Friday, October 31, 2014

Voice Activated Home Automation

Voice recognition is no longer in infancy, especially since Siri just launched on the new iPhone 4S (although similar technology is available on Android too. This project shows you how to use it to automate your home without spending big bucks.




The core of this project is a VRBot speech recognition module.  Next, the project creator used some low-cost wireless light switches (you could also use relays).  The VRBot speech recognition module recognizes 32 custom voice commands.  Once a command is recognized, a wireless switch is activated via a radio receiver.  The setup is simple after you spend some time understanding the wireless communication protocol between the different modules.  See the project blog to engineer the whole thing yourself.
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Thursday, October 30, 2014

Speech Filter Circuit Diagram

In communications receivers and microphone amplifiers for transmitting equipment, there is frequently a need for a narrow, low-frequency band-pass filter that lets only the voice band through. This band is usually defined to be the portion of the audio frequency spectrum between approximately 300 Hz and 3300 Hz. In order to implement such a filter, we have calculated the values for two fifth-order Butterworth filters having these corner frequencies and connected them in series. The result is a band-pass filter for the desired pass-band with a skirt steepness of 100 dB/decade.  The first opamp (IC1) acts as a buffer.

Speech Filter Image :

 
image


The circuit can be powered by a unipolar supply voltage between 5 V and 18 V, which is a broad enough range that it should always be possible to find a suitable voltage when building the filter into existing equipment. The current consumption of the filter is only a few milliampères, which should rarely pose a problem. There is fairly wide selection of suitable candidates for the opamps, since the circuit is not critical in this regard. In addition to the indicated OP27A, you could consider using a TL081N or even an old-fashioned 741.

Speech Filter Circuit diagram : 
Speech-Filter-Circut-Diagram
Speech Filter Circuit Diagram 


Due to unavoidable spreads in component values, the pass-band curve of the filter will never be completely perfect in actual practice. However, the deviations will be very small and in any case inaudible. In the pass-band region, the gain is approximately unity. The printed circuit board design shown here allows the speech filter to be built in a very compact form, which can be an important factor if it must be fitted into existing equipment. You can quickly check the fully assembled circuit by momentarily measuring the voltages at the inputs and out-puts of the three opamps. Half of the supply voltage should be present at all of these locations.

PCB Layout :


Parts LIST:
Resistors:
R1.R2 = 22kΩ
R3,R11,R12,R18,R19 = 100kΩ
R4 = 470Ω
R5 = 150Ω
R6 = 10kΩ
R7 = 18kΩ
R8 = 15kΩ
R9 = 33kΩ
R10 = 82kΩ
R13-R17 = 3kΩ3
Capacitors:
C1,C8,C14,C15 = 100nF
C2 = 1µF MKT
C3-C7,C11 = 22nF
C9 = 33nF
C10 = 18nF
C12 = 10nF
C13 = 4nF7
C16,C17 = 10µF 16V
Semiconductors:
IC1,IC2,IC3 = OP27A, TL081CN
Miscellaneous:
Bt1 = 9-V battery



Author: G.Baars
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Wednesday, October 29, 2014

Solar Voltage Regulator

Our energy-hungry and environmentally aware society has been dawdling to put together moral depletion of the suns "on the house" power. But at once its to finish taking inedible. Using the suns stage at once, in lieu of cooking and other applications, is already a public and current knowledge stylish countries so as to hold good weather. angry-hose panels are in this day and age used in many parts of the the human race, in combination with a chatter or else thrilling powered fill with tears radiator to help old hat what time the weather doesnt help. But next to the same schedule, thrilling solar panels are still expensive, mitigating their use barely while a novelty, or else fashionable locations everywhere minute power is wanted, and bringing in infomercial power would ensue even added expensive.

