months. Without going deep into the notification issued by the government, let us answer the queries that may have popped up in your mind.

Q. What exactly is DTH?

A. DTH (direct-to-home) basically is a distribution platform for multichannel TV programmes on Ku (high frequency of 11.7 to 14.55 Gigahertz) band by using a satellite system that will be transmits directly to subscriber premises.

Q. What is the difference between Ku-band and the present form of cable transmission in India?

A. The basic difference between Ku-band and the present C-band transmission (3.4 to 6.65 GHz.) lies in the fact that Ku-band signals require a very small dish antenna of 30 to 48 cm (12 to 18 inch) diameter while lower C-band requires a large dish antenna of 2.4-to 4.9 metre (8 to 16 feet) diameter. The Ku-band DTH service offers greater and direct connectivity to the viewer providing over 100 channels through a small dish antenna, doing away with intermediaries like cable operators. Moreover, this will have a better control over the revenue.

Q. What is the USP (unique selling point) of the service?

A. The subscribers can directly receive and view an array of channels available globally, doing away with intermediate cable-operators.

Q. Is the direct-to-home broadcasting legal in India?

A. After the removal of the existing three-year ban DTH broadcasting has been made legal from November 3, 2000 in India. The government has approved the service, reversing the 1997 prohibition of instituting measures for regulation of content and prevent monopoly.

Q. What will be the cost for an end-user?

A. The users will not have to pay much to view channels of their interest. A nominal investment of approximately Rs25,000 will link them to the multi-channel platform. This will include a pizza-sized 30 to 48 cm (12 to 18 inch) dish antenna for Rs.10,000 along with the TV set-top box, a digital decoder, for Rs. I5,000. Users will be provided with 'channel cards' which will act as pre-paid debit cards.

Q. What amount a subscriber will have to pay for the service?

A. All channels will be 'pay' channels and the subscriber will have to pay the fee to the service provider. Since DTH is now legal in India, the companies will recover the cost from the revenue accumulated via the 'channel cards'.

Q. What is the entry fee for DTH?

A. The applicants are bound to pay an initial deposit of Rs. 100 million beside Rs. 400 million as the bank guarantee for a licence from the government to provide DTH service. Out of this a minimum of ten percent of the total revenue collected by the platform owner will be shared with the government.

Q. Do we have the same investment policy for the foreign applicants?

A. Foreign investment in DTH companies has been capped at 49 per cent, which includes 20 per cent foreign direct investment. NRIs, FIIs and OCBs can invest up to 29 per cent. Broadcast and cable companies cannot own more than 20 per cent equity.

Q. What are the other terms and conditions that need to be followed?

A. The licencee will have to establish the uplink Earth Station in India within 12 months of issue of licence. It is mandatory for all the DTH service providers to get registered under the Indian Company Act (ICA) and the Indian Management Control (IMC) with majority representatives from the Indian board. The CEO should be an Indian resident. The companies will have to preserve the recordings for at least 90 days so that any complaint filed could be checked out.

Q. How many and for how long will the licence be issued?

A. At present there exists no restriction on the number of licences issued by the I&B (Information and Broadcasting) ministry for a period of ten years. Thus the licence issued will need to be renewed only after ten years from its issue date.

Q. Who all will be the DTH broadcasters?

A. From the current indications it appears that other than Doordarshan the other TV broadcasters such as Star TV, Zee TV, ESPN etc., are keen to join the direct-to-home bandwagon. But this will be subject to the regulatory supervision on the account of security concerns.

Q. What will happen if a broadcaster violates DTH notification?

A. In case of violation of rules, apart from the cancellation of licence issued the company will have to shell out a penalty of Rs. 500 million.

Q. What is the future of the DTH broadcasting in India?

A. Its too premature to comment on the conditions prevailing but it is true that DTH is definitely a step towards convergence. DTH has definitely got more power than Pokhran.

Direct to home tv Enjoy more channels with better quality!

2009-01-20T05:33:00.001+07:00

The recently approved direct-to-home (DTH) TV obviates the requirement of cable-wallahs and enables viewers to access error-free programmes with better picture- and audio-quality

The union cabinet has finally ap- proved direct-to-home (DTH) TV broadcasting, about three years after enthusiastic Indian TV viewers were tantalised by the prospect of access to hundreds of global channels.

