electronics

Train Detector

2009-12-22T16:02:00.001+07:00

Regulted dual power supply

2009-12-22T15:57:00.002+07:00

Astable Multivibrator

2009-12-22T15:38:00.001+07:00

Bell Controller PIC 16F628A

2009-12-22T15:05:00.007+07:00

Inductance Meter Schematic

2009-05-30T15:16:00.008+07:00

The schematic shown below converts an unknown inductance into a voltage that can be displayed on a DVM or VOM. Values between 3uH and 500uH are measured on the L (low) range and from 100uH to 7mH on the H (high) range. NAND gate ICA is a two frequency RC square-wave oscillator. The output frequency (pin 3) is approximately 60 KHz in the L (low) range and 6 KHz in the H (high) range. The square-wave output is buffered by ICB and applied to a differentiator formed by R3 and the unknown inductor; LX. The stream of spikes produced at pin 9 decay at a rate proportional to the time constant of R3-LX. Because R3 is a constant, the decay time is directly proportional to the value of LX. ICC squares up the positive going spikes, producing a stream of negative going pulses, at pin 8 whose width is proportional to the value of LX.

They are inverted by ICD (pin 11) and integrated by R4-C2 to produce a steady dc voltage at the + output terminal. The resulting dc voltage is proportional to LX and the repetition rate of the oscillator. R6 and R7 are used to calibrate the unit by setting a repetition rate that produces a dc voltage corresponding to the unknown inductance. D1 provides a 0.7 volt constant voltage source that is scaled by R1 to produce a small offset reference voltage for zeroing the meter on the L (low) inductance range.

When SW1 is L (low), mV corresponds to uH, and when H (high), mV corresponds to mH. A sensitive VOM may be substituted for the DVM with a sacrifice in resolution.

Test and Calibration...

Short the LX terminals with a piece of wire and connect a DVM set to the 200-mV range to the output. Adjust R1 for a zero reading. Remove the short and substitute a known inductor of approximately 400uH. Set SW1 to the L (low) position and adjust R7 for a reading equal to the known inductance. Switch SW1 to the H (high) position and connect a known inductor of about 5mH. Adjust R6 for the corresponding value. For instance, if the actual value of the calibration inductor is 4.76mH, adjust R6 so the DVM reads 476mV.

All components are 10% tolerance. 1N4146 or equivalent may be substituted for D1. An LM7805 may be substituted for the 78L05. All fixed resistors are 1/4 watt carbon composition. Capacitors are in uF. R3 value may need to be increased or decreased if calibration cannot be achieved as described in text.

Capacitance Meter Schematic

2009-05-30T14:57:00.005+07:00

The schematic shown below measures capacitance from 2.2pF to 1000pF in the L (low) range, and from 1000pF to 2.2uF in the H (high) range. ICD of the 74HC132 (pin 11) produces a 300 Hz square-wave clock. On the rising edge CX rapidly charges through D1. On the falling edge CX slowly discharges through R5 on the L (low) range and through R3-R4 on the H (high) range. This produces an asymmetrical waveform at pin 8 of ICC with a duty cycle proportional to the unknown capacitance; CX. This signal is integrated by R8-R9-C2 producing a dc voltage at the negative meter terminal proportional to the unknown capacitance. A constant reference voltage is produced at the positive meter terminal by integrating the square-wave at ICA, pin 3. R6 alters the symmetry of this square-wave producing a small change in the reference voltage at the positive meter terminal. This feature provides a zero adjustment on the L (low) range. The DVM measures the difference between the positive and negative meter terminals. This difference is proportional to the unknown capacitance.

Test and Calibration…

Without a capacitor connected to the input terminals, set SW2 to the L (low range) and attach a DVM to the output terminals. Set the DVM to the 2-volt range and adjust R6 for a zero meter reading. Now connect a 1000pF ‘calibration’ capacitor to the input and adjust R1 for a reading of 1.00 volt. Next, switch SW2 to the H (high) range and connect a 1.00uF ‘calibration’ capacitor to the input. Adjust R3 for a meter reading of 1.00 volt. The ‘calibration’ capacitors do not have to be exactly 1000pF or 1.00uF, as long as you know their exact value. Fro instance, if the ‘calibration’ capacitor is known to be .940uF, adjust the output for a reading of 940mV.