A solar stimulating power usage needs panels in place of generation, batteries for cargo space, a control device to keep the batteries surrounded by a safe operating range, and in a little luggage a power converter for AC output. For folks who need to position up a hardly any panels meant for a summer lodge, a ferry, a remote peak installation, or else whatever, Im enclosed provided that a version of the controller circuit with the purpose of I engage in used popular a luck of such installations.

Such a solar panel regulator be supposed to make by slightest two operations: The obvious solitary is defensive the battery from overprice next to period of deep sun and diminutive consumption, and the other is defensive it from unnecessary discharge in bad weather conditions. Both cheat and deep discharge are injurious to a battery.

In place of regulating a solar panels output, in attendance are several achievable ways. A linear progression supervisory body can be real used, but has the disadvantage of causing more or less voltage discontinue and having some home power consumption by era whilst the sun is weak and the load is serious. Its much better to practice a shove supervisor, which is dormant on such epoch, and springs to life no more than while present is surplus energy. in lieu of this rationalize, largely solar panel regulators use the push scheme, the lone presented at this point being refusal exception.

But such force regulators roll up in the sphere of two flavors: Most mercantile units are ON-inedible regulators. to income, they hold a regular switch device, as a rule often a transistor otherwise MOSFET, at times even absolutely a relay, to facilitate stays rancid until the battery reaches done voltage, and followed by switches in, shorting off the panel until the battery voltage has dropped rancid. Then the bursting panel current is switched on again. The simply lead of this method is tos second-rate. The power switch operates with very low power dissipation, allowing a miniature, low cost construction.

But the disadvantages of this routine are key: The voltage output is all the epoch irregular linking in the region of 13 and 14.5V. The battery is cycling between getting charge too much and having to transfer all the load current, which strictly reduces the batterys time. And in the field of the event of battery disengagement otherwise failure, the gauge cycles quickly, applying pulses of broad panel voltage to the output, which can raze aware equipment powered by the classification!

The circuit presented at this time uses linear jolt supervision. Simply verbal, it burns rotten all surplus energy from the panel, keeping output voltage constant. next to epoch what time the solar panel output is equal otherwise greater than the load, and the battery is fully charged, the load gets its power from the panel, while the battery rests by the side of jam-packed charge. Five years battery time are entirely usual with this usage, while the same batteries stay fresh no more than two to three years what time used with pulsing regulators!

The next dependability of the valve is watching in excess of the battery voltage, and dropping rotten the load whilst the battery gets discharged too much. main batteries are harshly damaged by deep discharges, so its far preferable to plunge rotten the load, in that case to hold the battery stop working hip a bad weather spell. This controller is designed in support of 12V systems employing panels of up to 7A whole current, and many of not finished 20A. It can subsist certainly modified for greater currents.



U1A compares an bendable sample of the offer battery voltage to a 5V reference from a highly committed source. According to the findings, it controls the power transistors Q1 and Q2, which push inedible the overload power generation from the panel. A diode (D1) avoids battery voltage to operate back to the panel under rejection-light condition. To avert blurred voltage control due to unreliable diode globule, the sample is taken from the battery boundary, even if this course a very minor power devastate.

The power resistors R1 and R2 are dimensioned now such a way so as to under greatest shunting, these resistors command dissipate almost all power (with reference to 100W utter), leaving the transistors running cool. The highest dissipation in the transistors happens as soon as the control device is dissipating partly of the panel output; in this litigation, all transistor desire dissipate about 12W.

U1B is a Schmitt trigger to facilitate compares the battery voltage to the same firm reference of the other section, but in favor of an additional end: It controls the load switch Q3. This circuit resolve disconnect the load if the battery gets close to deep discharge, and reconnect it simply whilst renew is well underway. The unconstructive bank of the load is switched, simply as N-channel MOSFETs are much cheaper and better than P-channel ones.