DTH is a concept whereby high-density TV signals in the Ku band from the satellite are beamed directly to the home of the viewer, obviating the need of a cable operator. Using 30cm to 46cm diameter small dish antennae and TV set-top boxes, subscribers are able to decode channels and view a plethora of programmes. Users are provided with channel cards that are essentially prepaid debit cards.

The price of set-top box and antenna varies from Rs 10,000 to Rs 25,000. However, the actual cost to a subscriber can be lower. In the countries where DTH is legal, satellite channels sell the system at a subsidised price to the consumers. Service providers recover their costs through the subscription fee that accrues to them from the sales of channel cards. The DTH operator offers a bouquet of channels, once the subscriber purchases the SIM card.

The elite class in the metropolitan cities is on the upbeat mode. Their kids would now be able to watch more Cartoon and Discovery type channels in English. Foreign diplomats, businessmen, and MNCs can watch channels of their choice in their own language. Users will have direct access to high-quality programmes on pay channels such as Star Movies. People in far flung areas,where cable operators are yet to provide the service, can use dish antennae for receiving channels. DTH can also provide satellite-based radio services.

Regulatory issues

The government has allowed DTH with certain safeguards. DTH players will have to beam from the home soil through earth stations within the country and adhere to the laid down programme and advertisement codes. The CEO and a majority of directors of DTH companies would have to be resident Indians. To obviate creation of monopolies, existing broadcasting companies and cable network owners are restricted to 20 per cent share in DTH ventures.

The companies will have to furnish an initial deposit of Rs 100 million and Rs 400 million as bank guarantee for the 10-year licence period. At least 10 per cent of the revenue generated will have to be shared with the government. There is a 49 per cent limit on foreign equity. Of this, up to 20 per cent is to be foreign direct investment (FDI) and the balance from institutional investors, overseas corporates, and NRI investments.

DTH operators are allowed to set up the satellite earth station only on Indian soil. The earth station will act as the nerve centre for receiving and distributing signals from the satellites. It will enable government agencies to monitor the programme content to ensure that it conforms to the country’s parameters of security and decency. “No porn channels or tobacco/liquor ads” is the first rule of DTH in India.
Content will have to pass through the common encryption and conditional access system located within the earth station. DTH operators will provide access to all facilities, including equipment, records, and systems, to government agencies. They will provide access to all content providers and channels on a non-discriminatory basis, but shall not carry prohibited channels.

The DTH is barred from being used for other means of communication, such as voice, fax, and data. In other words, DTH operators will not be able to provide the Internet and voice communication services, as of now. They are bound to carry all channels of Prasar Bharti on the most favoured financial terms extended to other channels.

Pros and cons

Many cable subscribers are unhappy with the erratic cable TV services that keep on shifting the channels, at the expense of other channels, without any prior notification. Archiac hardware, cable wires, poor-quality reception, electrical disturbances, etc are irritating. DTH obviates the requirement of cable-wallahs altogether and enables viewers to access error-free programmes with better picture- and audio-quality. It allows the broadcasters to tap homes in rural and remote uncabled areas. One can choose from as many as 300 to 800 channels.

Pay channels will have direct and better control over revenues. Broadcasters, particularly of pay channels, feel that cable operators misrepresent the number of actual subscribers, resulting in revenue leaks. Pay channels can now monitor how many homes are watching their channels, and collect their revenue accordingly. DTH enables people with niche interests to view channels that are normally not available with the cable operators.

Because of the high investment of Rs 10,000 to Rs 25,000 for pizza-sized satellite-dish and digital decoder, along with monthly fee of about Rs 1,000, this service may not be affordable for most of the existing cable homes. But, if one pays the premium, one is the recipient of better picture quality with a wider choice.
DTH also promises to benefit hardware manufacturers as increase in subscriber base would lead to increase in demand and supply of DTH components.

Cable operators claim that they can also provide the Internet, interactive facilities on TV, and video-on-demand. But subscribers cannot call up the DTH providers and ask them to repeat their favourite Bollywood blockbusters on TV.

Technically, the only drawback to DTH is the effect caused by the various modes of interference like rain, sun, adjacent satellites, and terrestrial interference. Moisture blocks signals and also absorbs some of them. Sun spots, the magnetic storm on the sun’s surface, can directly affect satellites placed outside the ionsphere. Users need a clear and unobstructed view of the satellite, and the dish must be aligned accurately.