All components are 10% tolerance. An LM7805 may be substituted for the 78L05. All fixed resistors are 1/4 watt carbon composition. Capacitors are in uF unless otherwise indicated.

Inverter 500V 12VDC

2009-04-24T13:49:00.006+07:00

see here!

LM3914 battery monitor

2009-04-24T13:43:00.008+07:00

All resistors are 5 or 10 percent tolerance, 1/4-watt.
This circuit should be powered by its own battery, otherwise you might get an innacurate reading. hook the to-be-monitored battery to pin 5 of the chip.

Low Power LED Voltmeter by LM3914

2009-01-14T15:32:00.002+07:00

This is a low power voltmeter circuit that can be used with alternative energy systems that run on 12 and 24 volt batteries. The voltmeter is an expanded scale type that indicates small voltage steps over the 10 to 16 volt range for 12 volt batteries and over the 22 to 32 volt range for 24 volt batteries. Power consumption can be as low as 14mw when operated from 12V and 160mw when operated from 24V. It is possible to set the meter to read equal steps across a variety of upper and lower voltages. The meter saves power by operating in a low duty-cycle blinking mode where the LED indicators are only on and consuming power briefly during a repeating 2 second cycle. The circuit may be switched to a high power mode where the active LED stays on at all times.

Different colored LEDs may be used for the voltage level indicators, this allows the battery state to be read in the dark. With the new blue LEDs, it is possible to have a nice looking rainbow of colors using two each of red, amber, yellow, green, and blue LEDs. The circuit will also work with inexpensive and common red LEDs. If the circuit is to be used in sunlight, ultra-bright LEDs should be used, although even those may be hard to read without some kind of sun shield.

Typical uses include the monitoring of portable battery operated systems and indoor wall mounted home power system charge indicators. The cost of the parts for the circuit is around $25.00 (US) and the parts are commonly available, except for the optional blue LEDs. If blue LEDS are used, expect to pay a premium for them, they cost several dollars each, compared to around 15 cents for the other colors. The blue LEDs do look nice in any case.

The circuit may be built with either the CMOS ICM7555 timer or the more common bipolar 555 timer. The 7555 timer will provide much more efficient operation and should be used for systems with small batteries. The volt meter works nicely with the solar charge controller and low voltage disconnect circuits described in the home-brew section of Home Power #60 and #63.

500W low cost 12V to 220V inverter

2009-01-14T14:41:00.005+07:00

Using this circuit you can convert the 12V dc in to the 220V Ac. In this circuit 4047 is use to generate the square wave of 50hz and amplify the current and then amplify the voltage by using the step transformer.

How to calculate transformer rating

The basic formula is P=VI and between input output of the transformer we have Power input = Power output
For example if we want a 220W output at 220V then we need 1A at the output. Then at the input we must have at least 18.3V at 12V because: 12V*18.3 = 220v*1
So you have to wind the step up transformer 12v to 220v but input winding must be capable to bear 20A.

12v Fluorescent Inverter by TIP3055

2009-01-14T14:27:00.002+07:00

This is a low-cost project for 20 or 40 watt fluorescent tubes. However the most efficient is to use a 40 watt tube (or two 20 watt tubes in series). It’s a circuit you can put together from junk box components or build from a kit. It’s very simple to build and requires no printed circuit board.
The transformer is hand-wound on a ferrite rod (from an old transistor radio) and the winding wire can be salvaged from an old transformer.

Simple Circuit Charges Lead-Acid Batteries

2009-01-14T14:03:00.003+07:00

Charges lead-acid batteries in the conventional way: A current-limited power supply maintains a constant voltage across the battery (2.4V/cell or so, as specified by the battery manufacturer) until the charging current decreases below a current threshold defined by the capacity of the battery. At this point, the charger is placed in a trickle-charge mode. The current threshold is typically 0.01C, where C refers to the battery capacity, which is specified in ampere-hours. When charging a battery, the term "C rate" refers to the current required, in theory, to charge a battery to its full battery capacity C in one hour. In actuality, power lost during the charge cycle ensures that all batteries charged at their C rate take more than an hour to reach full charge. Ideally, you could charge a 5A-hr battery in one hour if the charge current is 5A. Also, ideally, a C/10 charge rate (500mA) charges the same battery in 10 hours. However, the power loss mentioned previously increases these charge times beyond the two time spans stated above.
The charging voltage involves a trade-off between cell life and charging time. High voltage minimizes the time required, but at full charge it produces a large overcharging current that shortens the battery's life by oxidizing its grid. To save battery life at the expense of charging time, you can lower this current by decreasing the charging voltage.