Module annotations:

D1 can survive in the least diode with the purpose of can safely stay alive the panels current. If the panel has a very low voltage output (take away than 33 cells concerning cycle), it is an help to employ a Schottky diode in this place. Q1 and Q2 are conventional power Darlington transistors. They need to take place heatsinked meant for safe lengthy-call surgical treatment on the 12 Watt dissipation level. with the aim ofs trouble-free sufficient to act, but many newcomers misjudge how much thermal resistance is introduced by a mica insulator! map on 1K/W thermal resistance inside each one transistor, two epoch because much concerning the insulator (if you usage some), and 370K safe junction hotness. used for archetypal environmental conditions, this makes you need a heatsink having a thermal resistance of approaching 1.3K/W. If it is better, you induce new safety margin.

R1 and R2 want gain to be located made by combining a integer of power resistors concerning matching. no problem, you need to make up two resistor arrays of 4 Ohm, 80W all! This 80W dignitary includes a reasonable safety margin. These resistors long for give off a slice of part, and you may perhaps cook your coffee on them! transpire guaranteed to mount them in such a way with the aim of they have lots of ventilation, and that the heat from them will not contact the other components. R3 and R4 could to partake of be built from like combinations too, as of the low respect of merely 0.15 Ohm.

U2 is a voltage reference IC. You cannot exchange it by a standard Zener diode! Zeners are much too unstable! If you cant acquire this central processing unit locally, you could exhaust the ever-present 7805 manager as a substitute, but the power drain from the battery motivation stay upper. popular this project, of gush you dont need R8, but you would need a 1uF capacitor by the 7805 output. Q3 is a power MOSFET with the aim of has a very low Rds(on). You may well use a unusual lone, provided that it has a resistance to facilitates low an adequate amount of meant for your devotion. You could use several in equivalence. The individual I used has low loss even by the side of heaps of 20A, and can sell much added!

Calibration:

in the past the circuit is assembled, calibration is quite relaxed. unite the panel, leave the battery and load disconnected. With a pleasant sun on the panel, adjust RV1 designed for the desired voltage by the battery output. I advise 13.8V for sealed batteries, and 14 to 14.2V for uncluttered cell ones, to which wet can be alive added if vital.

instantly you need either a wavering power supply connected to the battery defenses, or else several kind of variable load. You may well in addition purpose your panel in the role of variable power supply, by sloping it away from the sun while a fixed load is connected to the battery position. The understanding is to adjust the voltage by the side of the battery ranks to the desired put down the lid-rancid evaluate (I put in a good word for 11.5V), and followed by move RV2 until Q3 shuts inedible, to the same degree indicated by a voltmeter across the load output, a 12V light bulb, before whatever you can use to detect it. similar to Q3 has go home for the day inedible, strengthen the voltage across the battery position and get by which level Q3 switches on again. This must occur on top of 12.6 and not more than 13.4 in opposition to. You may well enjoy to retouch RV2 and look on behalf of a compromise linking ON and rotten voltages. If your components are not too much revealed of regard, so therefore both potentiometers be supposed to have ended up rationally close to the base point.

Using additional panels:

You can handle this controller in support of bigger installations. Simply add lone band together like R1-Q1-R3 for both added 3.5A panel, and use a diode for D1 so as to handles the sum current. Remember that bulky diodes need high temperature sinks! U1A can drive by the side of slightest 8 such transistors. If you propose to build a really large arrangement, you might covet to add an emitter follower flanked by U1A and the power transistors. If you need to nickname large load currents, you can place to the same extent many MOSFETs wearing equivalent as you need. in attendance is rejection doable driving limitation in this basis.
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Tuesday, October 28, 2014

Cell Phone Detector circuit diagram

The circuit can detect both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The moment the bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases.
An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuit detecting gigahertz signals is required for a mobile bug.

Here the circuit uses a 0.22μF disk capacitor (C3) to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.
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Monday, October 27, 2014

600 Watt Quasi Amplifier With Mosfet IRFP460

Two versions of a robust module capable of delivering high powwer for extended periods.  The Actrk400 uses 6 n-channel Mosfets in the output stage to deliver around 400 watts into 4 ohms while the Actrk600 uses 12 n-channel Mosfets in the output stage to deliver power in excess of 600 watts into 4 ohms.  One constructor has achieved almost 900 watts with the Actrk600 layout using 12 IRFP460 Mosfets.