Commercial viability and the tremendous expense involved for both service providers as well as subscribers are the other major deterrents to DTH.

Enabling innovations

The idea to launch satellites for direct diffusion of TV programmes to ordinary people appeared economically realisable at the beginning of the 1970s. WARC (World Administration Radiocomm-unication Conference) defined the frequency bands, transponder power, and polarisation to be used, and allocated orbital positions, channels, and EIRP contours to each of the participating countries. The attributed frequency band (11.700 to 12.500 GHz), known as the direct broadcast satellite (DBS) band, was chosen, since it allows well delimited service areas with relatively small transmission antennae of less than 3m diameter on the satellite and does not suffer too much from meteorological conditions.

The specifications of DTH TV satellites have evolved considerably since then. The reception heads, the LNBs (low-noise boosters), have made enormous progress, with average noise factors in the range of 1.5 to 1.8 dB in 1990 and 0.8 to 1.1 dB in 1998, compared to 3 to 5 dB in 1977. This ensures the same service area with much smaller transmission power (50 to 100W instead of 200W) at identical or smaller antenna dimensions.

The increase in useful load of the satellite-launching rockets allowed an important increase in the total electrical power available, which, coupled with the satellite transponder power reduction, allowed the number of transponders per satellite to be increased by a factor of four. These satellites now have sufficient electrical autonomy to ensure an uninterrupted and normal service during equinox solar eclipses. The lifetime of satellites has doubled, approximately to 15 years, partly due to the better accuracy of the launches, which strongly influences the quantity of remaining propellent necessary for position control of the satellite.

Circular polarisation is preferred over the linear polarisation used by telecommunication satellites. This is because, for circular polarisation, the orientation of the reception head (LNB) around the propogation axis is unimportant, and therefore does not require any precise adjustment. This point is especially important for motorised antennae which, with linear polarisation, require a polarisation adjustment for each different satellite.

In the early 1980s, the home satellite systems started sprouting up all over the US. Japan became the first country to launch a DTH service in 1984. Subsequent innovative developments over the years started modernising and changing the lifestyles of people. DTH transmission has been operational in most Western countries since 1986.

Although the US had a long history of DTH satellite reception on large ‘TV receive only’ (TVRO) satellite antennae, it was not until the launch of theHughes DBS-I satellite in December 1993, and subsequent availability of diminutive satellite dishes with digital receivers, that the services started developing. The year 1994 brought a breakthrough in the debut of the first DBS service. In June 1994, consumers in Jackson, Mississipi, USA became the first Amercians to purchase the 45.7cm satellite dish and digital satellite receiver, enabling DBS reception.

By the end of 1994, over one million homes had subscribed to DBS multichannel services. Hughes developed DirectTV, a high-power DBS, to supply digitally compressed TV to the home. Contracts were issued to Thomson Consumer Electronics for the supply of antennae, home receiving equipment, and digital compression hardware. Individual antenna reception of TV was used in most homes in nine out of sixteen European countries, including the UK, France, Italy, and Spain—containing 70 million TV homes and 55 per cent of all Europe. These homes formed the target for DTH satellite broadcasting. The number of individual satellite receiving antennae and receivers increased dramatically in the UK and Germany, and higher power transmitted by satellites were receivable by antennae as small as 30 cm in diameter.

Functioning

A DTH network includes a broadcasting centre, one or more high-power satellites, and DTH receivers. Broadcasting centre, the hub of the network, is a complex production/playback studio with satellite uplink facility, subscriber management/access control system, and access network management system in the case of interactive and data services. Analogue or digital video programmes are received through satellite contribution links, fibre-optics, or microwave terrestrial connections. Audio-video(AV) programmes are also available through tape library or CD/DAT media. In case data or the Internet services are offered, connections to servers are used via cable or satellite links.

AV programmes are edited at the edit suite and final control centre (FCC). The final control centre consists of a mixing desk in which video, audio, and data programme scheduling and management are performed, with the insertion of commercials and promotionals, as required.

A typical DTH service provider leases Ku-band transponders from the satellite, accommodating the TV channels using compression techniques, after he has negotiated with different broadcasters (A1, A2, A3, A4) for these channels. The source of analogue signals from these channels could be camera, tape, or disk. Video, audio, data, or text signals are converted into digital format by encoder and mixed in multiplexer. Different programmes from various channels are mixed on a DTH platform. Subscriber management system manages various requests for different channels from the users.