The ideal compromise is to charge at high voltage until the current drops to 0.01C or so and then lower the voltage to maintain a low trickle-charge current (<0.001c)>IN and allows V_IN to range above and below the charging voltage. To begin a charge cycle, press the Start switch momentarily.

IC2 measures the battery-charging current by generating a smaller (1/2000) but proportional current at the OUT terminal (pin 8). The resulting drop across R2 produces a voltage at IC1 pin 5. When the charging current drops below 0.01C, for instance, this voltage crosses the internal comparator threshold and drives LBO low. By turning off Q2, the feedback level is shifted, which changes the charging voltage to about 13.6V. The maximum available charging current depends on V_IN, the transformer's saturation current, and the current-sense resistor R1.

Eye Candy 2 A bargraph display

2009-01-14T10:55:00.003+07:00

The LM3914N is a chip specially designed to drive a bargraph display. The basic IC operates as a linear voltmeter with a 1.2v range, using 10 leds as the scale. Our need is to get full power showing all 10 leds lit, with just the bottom led lit when the fan controller is turned right down to around 6v.

To get the maximum input scaled down to suit, we use a variable potential divider VR1 on the input from the fan controller.

To raise the zero point, we add a resistor R2 between ground and the "lo" pin (#4) of the IC. The chip contains a chain of 10 x 1k resistors marking off the lit steps between zero and full-scale; we add to the first resistor in the chain (between zero and the first led pin) so there is another potential divider with 9k above the "led1 on" point, 1k+R2 below it.

The only other components needed are the 1k2 resistor R1 that sets the led current to 10mA and a couple of capacitors to get rid of any noise on the power or signal lines.

Here's a stripboard layout and construction plan with the parts list.

IC1	LM3914 and 18-pin DIL socket
C1	2.2uF 16v tantalum bead capacitor.
C2	0.47uF 16v tantalum bead capacitor
R1	1k2 resistor, 0.25W
R2	6k8 resistor, 0.25W (see notes)
VR1	10k horiz preset eg Maplin UH03D
D1-10	LEDs to suit.

Electronic, Mech, Maths and Computing 9.2 download free

2008-12-13T14:31:00.006+07:00

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Discharge Indicator for 12V Battery (Lead Acid)

2008-12-12T12:24:00.003+07:00

In perfect discharge the batteries of lead acid, exists the fear they are destroyed. This circuit makes the work, this detection of discharge, protecting the batteries from destruction. At the discharge the polar voltage of batteries 12V of Lead acid, is not allowed it is decreased under 10.8V, thus we have warning with the turn on of Led, when the voltage falls under this price. In order to we achieve the control, we needed a stability voltage and a circuit that could him compare with the checked voltage. And two these condition provide to IC1- LM 723. The terminal entry of circuit they are connected in the poles of battery. In the IC1/6 is presented a stability voltage + 7.15V, when the voltage of entry is bigger than + 9.5V. The stability voltage is applied in the IC1/5. In the IC1/4 is applied a part of voltage of entry, that is checked by the trimmer TR1. The IC1 is as voltage comparator. Thus when the voltage in the IC1/4 is bigger than the voltage in the IC1/5, then the exit of IC1/9, has low level [ L ], the Q1 is in cutting off and the LED it is off. In order to it turns on the LED should the voltage in the IC1/4 become smaller than the voltage of reference in the IC1/6, whenever the exit of IC1/9 will become high [ H ], Q1 conduct and the LED turn on. For the regulation of circuit we will need exterior supply, which is regulated has expense + 10.8V, him applies in the entry of him circuit. We regulate the trimmer TR1, so as to it turns on LED,me the least alleviation of voltage from supply. The terminal entry of circuit, it should they are connected directly in the poles of battery, it can be used where we use batteries 12V of (Lead acid).