600 Watt Quasi Amplifier With Mosfet IRFP460

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Friday, October 24, 2014

Alarm Digital Clock

The circuit diagram on behalf of the digital watch. 2x16 LCD is connected to the docks 2 of AT89C51. P1.0 of uC force provide the SCL (entertainment in installments clock) and P1.1 SDA (serial data) for I2C message.

Nearby are four switches connected to the uC, having the status of given away within the think. Function of the keys are same as unmistakable from their names.

at what time the power supply is switched on it willpower confer you the default appointment and schedule, but anon you can convert it to the desired appraise. taking into account setting as soon as, the backup battery will keep the timepiece ticking even after the power is not near.

Alarm Digital Clock Schematic

A trivial a propos I2C:
There are basically four most important conditions wearing I2C protocol.
1) Start Condition
2)block Condition
3)Data Validity
4)Acknowledgement

1)Start Condition:
while SCL is sharp and SDA H->L, force take place taken since start condition meant for the exchange of ideas.
2)Stop Condition:
once SCL is high and SDA L->H, will generate a stop condition.
3)Data Validity:
after SCL is high at hand must be refusal chande voguish SDA line lone followed by the data is legally binding, the data loose change be supposed to be there made only as SCL is low.
4)Acknowledgement:
taking into account transfer of lone byte of data the reciever has to acknowledge the sender pro the winning reception. for this the sender add up to the SDA line soaring and reciever pulls down the SDA low, which tells the sender so as to data has reached safely.
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Thursday, October 23, 2014

12V Lead Acid Battery Charger

The follow-on charge voltage is something like 14.4V, which is fine on behalf of charging a gelled before AGM battery to full up charge, but is too high point in the same way as a seep mount, so dont leave this charger permanently connected to a battery. If you would like to make sure of truthful to facilitate, in that case add a subsequent diode in vogue string with D3! at hand is a LED connected as a charge indicator. It hope against hope light after the charge current is senior than a propos 150mA. The utmost charge current will ensue roughly 400mA. in attendance is an supplementary output, with the aim of provides about 20V on thumbs down load (depending on input voltage), and comes down for example the load increases. I incorporated this designed for charging 12V, 4Ah NiCd packs, which require precisely a restricted current but not a limited voltage for charging.

12V Lead Acid Battery Charger Circuit Schematic Diagram

document so as to if the charge output is abruptly-circuited, the overcurrent protection of U2 command kick arrived, but the current is still soprano an adequate amount of to impairment the diodes, if it lasts. So, dont short the output! If in its place you short the assisting output, the fuse be supposed to blow. I built this steed into a slight home-grown aluminum sheet corral, using dull-bug construction variety. Not very tidy, but it machinery. take notice of the elongated leads on the power resistors. They are needed, as with shorter leads the resistors long for unsolder themselves, to the same extent they contract pretty intense! The transistors and the control device IC are bolted to the indictment, which serves as stage sink. The transistors dont heat up very much, but the IC does.
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Sunday, October 19, 2014

LM2577 12V to 16V DC DC Converter

The circuit is a boost step-up regulator based around an LM2577-ADJ voltage regulator chip and a few other discrete components. Resistors R1 and R2 set the regulated output voltage.