Each decoder with the subscriber can be controlled individually at the multiplxer. A modulator superimposes the information on an RF carrier for transmission to the earth station. The earth station uplinks the signals to the satellite transponder for subsequent downlinking to individual households, apartments, hotels, and cable head-ends. Small-diameter antannae on the rooftops (or flats’ outer walls) transmit the signals to the integrated receiver decoder (IRD). A smart card inserted into the IRD completes the conditional access progress, enabling the system to descramble the encrypted signals. The IRD is connected to the TV set.

DTH vs Cable

Installation cost	Rs 10,000 for 30cm Rs antenna + Rs 10,000 to 15,000 for decoder	150 for wiring
Monthly charges	Rs 500-1,000	Rs 100-200
Number of channels	More than 200	50-100
Reception quality	Excellent	Average to good
Other services	Fax, Internet, teleshopping time	Not available for being

Digital TV decoder/receiver

Signal frequencies ranging from 10.7 to 12.75 GHz received from the satellite are amplified and down-converted into the 950MHz to 2,150MHz range by the LNB. The tuner selects the required RF channel in this range and converts it into a 480MHz IF. This signal is then

amplified and demodulated. The analogue signals are converted by A-to-D converter and error correction is realised by forward error correction (FEC) block.
Selection and descrambling of the packets for the required programme are performed by the descrambler under the control of the conditional access device that includes smart card readers. The demultiplxer selects the packets corresponding to the programme selected by the user, using programmable filters. The audio and video outputs of the demultiplexer are applied to the MPEG block that combines MPEG AV and graphic-control functions for the electronic programme guide. MPEG decoding generally requires at least 16 Mbps of DRAM and SDRAM. Video signals reconstructed by the MPEG decoder are then applied to digital video recorder for conversion and subsequent display on a TV set. A 32-bit microprocessor controls the circuitry, interprets user commands from the remote control, and manages the smart card readers and the communications interfaces. Software is located in a Flash-EPROM.

Security Monitor

2009-01-17T05:00:00.001+07:00

Circuit : Aryanto
Email : arie.emdee@yahoo.com

Description:
A remote listening circuit. The area to be monitored is connected via a cable and allows remote audio listening.

Click on the image to view the full

Notes:
You can use this in your garden and listen for any unusual sounds, or maybe just wildlife noises. If you have a car parked in a remote location, the microphone will also pick up any sounds od activity in this area. The cable may be visible or hidden, screened cable is not necessary and you can use bellwire or speaker cable if desired.

Circuit Description:
Starting from the right hand side, the power supply. I have used 12V as a standard power supply voltage, or a 12V car battery may be used. The circuit is in two halves, a remote microphone preamp, and an audio amplifier based around the National Semiconductor LM386 audio amplifier.

The remote preamp uses an ECM microphone to monitor sound. A direct coupled 2 stage amplifier built around Q1 and Q2 amplify the weak microphone signal. Preset resistor R2 acts as a gain control, and C1 provides some high frequency roll off to the overall audio response. Q1 is run at a low collector current for a high signal to noise ratio, whilst Q2 collector is biased to around half the supply voltage for maximum dynamic range.

The power supply for this preamp is fed via R10 and R6 from the 12V supply. C4 ensures that the preamp power supply is decoupled and no ac voltages are present on the power lines. The amplified audio output from Q2 collector is fed onto the supply lines via C6 a 220u capacitor. The output impedance of Q2 is low, hence the relatively high value of C6. C6 also has a second purpose of letting the output audio signals pass, whilst blocking the dc voltage of the power supply.

At the opposite end, C7 a 10u capacitor, brings home the amplified audio to the listening location. The signal is first further amplifier by a x10 voltage gain amplified using the TL071. C8, a 22p capacitor again rolls off some high frequency response above 100kHz. This is necessary as long wires may pick up a little radio interference.

After amplification by the op-amp, the audio is finally passed to the LM386 audio amplifier. R14 acts as volume control. R13 and C12 prevent possible instability in the LM386 and are recommended by the manufacturer.

Audio output is around 1 watt into an 8 ohm loudspeaker, distortion about 0.2%. If preferred headphones could be used, although I'd recommend a series resistor of the same value impedance as the headphones.