Electronic Door Release

2008-10-12T11:00:00.004+07:00

Description
This circuit is designed to operate an electrical door-release mechanism - but it will have other applications. Enter the four-digit code of your choice - and the relay will energize for the period of time set by C4 & R4. Use the relay contacts to power the release mechanism. The standby current is virtually zero - so battery power is a realistic option.

The circuit is drawn with a 12-volt supply - but it will work at anything from 5 to 15-volts. All you have to do is choose a relay suitable for the supply voltage you want to use. Replace the SPCO/SPDT relay with a multi-pole relay - if it suits your application.

Notes:
Choose the four keys you want to use as your code - and connect them to "A B C & D". Wire the common to R1 and all the remaining keys to "E". When you press your four keys - in the right order - the relay will energize.

With the values of C4 & R4 as shown - and with R4 set to its maximum - the relay will de-energized about one minute after "D" is released. However - if you replace C4 with a 100nF capacitor - and replace R4 with a 4k7 fixed resistor - the relay will de-energize the moment "D" is released.

Any keys not wired to "A B C & D" are connected to the base of Q2. Whenever one of these "Wrong" keys is pressed - Q2 takes pin 1 low and the code entry fails. Similarly, if "C" or "D" is pressed out of sequence - Q4 or Q3 will take pin 1 low and the code entry will fail. If you make a mistake while entering the code - simply start again.

The Keypad must be the kind with a common terminal and a separate connection for each key. On a 12-key pad, look for 13 terminals. The matrix type with 7 or 8 terminals will NOT do. A 12-key pad has eight "Wrong" keys connected to "E". If you need a more secure code - use a bigger keypad with more "Wrong" keys.

NiCAD Battery Charger

2008-08-27T16:32:00.002+07:00

This circuit is an AC-powered current source designed for recharging batteries. It can crank out as much as 1 amp and can be modified to go even higher by choosing different devices for Q1. Since this circuit uses AC line voltages and currents, please exercise extreme caution during assembly, turn-on, and test.

NiCAD batteries have a capacity specification called milliamp-hours. This value called "C" is a measure of how much total current they can provide in one hour. Milliamp-hours is another way to express the energy contained in the battery. To recharge a NiCAD battery conservatively, it is common practice to pump a current of 0.1 C into the anode or positive terminal for about 12 hours.. Therefore, if you had a D-size NiCAD with a capacity of 4000mAh, you would want to charge it at 400mA for about 12 hours. Another advantage of this charging technique is that it is gentle on batteries and doesn't cause them to lose capacity as quickly as the fast charge techniques.

The output current of this device is controlled by the summation of the bandgap reference diode and the base-emitter junction of the PNP transistor. The PNP transistor provides negative feedback to the gate of the MOSFET. As noted in the schematic, the batteries being charged can have a total of 12V which is equivalent to about 8 NiCAD's in series. The output current is determined by the value of R1 which is determined by:

R1=3.2Volts/Iout

The power dissipation of R1 will equal:

Pr1=3.2Volts*Iout

Be sure to provide pleanty of heatsink for Q1 and choose an appropriately sized resistor for R1. The following table summarizes some of the resistor current combinations that are possible:

Iout	Resistor Value	Resistor Power
100mA	33 ohms	1 watt
500mA	6.2 ohms	2 watt
1Amp	3.3 ohms	5 watt

The power dissipation of Q1 as a function of Output current and load voltage can be shown as: see battery charger schematic

PIC Temperature Meter with Thermoster

2008-08-13T09:41:00.001+07:00

This is our upcoming project that is similar to PIC dual temperature meter but with the thermostat option built-in. Besides displaying customized temperature readings in Celsius and/or Fahrenheit degrees it will turn on heater if the temperature drops below specified temperature or it can be set to turn on the fan or air conditioning system if the temperature reaches above specified temperature that is set by UP / DOWN buttons. Thermostat can display both Celsius and Fahrenheit values (together or individually) and is capable of measuring temperatures from -55 to 125 degrees Celsius (-67 to 257 degrees Fahrenheit)