A switch inside the voltage regulator closes between pins 4 and 3, causing current to flow through the inductor to ground. When the switch is released a few microseconds later, a back-EMF ‘kick’ is produced by the inductor, resulting in a positive pulse with respect to the input voltage. This pulse charges the output capacitor via the schottky diode, which tends towards an equilibrium voltage.
The switch continues to oscillate, the diode preventing the switch from shorting the output capacitor during the ‘on’ phase. The output voltage is monitored via the voltage divider R1/R2, causing the duty cycle of the switch oscillator to be continuously regulated in order to maintain a constant output voltage under varying loads.
Source: http://www.mcrent.com/workbench/dcdc1216/
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Saturday, October 18, 2014

Wurlitzer 680 Schematic


Wurlitzer 680 Schematic
Schematics for Wurlitzer 680
Wurlitzer Model 680 Amplifier Schematic
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Thursday, October 16, 2014

Build a Period To Voltage Converter Circuit Diagram

The input signal drives ICD. Because ICD`s positive input (V+) is slightly offset to + 0.1 V, its steady stateoutput will be around +13 V. This voltage is sent to ICC through D2, setting ICC`s output to +13 V. Therefore, point D is cut off by Dl, and CI is charged by the current source. Assuming the initial voltage on CI is zero, the maximum voltage (^Cinax) is given by: 

When the input goes from low to high, a narrow positive pulse is generated at point A. This pulse becomes -13 V at point B, which cuts off D2. ICC`s V+ voltage becomes zero. The charge on CI will be absorbed by ICC on in a short time. 

The time constant of C2 and R5 determines the discharge period— about 10 /is. ICB is a buffer whose gain is equal to (R& + R9)~Rg = lM5. ICD`s average voltage will be (1362f 1.545) + 2 = 1052/. RIO and C3 smooth the sawtooth waveform to a dc output.


Period-To-Voltage Converter Circuit Diagram

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Wednesday, October 15, 2014

Automatic Night Light Feeds Directly From the AC Line

There are many approaches to the problem of activating a light when it becomes dark, and a recent Design Idea covers this topic (Reference 1). Some approaches require a dc power supply and an electromechanical relay, but a better approach involves feeding the device directly from the ac line, minimizing the number of components 

(Figure 1).


Figure 1. The photoresistor activates the TRIAC and the load when darkness falls.

The heart of the device is a light-sensitive cadmium-sulphide resistor, PR, with a resistance of approximately 200 kΩ in the dark and decreasing to a few kilohms in the light. PR and capacitor C1 form an ac-voltage divider. In daylight, the voltage across PR is too low to generate the required gate-trigger current to turn on bidirectional ac switch Q1, thus keeping the load – usually a lamp – off. When it becomes dark, PR’s resistance rises, resulting in an increase in the TRIAC’s gate current that triggers the TRIAC and lights the lamp.

The circuit uses inexpensive, off-the-shelf components, including the VT90N1 photoresistor; a 0.1-μF, 275V capacitor; and an L2004F61 TRIAC with a load current of 4A rms, a peak blocking voltage of 200V, and a gate-trigger current of 5 mA. The exact specifications of these components are not critical; you could use others instead.

Editor’s note:
Attributes worth mentioning include the fact that the capacitor introduces a phase shift, which places the peak of the gate voltage close to the zero crossing of the load’s sine wave for optimum turn-on timing. Another benefit is thermal hysteresis, which occurs due to the reduction of the required triggering voltage and current as the TRIAC warms up after the initial turn-on. 
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Monday, October 13, 2014

LM35 Digital Remote Thermometer circuit and explanation

Digital Remote Thermometer circuit with very high accuracy temperature measurement features are set using the IC package numbers
Receiver Module Functional
LM35 (Linear temperature sensor IC) through a power amplifier and voltage change as a form of high frequency. To signal through the AC power cord.
For the receiver circuit and display module will receive the high frequency AC power cord and decode out to be the actual temperature figures with digital IC No. CD4553 Three-digit BCD Counter IC) and IC-CD4511 (BCD-to-7-Segment Latch / Decoder / Driver IC).
 Electronic Part List
Receiver Module
R1   100K 1/4W
C1,C2  47nF 400V
R2  1K 1/4W
C3,C7  1nF 63V
R3,R4,R6-R8  12K 1/4W
C4  10nF 63V
R5  47K 1/4W
C5,C6,C10  220nF 63V
R9-R15  470R 1/4W
C8  1000uF 25V
R16  680 1/4W
C9  100pF 63V
D1,D2,D5  1N4148
D3,D4  1N4002
D6-D8  Common-cathode 7-segment LED mini-displays
IC1  403
IC2   4518
IC3  78L12
IC4  4017
IC5  4553
IC6  4511
Q1 BC239C
Q2-Q4  BC327

