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email Colin Mitchell: firstname.lastname@example.org
This e-book covers the Light Emitting Diode.
The LED (Light Emitting Diode) is the modern-day equivalent to the light-globe.
It has changed from a dimly-glowing indicator to one that is too-bright to look at.
However it is entirely different to a "globe."
A globe is an electrical device consisting of a glowing wire while a LED is an electronic device.
A LED is more efficient, produces less heat and must be "driven" correctly to prevent it being damaged.
This eBook shows you how to connect a LED to a circuit plus a number of projects using LEDs.
It's simple to use a LED - once you know how.
CONNECTING A LED
A LED must be connected around the correct way in a circuit and it must have a resistor to limit the current.
The LED in the first diagram does not illuminate because a red LED requires 1.7v and the cell only supplies 1.5v. The LED in the second diagram is damaged because it requires 1.7v and the two cells supply 3v. A resistor is needed to limit the current to about 25mA and also the voltage to 1.7v, as shown in the third diagram. The fourth diagram is the circuit for layout #3 showing the symbol for the LED, resistor and battery and how the three are connected. The LED in the fifth diagram does not work because it is around the wrong way.
The voltage dropped across this resistor, combined with the current,
constitutes wasted energy and should be kept to a minimum, but a small
HEAD VOLTAGE is not advisable (such as 0.5v). The head voltage should be a minimum of
1.5v - and this only applies if the supply is fixed.
TESTING A LED
IDENTIFYING A LED
LEDs ARE CURRENT DRIVEN DEVICES
Here is a set of strings for a
supply voltage of 5v to 12v and two LEDs:
LED VOLTAGE AND CURRENT
THE DANGERS OF USING A "LED WIZARD"
Here is an example, provided by a reader. Can you see the major fault?
The characteristic voltage (the colour of the LED) is not important in this
discussion. Obviously white LEDs will not work as they require 3.4v to 3.6v
BI-COLOUR, TRI-COLOUR, FLASHING LEDS and 7-colour LEDs
LEDs can also be obtained in a range of novelty effects as well as a red and green LED inside a clear or opaque lens. You can also get red, blue, white, green or any combination inside a LED with 2 leads.
Simply connect these LEDs to a 6v supply and 330R series dropper resistor to see the effects they produce.
Some LEDs have 3 leads and the third lead needs to be pulsed to change the pattern.
Some LEDs can be reversed to produce a different colour. These LEDs contain red and green and by reversing the voltage, one or the other colour will illuminate.
When the voltage is reversed rapidly, the LED produces orange.
Sometimes it is not convenient to reverse the voltage to produce orange.
In this case three leaded LEDs are available to produce red, green and orange.
Flashing LEDs contain a chip and inbuilt current-limiting resistor. They operate from 3.5v to 12v. The flash-rate will alter slightly on different supply voltage. You can get 3mm and 5mm versions as well as high-bright types and surface-mount.
Novelty LEDs can have 2 or three leads. They contain a microcontroller chip, inbuilt current-limiting resistor and two or three colours.
The two leaded LEDs cycle through a range of colours, including flashing and fading.
The three leaded LEDs have up to 16 different patterns and the control lead must be taken from 0v to rail volts to activate the next pattern.
LEDs LEDs LEDs
There are hundreds of circuits that use a LED or drive a LED or flash a LED and nearly all the circuits in this eBook are different.
Some flash a LED on a 1.5v supply, some use very little current, some flash the LED very brightly and others use a flashing LED to create the flash-rate.
You will learn something from every circuit. Some are interesting and some are amazing. Some consist of components called a "building Block" and they can be added to other circuits to create a larger, more complex, circuit.
This is what this eBook is all about.
It teaches you how to build and design circuits that are fun to see working, yet practical.
You will learn a lot . . . . even from these simple circuits.
How good is your power of observation?
Can you find the LED:
Infrared LEDs are just like ordinary LEDs but the light output cannot be seen. To view an infrared LEDs, turn it on with the appropriate battery and dropper resistor and view it with a camera. You will see the illumination on the screen.
Infrared LEDs are sometimes clear and sometimes black. They operate just like a red LED with the same characteristic voltage-drop of about 1.7v.
Sometimes an infrared LED is pulsed with a high current for a very short period of time but the thing to remember is the wattage-dissipation of a 5mm LED is about 70mW. This means the constant-current should be no more than 40mA.
Infrared LEDs are also called TRANSMITTING LEDs as they emit light. These are given the term Tx (for transmitting). An infrared LED can be connected to a 5v supply via a 220R current-limiting resistor for 15mA current.
Infrared receivers (Rx) can look exactly like infrared LEDs, but they do not emit IR light. They detect Infrared illumination and must be connected the correct way in a circuit.