Presented PIC temperature meter with thermostat uses very exciting DS1820 1-Wire digital temperature sensor. Unlike regular sensors where temperature readings are passed as varying voltage, DS1820 passes temperature information in a digital format as data. This brings many new possibilities and enables to pass temperature information over much longer distances just over a two wire cable.

input 0-70V PIC Voltmeter

2008-08-13T09:33:00.002+07:00

This is a sneak preview of upcoming PIC voltmeter project. You may use this PIC voltmeter for your power supply, as a battery meter for car, RC cars, RC helicopters, to monitor voltages in your computer or it could be used as a small portable voltmeter.

This is extremely simple to build PIC Temperature meter that allows to measure temperature in two different locations at the same time. Meter can display both Celsius and Fahrenheit values (together or individually) and is capable of measuring temperatures from -55 to 125 degrees Celsius (-67 to 257 degrees Fahrenheit). Never before such a useful and powerful circuit could be built with so little components and yet provide endless possibilities. This is all possible thanks to the use of PIC16F628 microcontroller and 2x16 character LCD display that act like a small computer which can be customizable thanks to upgradeable hex firmware.

Presented PIC temperature meter uses two very exciting DS18S20 1-Wire digital temperature sensors. Unlike regular sensors where temperature readings are passed as varying voltage, DS18S20 passes temperature information in a digital format as data. This brings many new possibilities and enables to pass temperature information over much longer distances just over a two wire cable

DC power supply 10 A

2008-08-09T17:26:00.003+07:00

Today a friend of me visits at a house. He leads old equipment come to give very plentiful and tell beg for DC power supply 10A circuit. That be simple please , when I sees the equipment that him has had already. Then take this circuit see use 78XX integrated circuits. Have many the number and Transistor power number M15004 that can enhance current. Get tall arrive at 10A.The important aspect is must use Transfermer that be appropriate give current 10A More than. And use C-Filter 10000uF sizes go up with. The detail is other see in the circuit has leisurely sir.

Equivalent Circuit and Vector Diagram

2008-06-30T13:10:00.002+07:00

For purposes of analysis the transformer may be represented by a 1:1 truns ratio equivalent circuit. This circuit is based on the following assumptions:

(a) Primary and secondary turns are equal in number. One winding is chosen as the reference winding; the other is the referred winding. The voltage in the referred winding is multiplied by the actual turns ratio after it is computed from the equivalent circuit. The choice between primary and secondary for the reference winding is a matter of convenience.

(b) Core loss may be represented by a resistance across the terminals of the reference winding.

(d) Primary and secondary IR and IX voltage drops may be lumped together; the voltage drops in the referred winding are multiplied by a factor derived at the end of this section, to give them the correct equivalent value.

(e) Equivalent reactances and resistances are linear.

As will be shown later, some of these assumptions are approximate, and the analysis based on them is only accurate so far as the assumptions are justified. With proper attention to this fact, practical use can be made of the equivalent circuit.

With many sine-wave electronic transformers, the transformer load is resistive. A tube filament heating load, for example, has 100 per cent power factor. Under this condition the relations between voltages and currents become appreciably simplified in comparison with the same relations for reactive loads. In what follows, the secondary winding will be chosen as the reference winding. At low frequencies such a transformer may be represented by Fig. 3 (a).

Fig. 3. (a) Transformer with resistive load; (b) equivalent circuit; (c) vector diagram.

The transformer equivalent circuit is approximated by Fig. 3(b), and its vector diagram for 100 per cent p-f load by Fig. 3(c). Secondary load voltage E_L and load current I_L are in phase. Secondary induced voltage E_sis greater than E_L because it must compensate for the winding resistances and leakage reactances. The winding resistance and leakage reactance voltage drops are shown in Fig. 3(c) as IR and IX, which are respectively in phase and in quadrature with I_L and E_L. These voltage drops are the sum of secondary and primary winding voltage drops, but the primary values are multiplied by a factor to be derived later. If voltage drops and losses are temporarily forgotten, the same power is delivered to the load as is taken from the line. Let subscripts 1 and 2 denote the respective primary and secondary quantities.