L1  Primary (Connected to C1 & C2): 10 turns
Secondary: 100 turns
Wire diameter: 0.02mm. enameled
Plastic former with ferrite core. Outer diameter: 4mm.

T1 220V Primary, 12+12V Secondary 3VA Mains transforme
Remote Module Functional
    The frequency pulses from the wall and safely isolated by C1, C2 and L1 are amplified by Q1, diodes D1 and D2 of the peak limit his admission. The pulses are filtered by C5, IC1b square, divided by 10 in IC2B and sent for final countdown clock input of IC5. IC4 is the time base generator: it provides IC1b and reset pulses to the making and IC5 locks and opening time of IC5 at 1Hz frequency. It is powered by a 5Hz square wave obtained from the frequency 50Hz sector Retrieved from T1 secondary, squared and divided by 10 in IC1C IC2A. IC5 readers across the cathodes scenes Q2, Q3 and Q4 with a rate determined by frequency multiplexing C7. Readers also shows three anodes in parallel by the BCD to 7 segments decoder IC6. In summary, the input pulse power on, say, 2kHz frequency be divided by 10 and displayed as 20.0 ° C



Remote Circuit
Electronic Part List


Remote Module
R1,R3  100K 1/4W
C1  220nF 63V
R2  47R 1/4W
C2  10nF 63V
R4  5K 1/2W Trimmer Cermet
C3  1uF 63V
R5  12K 1/4W
C4,C6  1nF 63V
R6  10K 1/4W
C5  2n2 63V
R7  6K8 1/4W
C7,C8  47nF 400V
R8,R9  1K 1/4W
C9  1000uF 25V
D1  1N4148
IC1  LM35
D2,D3  1N4002
IC2  LM331
D4   Red LED
IC3  78L06
Q1  BC238
Q2 BD139

L1   Primary (Connected to Q2 Collector): 100 turns
Secondary: 10 turns
Wire diameter: 0.02mm. enameled
Plastic former with ferrite core. Outer diameter: 4mm.

T1   220V Primary, 12+12V Secondary 3VA Mains transformer
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Sunday, October 5, 2014

5 to 15V 400mA Regulated Power Supply

5 to 15V 400mA Regulated Power Supply

This project is a simple DC regulated power supply that has a variable DC voltage range from 5V to 15V. It can supply current up to a 400mA to power the various circuits for your electronic projects. The voltage output is varied by using the potentiometer VR1. In this circuit, the input line power supply is designed for 240VAC. If 110VAC input is used, change the ratings of the varistor to 150VAC and the transformer ratio to 110V/12V.

Fuse F1 is used as a protection in case there is any short circuit in the circuit. Varistor V1 is connected in parallel to the input of the line voltage to clamp the surge voltage from the line to a reasonable level that helps to protect the transformer and other circuitry. Once the voltage level surge to a high level beyond the ability of the varistor to absorb it, fuse F1 or varistor V1 or both will burn. If this circuit failed after a period of operation, check that the fuse and the varistor are still in good condition or else replace them.