AUTOMATIC SCHOOL BELL

2009-01-16T13:08:00.001+07:00

Consider that a school has a total of eight periods with a lunch break after the fourth period. Each period is 45 minutes long, while the duration of the lunch break is 30 minutes. To ring this automatic school bell to start the first period, the peon needs to momentarily press switch S1. Thereafter, the bell sounds every 45 minutes to indicate the end of consecutive periods, except
immediately after the fourth period,when it sounds after 30 minutes to indicate that the lunch break is over.

When the last period is over, LED2 glows to indicate that the bell circuit should now be switched off manually.In case the peon has been late to start the school bell, the delay in minutes can be adjusted by advancing the time using switch S3. Each pushing of switch S3 advances the time by 4.5 minutes. If the school is closed early, the peon can turn the bell circuit off by momentarily pressing switch S2.The bell circuit contains timer IC NE555 (IC1), two CD4017 decade counters (IC2 and IC3) and AND gate CD4081 (IC4). Timer IC1 is wired as an astable multivibrator, whose clock output pulses are fed to IC2. IC2 increases the time periods of IC1 (4.5 and 3 minutes) by ten times to provide a clock pulse to IC3 every 45 minutes or after 30 minutes, respectively.

When the class periods are going on, the outputs of IC3 switch on transistors
T1 and T2 via diodes D4 through D12. Resistors R4 and R5 connected in series to the emitter of npn transistor T2 decide the 4.5-minute time period of IC1. The output of IC1 is further connected to pin 14 of IC2 to provide a period with a duration of 45 minutes. Similarly, resistors
R2 and R3 connected in series to the emitter of npn transistor T1 decide the 3minute time period of IC1, which is further given to IC2 to provide the lunchbreak duration of 30 minutes.
Initially, the circuit does not ground to perform its operation when 12V power supply is given to the circuit. When switch S1 is pressed momentarily, a high enough voltage to fire silicon controlled resistor SCR1 appears at its gate. When SCR1 is fired, it provides ground path to operate the circuit after resetting both decade counters IC2 and IC3. At the same time, LED1 glows to indicate that school bell is now active. When switch S2 is pressed momentarily,the anode of SCR1 is again grounded and the circuit stops operating.

In this condition, both LED1 and LED2 don’t glow.When the eighth period is over, Q9
output of IC3 goes high. At this time, transistors T1 and T2 don’t get any voltage through the outputs of IC2. As a result, the astable multivibrator (IC1) stops working.The school bell sounds for around 8 seconds at the end of each period. One can increase/decrease the ringing time of the bell by adding/removing diodes connected in series across pins 6 and 7 of IC1.

The terminals of the 230V AC electric bell are connected to the normally open (N/O) contact of relay RL1.The circuit works off a 12V regulated power supply. However, a battery source for back-up in case the power fails is also recommended.

QUALITY FM TRANSMITTER

2009-01-15T10:45:00.002+07:00

This FM transmitter for your stereo or any other amplifier provides a good signal strength up to a distance of 500 metres with a power output of about 200 mW. It works off a 9V battery. The audio-frequency modulation stage is built around transistor BF494 (T1), which is wired as a VHF oscillator and modulates the audio signal present at the base. Using preset VR1, you can adjust the audio signal level.The VHF frequency is decided by

coil L1 and variable capacitor VC1. Reduce the value of VR2 to have a greater power output.The next stage is built around transistor BC548 (T2), which serves as a Class-A power amplifier.

This stage is inductively coupled to the audio-frequency modulation stage. The antenna matching network consists of variable capacitor VC2 and capacitor C9. Adjust VC2 for the maximum transmission of power or signal strength at the receiver.

For frequency stability, use a regulated DC power supply and house the transmitter inside a metallic cabinet. For higher antenna gain, use a telescopic antenna in place of the simple wire. Coils L1 and L2 are to be wound over the same air core such that windings for coil L2 start from the end point for coil L1. Coil winding details are given below: L1: 5 turns of 24 SWG wire closely

wound over a 5mm dia. air core L2: 2 turns of 24 SWG wire closely wound over the 5mm dia. air core L3: 7 turns of 24 SWG wire closely wound over a 4mm dia. air core L4: 5 turns of 28 SWG wire on an intermediate-frequency transmitter (IFT) ferrite core

Mobil Cell Phone Charger

2009-01-15T10:44:00.002+07:00

Charging of the cellphone battery is a big problem while travelling as power supply source is not generally accessible. If you keep your cellphone switched on continuously, its battery will go flat within five to six hours, making the cellphone useless. A fully charged battery becomes necessary especially when your distance from the nearest relay station increases. Here’s a simple charger that replenishes the cellphone battery within two to three hours.