E₁I₁ = E₂I₂ (5)

so that the voltages are inversely proportional to the currents. Also, from equation 2, they are directly proportional to their respective turns.

Now the transformer may be replaced by an impedance Z₁ drawing the same current from the line, so that

where Z₂ is the secondary load impedance, in this case R_L , If these expressions for current are substituted in equation 6,

Equation 7 is strictly true only for negligible voltage drops and losses. It is approximately true for voltage drops up to about 10 per cent of the winding voltage or for losses less than 20 per cent of the power delivered, but it is not true when the voltage drops approach in value the winding voltage or when the losses constitute most of the primary load.

Not only does the load impedance bear the relation of equation 7 to the equivalent primary load impedance; the winding reactance and resistance may also be referred from one winding to the other by the same ratio. This can be seen if the secondary winding resistance and reactance are considered part of the load, across which the secondary induced voltage E_s appears. Thus the factor by which the primary reactance and resistance are multiplied, to refer them to the secondary for addition to the secondary drops, is (N₂/N₁)². If the primary had been the reference winding, the secondary reactance and resistance would have been multiplied by (N₂/N₁)².

In Fig. 3(c) the IR voltage drop subtracts directly from the terminal voltage across the resistive load, but the IX drop makes virtually no difference. How much the IX drop may be before it becomes appreciable is shown in Fig. 4.

Fig. 4. Relation between reactive voltage drop and load voltage.

If the IX drop is 30 per cent of the induced voltage, 4 per cent reduction in load voltage results; 15 per cent IX drop causes but 1 per cent reduction.

Uninterruptible Alarm Power Supply

2008-06-26T17:12:00.001+07:00

Although this power supply was designed for the Modular Burglar Alarm - it has other applications. It provides an output of 12 V at a current of up to 1 A. In the event of a mains failure - the back-up battery takes over immediately. And when mains power is restored - the battery recharges automatically.

Schematic Diagram

The input from the transformer is rectified by BR1 and smooth by C1. There is generally a limit on the size of smoothing capacitor that may be connected across a rectifier. However, they all seem to be able to cope with 2200uF. Depending on the transformer, the output from the rectifier could be as much as 20-volts DC. Therefore, C1 should be at least 35-volts.

The output from the rectifier is fed to pin 1 of the 7805 regulator. Its output - at pin 3 - is always 5-volts above whatever voltage is on pin 2. Since pin 2 is generally connected to ground - the 7805 is usually referred to as a 5-volt regulator. However, R1, C1 & Z1 hold pin 2 at 9v1. Therefore the output at pin 3 will be ( 9v1 + 5 = 14v1 ). Additional smoothing is provided by C3. Current through R2 lights the red LED. This gives visual conformation that the mains supply is present - or not, as the case may be.

D1 provides a one-way path that allows the incoming current to pass; while - during periods of mains failure - it prevents current from the battery finding its way back to the red LED and the regulator output pin. There is a forward voltage-drop of about 0v6 across D1 and this reduces the incoming voltage to 13v5. Alarm back-up batteries are designed to be charged at a constant 13v5 to 13v8.

The terminal voltage of a fully-charged 12-volt battery is between 13v5 and 13v8. When the terminal voltage of the battery reaches 13v5, it is then equal to the charging voltage coming from D1. Since there is now no difference in potential between the battery voltage and the charging voltage - no more current will flow into the battery. In other words, once it's fully-charged the energy flow into the battery stops. This is sometimes referred to as a "Float" or "Trickle" charge.

Alarm back-up batteries are designed to be charged like this; and their terminals can be held at 13v5 to 13v8 for many years with no apparent ill-effects. They are maintenance-free and have a life expectancy of 5 years. However, they tend not to recover from a very deep discharge.

If the mains fails, the battery will automatically take over; and the output from the power supply will not be interrupted. When the mains returns, the battery will be recharged automatically.

Batteries of this size hold a lot of charge and can supply a very heavy current. If they are short-circuited they can cause personal injury and/or a fire. Under normal circumstances - if the output from the supply is shorted - FS2 will blow. If for some reason there is a short in the supply itself, FS1 will blow. If the battery is accidentally connected the wrong way round, D2 will cause a deliberate short and FS1 will blow, instantly isolating the battery.