Diodes D1, D2, D3 and D4 are used to rectify the 12VAC voltage to DC voltage. Electrolytic capacitor E1 is used as a smoothing capacitor to reduce the ripple of the DC voltage. The DC voltage is fed into the input of 7805 regulator where the output DC voltage is obtained. Changing the value of VR1 will change the output of the DC voltage. Capacitor C1 is used to filter out high frequency component from the power supply.
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Friday, October 3, 2014

FM Radio Using TDA7000

Here’s a design simple circuit using TDA7000 that can build an FM radio with a minimum of components; most of them so easy to manage, like the ceramic capacitors that do not require polarization and only two resistors. Here’s the figure of the circuit;


The components you will employ will be of small dimensions and low cost. Even the integrated circuit itself, the TDA 7000, is not of great cost.
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Thursday, October 2, 2014

Temperature Data Logger 4 Channel Four Channel

Heres a temperature data logger circuit, an 8-pin microcontroller based circuit for temperature data logging via the serial port of any computer from between 1 and 4 remote digital temperature sensors, DS1820/DS18S20 made by Dallas.

Features of the Temperature Data Logger are are follow:
  • Provides realtime data via the serial port.
  • Interfaces up to 4 x DS1820 sensors.
  • Accuracy to 0.5 degrees centigrade
  • Fahrenheit scale selected by a jumper
  • No external power required
  • Data stream easily logged & processed.
  • Over 200m distance for each sensor


The sensors of temperature data logger may be located more than 200 yards from the PCB. Just check that the remote VDD is above 4.4Volts. Its recommends 10uF tantalum capacitors be placed across the power supply pins (not supplied) to minimise random errors and possibly get increased range.


Download :
- Circuit Schematic and Parts List
- Sensor Specs DS1820 DS18S20
- Software for Windows and DOS
- Source Code
- Terminal Emulator 
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Wednesday, October 1, 2014

Stereo Cassette Player Circuit Diagram

As the equipment is mains operated, the battery compartment can be removed. The loudspeaker can also be removed as it is no longer required; the mains transformer may be fitted in its position. lf so desired, you · may add a neon indicator and a mains switch as well. Assembly cost and time As to the cost, you are lucky if you are in Delhi and can shop around Lajpatrai Market. The cost would be around Rs 400 or even less if you know the way the Delhi market works. Outside Delhi, the cost would be around Rs 450 much less than you would pay for a mono cassette recorder. lt is certainly worth every rupee that you spend as it would enable you to use the stereo system that you already own. Assembly time would depend on your familiarity with the electronic circuitry, but it would be a weekend well spent. (Time you need for shopping for the mechanism and parts would be extra.)  Two direct·coupled transistors are used in the preamplifier, with base bias for the first provided from the emitter of the second. This system of biasing results in heavy DC ° feedback that ensures stable operation of the circuit.


With the DC operating conditions stabilised, as mentioned, the AC characteristics of the preamplifier are also stabilised. The AC feedback from the collector of the second A transistor to the emitter of the first mainly provides frequency equalisation high frequency de-emphasis to compensate for the recording characteristics. To keep up a high signal- to-noise ratio in the high frequency range, the highs are boosted during recording. (This is called pre-emphasis.) A reverse treatment is therefore essential in playback to reproduce the original high-frequency signal correctly. An R- C combination, 22k with 5nF capacitor, carries out this function adequately.

Output from the preamplifier is fed to a 741 op-amp set for a gain of about ten. The total gain is thus around 1000 so as to provide a 100mV signal from 100uV input normally expected from a cassette head. Use of discrete preamplifier at the input ensures a good signal-to-noise ratio, and the op-amp at the output with a wide dynamic range ensures that there is no chance of signal being driven into saturation. The signal from the preamp is fed to inverting terminal number 2 of the op-amp through 5.6k resistor. Negative feedback from the output terminal number 6 through 68k is also provided to terminal number 2. The non-inverting terminal number 3 is set to half the supply voltage. That sets the DC voltage at terminal number 6. Thus an optimum swing of the output is possible at terminal number6. Positive supply terminal number 7 is taken to the supply positive, and terminal number 4 returned to ground. The output from the op-amp is taken out through a47k resistor to the standard 3.5 mm socket used in cassette recorders. The socket fits in a window provided in a ‘Sony’ cabinet for mono cassette recorder. `  
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