Basically, the charger is a current-limited voltage source. Generally, cellphone battery packs require 3.6-6V DC and 180-200mA current for charging. These usually contain three NiCd cells, each having 1.2V rating. Current of 100mA is sufficient for charging the cellphone battery at a slow rate. A 12V battery containing eight pencells gives sufficient current (1.8A) to charge the battery connected across the output terminals. The circuit also monitors the voltage level of the battery.It automatically cuts off the charging process when its output terminal voltage increases above the predetermined voltage level. Timer IC NE555 is used to charge and monitor the voltage level in the battery.Control voltage pin 5 of IC1 is provided with a reference voltage of 5.6V by zener diode ZD1. Threshold pin 6 is supplied with a voltage set by VR1 and trigger pin2 is supplied with a voltage set by VR2. When the discharged cellphone battery is connected to the circuit, the voltage given to trigger pin 2 of IC1 is below 1/3Vcc and hence the flip-flop in the IC is switched onto take output pin 3 high. When the battery is fully charged, the output terminal voltage increases the voltage at pin 2 of IC1 above the trigger point threshold. This switches off
the flip-flop and the output goes low to terminate the charging process. Threshold pin 6 of IC1 is referenced at 2/3Vcc set by VR1. Transistor T1 is used to enhance the charging current. Value of R3 is critical in providing the required current for charging.

With the given value of 39-ohm the charging current is around 180 mA. The circuit can be constructed on a small general-purpose PCB. For calibration of cut-off voltage level, use a variable DC power source. Connect the output terminals of the circuit to the variable power supply set at 7V. Adjust VR1 in the middle position and slowly adjust VR2 until LED1 goes off, indicating low output. LED1 should turn on when the voltage of the variable power supply reduces below 5V.Enclose the circuit in a small plastic case and use suitable connector for connecting to the cellphone battery.

Note. At EFY lab, the circuit was tested with a Motorola make cellphone battery rated at 3.6V, 320 mAH. In place of 5.6V zener, a 3.3V zener diode was used. The charging current measured was about 200mA.The status of LED1 is shown in the table.

LED Status for Different Charging Conditions

Load across the output	Output frequency (at pin 3)	LED1
No battery connected	765 kHz	On
Charging battery	4.5 Hz	Blinks
Fully charged battery	0	Off

Long Range IR Transmitter

2009-01-15T10:43:00.001+07:00

Most of the IR remotes work reliably within a range of 5 metres. The circuit complexity increases if you design the IR transmitter

for reliable operation over a longer range, say, 10 metres. To double the range from 5 metres to 10 metres, you need to increase the transmitted power four times. If you wish to realise a highly directional IR beam (very narrow beam), you can suitably use an IR laser pointer as the IR signal source. The laser pointer is readily available in the market.

However, with a very narrow beam from the laser pointer, you have to take extra care, lest a small jerk to the gadget may change the beam orientation and cause loss of contact.
Here is a simple circuit that will give you a pretty long range. It uses

three infrared transmitting LEDs (IR1 through IR3) in series to increase the radiated power. Further, to increase the directivity and so also the power density, you may assemble the IR LEDs inside the reflector of a torch.For increasing the circuit efficiency,a MOSFET (BS170) has been used,which acts as a switch and thus reduces the power loss that would result if a transistor were used.

To avoid any dip during its ‘on’/‘off’ operations, a 100µF reservoir capacitor C2 is used across the battery supply. Its advantage will be more obvious when the IR transmitter is powered by ordinary batteries. Capacitor C2 supplies extra charge during ‘switching on’ operations.

As the MOSFET exhibits large capacitance across gate-source terminals,a special drive arrangement has been made using npn-pnp Darlington pair of BC547 and BC557 (as emitter followers), to avoid distortion of the gate drive input. Data(CMOS-compatible) to be transmitted is used for modulating the 38 kHz frequency generated by CD4047 (IC1).

However, in the circuit shown here, tactile switch S1 has been used for modulating and transmitting the IR signal. Assemble the circuit on a general-purpose PCB. Use switch S2 for power ‘on’/‘off’ control. Commercially available IR receiver modules (e.g., TSOP1738) could be used for efficient reception of the transmitted IR signals.