After a long period of mains failure, when the battery terminal voltage is low, the initial charge current can be high. The 2-amp version of the 7805 will limit the maximum current to 2-amps. If you can't find a 2-amp 7805 then the 1-amp version will work just as well. In either case, 3-amps should be a sufficient rating for FS1.

The voltage drop across the 7812 reduces the 13v5 to about 12-volts. Current through R3 lights the green LED and gives a visual indication that the PSU output is working.

The regulators also have built-in thermal cut-outs. To prevent them from overheating, they need to be bolted to heatsinks. The power each regulator must dissipate is determined by the current through it and the voltage-drop across it. In the worst possible case, the 7805 could have ( 20 - 14 = 6-volts ) across it and a current of 2-amps flowing through it. Therefore it may need to dissipate ( 6 x 2 = 12-watts ). On the other hand, the 7812 can only have about 2-volts across it; and 1-amp flowing through it. So it should never need to dissipate more than ( 2 x 1 = 2-watts ).

The metal part of the regulator body is connected to pin 2. Therefore, the heatsink will be at the same potential as pin 2. In the case of the 7805 this means that its heatsink will be at 9v1. So it must NOT be connected to ground.

The Modular Burglar Alarm will work at up to 15-volts. So the 7812 is not strictly necessary. However, because the PSU will have other applications - where an output of 13 v 5 may be too high - the 7812 is included in the circuit.

The 7812 is not working as a regulator. The output voltage is in fact regulated by the back-up battery. Instead, the circuit makes use of the voltage drop across the 7812 - typically 2-volts - as a convenient way of lowering the output voltage. It's a simple, cheap and effective solution; with the added bonus that it also provides thermal-overload protection and a short-circuit current-limit.

The two 2 k 2 resistors and the red and green LEDs are just indicators. They are not necessary to the operation of the circuit. You may leave them out if you wish.The 7805 needs the larger heatsink because it has to dissipate a lot of energy - especially when called upon to recharge a flat battery. Its heatsink is at 9 v 1 - and must NOT be connected to ground. The 7812 never has to dissipate more than 2-watts - so its heatsink can be smaller.

Analog to Digital Converter

2008-06-26T17:03:00.000+07:00

Normally analogue-to-digital con-verter (ADC) needs interfacing through a microprocessor to convert analogue data into digital format. This requires hardware and necessary software, resulting in increased complexity and hence the total cost.
The circuit of A-to-D converter shown here is configured around ADC 0808, avoiding the use of a microprocessor. The ADC 0808 is an 8-bit A-to-D converter, having data lines D0-D7. It works on the principle of successive approximation. It has a total of eight analogue input channels, out of which any one can be selected using address lines A, B and C. Here, in this case, input channel IN0 is selected by grounding A, B and C address lines.
Usually the control signals EOC (end of conversion), SC (start conversion), ALE (address latch enable) and OE (output enable) are interfaced by means of a microprocessor. However, the circuit shown here is built to operate in its continuous mode without using any microprocessor. Therefore the input control signals ALE and OE, being active-high, are tied to Vcc (+5 volts). The input control signal SC, being active-low, initiates start of conversion at falling edge of the pulse, whereas the output signal EOC becomes high after completion of digitisation. This EOC output is coupled to SC input, where falling edge of EOC output acts as SC input to direct the ADC to start the conversion.
As the conversion starts, EOC signal goes high. At next clock pulse EOC output again goes low, and hence SC is enabled to start the next conversion. Thus, it provides continuous 8-bit digital output corresponding to instantaneous value of analogue input. The maximum level of analogue input voltage should be appropriately scaled down below positive reference (+5V) level.
The ADC 0808 IC requires clock signal of typically 550 kHz, which can be easily derived from an astable multivibrator constructed using 7404 inverter gates. In order to visualise the digital output, the row of eight LEDs (LED1 through LED8) have been used, wherein each LED is connected to respective data lines D0 through D7. Since ADC works in the continuous mode, it displays digital output as soon as analogue input is applied. The decimal equivalent digital output value D for a given analogue input voltage Vin can be calculated from the relationship

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2008-03-04T15:50:00.001+07:00

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