They have a very high resistance when no receiving IR illumination and the resistance decreases as the illumination increases.
This means they are connected to a 5v supply via a resistor and when the resistance of the infrared receiver decreases, current will flow thought it and the resistor. This will produce a voltage across the resistor and this voltage is fed to the rest of the circuit.
Here is a circuit to show how to connect an infrared LED and Infrared (diode) receiver:
You cannot use an IR LED as a receiver or an
Infrared diode as an illuminator. They are constructed differently.
An infrared LED has a characteristic voltage drop of 1.7v
The safest way to power a project is with a battery. Each circuit requires a voltage from 3v to 12v. This can be supplied from a set of AA cells in a holder or you can also use a 9v battery for some projects.
If you want to power a circuit for a long period of time, you will need a "power supply."
The safest power supply is a Plug Pack (wall-wort, wall wart, wall cube, power brick, plug-in adapter, adapter block, domestic mains adapter, power adapter, or AC adapter). Some plug packs have a switchable output voltage: 3v, 6v, 7.5v, 9v, 12v) DC with a current rating of 500mA. The black lead is negative and the other lead with a white stripe (or a grey lead with a black stripe) is the positive lead.
This is the safest way to power a project as the insulation (isolation) from the mains is provided inside the adapter and there is no possibility of getting a shock.
The rating "500mA" is the maximum the Plug Pack will deliver and if your circuit takes just 50mA, this is the current that will be supplied. Some pluck packs are rated at 300mA or 1A and some have a fixed output voltage. All these plug packs will be suitable.
Some Plug Packs are marked "12vAC." This type of plug pack is not suitable for these circuits as it does not have a set of diodes and electrolytic to convert the AC to DC. All the circuits in this eBook require DC.
Simplest LED Circuit
Connect a LED to a piezo diaphragm and tap the piezo with a screwdriver at the centre of the disc and the LED will flash very briefly.
This multivibrator circuit will flash the Robot Man's eyes as shown in the photo. The kit of components is available from Talking Electronics for $8.50 plus postage. Send an email to find out the cost of postage:
Here is the circuit from Velleman Kits. The two 10k resistors are replaced with a resistor and pot so the "flip flop" can be altered.
FLASHING A LED
These 7 circuits flash a LED using a supply from 1.5v to 12v.
They all have a different value of efficiency and current consumption. You will find at least one to suit your requirements.
The simplest way to flash a LED is to buy a FLASHING LED as shown in figure A. It will work on 3v to 9v but it is not very bright - mainly because the LED is not high-efficiency.
A Flashing LED can be used to flash a super-bright red LED, as shown in figure B.
Figure C shows a flashing LED driving a buffer transistor to flash a white LED. The circuit needs 4.5v - 6v.
Figure D produces a very bright flash for a very short period of time - for a red, green, orange or white LED.
Figure E uses 2 transistors to produce a brief flash - for a red, green, orange or white LED.
Figure F uses a single cell and a voltage multiplying arrangement to flash a red or green LED.
Figure G flashes a white LED on a 3v supply.
These four circuits delivers a constant 12mA to any number of LEDs connected in series (to the terminals shown) in the following arrangements.
The circuits can be connected to 6v, 9v or 12v and the brightness of the LEDs does not alter.
You can connect:
1 or 2 LEDs to 6v,
1, 2 or 3 LEDs to 9v or
1, 2, 3 or 4 LEDs to 12v.
The LEDs can be any colour.
The constant-current section can be considered as a MODULE and can be placed above or below the load:
WHITE LED on 1.5v SUPPLY
This circuit will illuminate a white LED using a single cell.
See LED Torch Circuits article for more details.
2 WHITE LEDs on 1.5v SUPPLY
This circuit will illuminate two white LEDs using a single cell.
See LED Torch Circuits article for more details.
This circuit will flash a white LEDs using a single cell.
See LED Torch Circuits article for more details.
10 LEDs on a 9v BATTERY
This circuit will illuminate 10 LEDs on a 9v battery.
It was designed in response to a readers request:
SHAKE TIC TAC LED TORCH
Here is a request from one of our readers:
I want to build a solar powered flashlight. It will contain 3-AAs nickel hydride batteries of 1.2v each. I want many ultrabright white LEDS @ 25mA. I also need a voltage regulator circuit so the batteries won't overcharge. The batteries are 800 mAH capacity. I need a high-low beam too. Do you have a schematic for this?
Here is a very simple circuit.
The circuit produces a voltage higher than 3.6v, from a supply of 4.5v to 6v to illuminate 3 super-bright LEDs in series.