Medium Power FM Transmitter

2009-01-15T10:42:00.002+07:00

The range of this FM transmitter is around 100 metres at 9V DC supply.
The circuit comprises three stages.The first stage is a microphone preamplifier built around BC548 transistor.The next stage is a VHF oscillator wired around another BC548. (BC series transistors are generally used in low-frequency stages. But these also work fine

in RF stages as oscillator.) The third stage is a class-A tuned amplifier that boosts signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter.
Coil L1 comprises four turns of 20SWG enamelled copper wire wound to 1.5cm length of a 4mm dia. air core. Coil L2 comprises six turns of 20SWG enamelled copper wire wound on a 4mm dia. air core.

Use a 75cm long wire as the antenna. For the maximum range, use a sensitive receiver. VC1 is a frequency-adjusting trimpot. VC2 should be adjusted for the maximum range. The transmitter
unit is powered by a 9V PP3 battery. It can be combined with a readily available FM receiver kit to make a walkie-talkie set as shown in Fig.3.?

Download Scematic Diagram

2009-01-15T10:03:00.001+07:00

StaiRcaSeliGhtWith.PDF

STRESS METER.pdf

TELE CONFERENCING SYSTEM.pdf

TELEPHONE RECEIVER.pdf

TELEPHONE OPERATED.PDF

SPEED CONTROL OF DC MOTOR USING.PDF

SOLAR LIGHTING SYSTEM.PDF

SONG NUMBER DISPLAY.pdf

SOLID STATE REMOTE CONTROL SWITCH.pdf

STABILISED POWER SUPPLY WITH SHORT-CIRCUIT INDICATION.pdf

Sine Wave Generator.pdf

SMART CELL PHONE HOLDER.pdf

SOLAR BUG.pdf

SMART FOOT SWITCH.pdf

SmarT VibraTion SenSor.PDF

SENSITIVE VIBRATION DETECTOR.pdf

SHADOW ALARM.PDF

SHUTTER GUARD.PDF

SIMPLE SHORT WAVE.PDF

SIMPLE DIGITAL SECURITY SYSTEM.pdf

NUMBER GUESSING GAME.pdf

PARALLEL TELEPHONE WITH SECRECYAND CALL PREVENTION.pdf

MUSICAL LIGHT CHASER.pdf

PARROT SOUNDING.PDF

NON CONTACT POWER MONITOR.pdf

MULTI BAND CW TRANSMITTER.pdf

MOTOR BIKE ALARM.PDF

MULTI SWITCH DOOR BELL WITH INDICATORS.pdf

MULTI MELODY GENERATOR.PDF

Muscul Arsti MulAtor.PDF

MOBILE CELL PHONE CHARGER.pdf

MICRO MOTOR CONTROLLER.pdf

MOSFET BASED PREAMPLIFIER FOR FM RADIO DXing.pdf

MOBILE SHIELD.PDF

MOBILE BUG.PDF

MAT SWITCH.PDF

MEDIUM POWER FM TRANSMITTER.pdf

MAINS OPERATED CHRISTMAS STAR.pdf

MAINS SUPPLY FAILURE ALARM.pdf

MAINS MANAGER.pdf

LITTLE DOOR GUARD.pdf

LIGHT FENCE.PDF

LOW POWER VOLTAGE DOUBLER.PDF

Long RangE IR TRansmITTER.PDF

LOW COST ELECTRONIC QUIZTABLE.pdf

LAPTOP PROTECTOR.PDF

LED BASED MESSAGE DISPLAY.pdf

LEAD ACID BATTERY CHARGER WITH VOLTAGE ANALYSER.pdf

LED LIGHTING FOR CHRISTMAS.PDF

KNOCK ALARM.pdf

INFRARED REMOTE CONTROL TIMER.pdf

IR MUSIC TRANSMITTER AND.PDF

INFRARED TOYCAR MOTOR CONTROLLER.pdf

IR BURGLAR DETERRENT.pdf

INTRUDER RADIO ALERT SYSTEM.pdf

INFRARED FIRE CRACKER IGNITER.PDF

INFRARED BUG.PDF

HIT SWITCH.pdf

Infrared Cordless Headphone.pdf

IC Controlled Emergency Light with Charger.pdf

GLOW PLuG CONTROLLER.pdf

FULLY AUTOMATIC EMERGENCY LIGHT.PDF

HIGH WAY ALERT SIGNAL LAMP.pdf

HEAR INGAID.PDF

HEAT SENSITIVE SWITCH.pdf

FLASH INGCUM RUNNING LIGHT.pdf

FUEL RESERVE INDICATOR FOR VEHICLES.pdf

FRIENDLY CHARGER FOR.PDF

FRONT DOOR GUARD.PDF

FLYING SAUCER.PDF

FIRE ALAR MUSING THERMISTOR.pdf

ELECTRONICSECURITY SYSTEM.pdf

ELECTRONIC MOTOR STARTER.pdf

ELECTRONIC WATCHDOG.pdf

FLASHING BEACON.pdf

Electronic Card Lock System.pdf

Electronic Jam.pdf

DTMF RECEIVER ICMT8870 TESTER.pdf

Dual Channel Digital Volume Control.pdf

ELECTRONIC HORN.PDF

DIGITAL DICE.pdf

Drinking Water alarm.PDF

digital stopwatch.pdf

DICE WITH 7 SEGMENT DISPLAY.PDF

DIGITAL SPEEDO METER.pdf

CRYSTAL CONTROLLED TIME BASE GENERATOR.pdf

CURRENT SENSOR.PDF

DC MOTOR CONTROL USING A SINGLE SWITCH.pdf

COMPUTERISED UNIVERSAL TIMER.pdf

DESKTOP POWER SUPPLY.PDF

Car anTi THefT Guard.PDF

Charge Monitorfor 12V Recharge ableLead-acid Battery.pdf

CHILDS LAMP.pdf

CLOCK TIMER.PDF

CLAP SWITCH.pdf

BAttery level indicAtor.pdf

AUTOMATIC OFF TIMER FORCD PLAYERS.pdf

BRAKE FAILURE INDICATOR.pdf

BELL CUM LIGHT CONTROLLER.PDF

Bicycle indicaTor.PDF

AUTOMATIC SCHOOL BELL

Laptop Protector

2009-01-11T16:43:00.001+07:00

Protect your valuable laptop against theft using this miniature alarm generator. Fixed inside the laptop case, it will sound a loud alarm when someone tries to take the laptop. This highly sensitive circuit uses a homemade tilt switch to activate the alarm through tilting of the laptop case. The circuit uses readily available components and can be assembled on a small piece of Vero board or a general purpose PCB. It is powered by a 12V miniature battery used in remote

control devices. IC TLO71 (IC1) is used as a voltage comparator with a potential divider comprising R2 and R3 providing half supply voltage at the non-inverting input (pin 3) of IC1. The inverting input receives a higher voltage through a water-activated tilt switch only when the probes in the tilt switch make contact with water.

When the tilt switch is kept in the horizontal position, the inverting input of IC1 gets a higher voltage than its noninverting input and the output remains low. IC CD4538 (IC2) is used as a monostable with timing elements R5 and C1. With the shown values, the output of IC2 remains low for a period of three minutes. CD4538 is a precision monostable multivibrator free from false triggering and is more reliable than the popular timer IC 555.

Its output becomes high when power is switched on and it becomes low when the trigger input (pin 5) gets a low-tohigh transition pulse. The unit is fixed inside the laptop case in horizontal position. In this position, water inside the tilt switch

effectively shorts the contacts, so the output of IC1 remains low. The alarm generator remains silent in the standby mode as trigger pin 5 of IC2 is low. When someone tries to take the laptop case, the unit takes the vertical position and the tilt switch breaks the electrical contact between the probes. Immediately the output of IC1 becomes high and monostable IC2 is triggered. The low output from IC2 triggers the pnp transistor (T1) and the buzzer starts beeping.

Assemble the circuit as compactly as possible so as to make the unit matchbox size. Make the tilt switch using a small (2.5cm long and 1cm wide) plastic bottle with two stainless pins as contacts. Fill two-third of the bottle with water such that the contacts never make electrical path when the tilt switch is in vertical position. Make the bottle leakproof with adhesive or wax.

Fix the tilt switch inside the enclosure of the circuit in horizontal position. Fit the unit inside the laptop case in horizontal position using adhesive. Use a miniature buzzer and a micro switch (S1) to make the gadget compact. Keep the laptop case in horizontal position and switch on the unit. Your laptop is now protected.