The flyback transformer consists of 30 turns and 30 turns wound on an old ferrite antenna slab. Reverse the feedback winding if the LEDs do not illuminate.
Some solar panels will drain a small current from the battery when not illuminated, so a "protection diode" can be added.
You can also use a single 3.7v Li-Ion cell.
LED DETECTS LIGHT
The LED in this circuit will detect light to turn on the oscillator. Ordinary red LEDs do not work. But green LEDs, yellow LEDs and high-bright white LEDs and high-bright red LEDs work very well.
The output voltage of the LED is up to 600mV when detecting very bright illumination.
When light is detected by the LED, its resistance decreases and a very small current flows into the base of the first transistor. The transistor amplifies this current about 200 times and the resistance between collector and emitter decreases. The 330k resistor on the collector is a current limiting resistor as the middle transistor only needs a very small current for the circuit to oscillate. If the current is too high, the circuit will "freeze."
The piezo diaphragm does not contain any active components and relies on the circuit to drive it to produce the tone.
|8 MILLION GAIN!
This circuit is so sensitive it will detect "mains hum." Simply move it across any wall and it will detect where the mains cable is located. It has a gain of about 200 x 200 x 200 = 8,000,000 and will also detect static electricity and the presence of your hand without any direct contact. You will be amazed what it detects! There is static electricity EVERYWHERE! The input of this circuit is classified as very high impedance.
I do not like any circuit connected directly to 240v mains. However Christmas tress lights (globes) have been connected directly to the mains for 30 years without any major problems.
Insulation must be provided and the lights (LEDs) must be away from prying fingers.
You need at least 50 LEDs in each string to prevent them being damaged via a surge through the 1k resistor - if the circuit is turned on at the peak of the waveform. As you add more LEDs to each string, the current will drop a very small amount until eventually, when you have 90 LEDs in each string, the current will be zero.
For 50 LEDs in each string, the total characteristic voltage will be 180v so that the peak voltage will be 330v - 180v = 150v. Each LED will see less than 7mA peak during the half-cycle they are illuminated (because the voltage across the 0.22u is 150v and this voltage determines the current-flow). The 1k resistor will drop 7v - since the RMS current is 7mA (7mA x 1,000 ohms = 7v). No rectifier diodes are needed. The LEDs are the "rectifiers." Very clever. You must have LEDs in both directions to charge and discharge the capacitor. The resistor is provided to take a heavy surge current through one of the strings of LEDs if the circuit is switched on when the mains is at a peak. This can be as high as 330mA if only 1 LED is used, so the value of this resistor must be adjusted if a small number of LEDs are used. The LEDs above detect peak current. The LEDs are turned on and off 50 times per second and this may create "flickering" or "strobing." To prevent this flicker, see the DC circuit below:
A 100n cap will deliver 7mA RMS or 10mA peak in full wave or 3.5mA RMS (10mA peak for half a cycle) in half-wave. (when only 1 LED is in each string).
The current-capability of a capacitor needs more explanation. In the diagram on the left we see a capacitor feeding a full-wave power supply. This is exactly the same as the LEDs on 240v circuit above. Imagine the LOAD resistor is removed. Two of the diodes will face down and two will face up. This is exactly the same as the LEDs facing up and facing down in the circuit above. The only difference is the mid-point is joined. Since the voltage on the mid-point of one string is the same as the voltage at the mid-point of the other string, the link can be removed and the circuit will operate the same.
This means each 100n of capacitance will deliver 7mA RMS (10mA peak on each half-cycle).
In the half-wave supply, the capacitor delivers 3.5mA RMS (10mA peak on each half-cycle, but one half-cycle is lost in the diode) for each 100n to the load, and during the other half-cycle the 10mA peak is lost in the diode that discharges the capacitor.
You can use any LEDs and try to keep the total voltage-drop in each string equal. Each string is actually working on DC. It's not constant DC but varying DC. In fact is it zero current for 1/2 cycle then nothing until the voltage rises above the total characteristic voltage of all the LEDs, then a gradual increase in current over the remainder of the cycle, then a gradual decrease to zero over the falling portion of the cycle, then nothing for 1/2 cycle. Because the LEDs turn on and off, you may observe some flickering and that's why the two strings should be placed together.
SINGLE LED on 240v
A single LED can be illuminated by using a 100n or 220n capacitor with a rating of 400v. These capacitors are called "X2" and are designed to be connected to the mains.
MAINS NIGHT LIGHT
The circuit illuminates a column of 10 white LEDs. The 10u prevents flicker and the 100R also reduces flicker (it allows the 10u to charge to a slightly higher value and this extra energy is delivered to the LEDs during each of the low portions of the AC cycle.)
This circuit is classified as a CONSTANT CURRENT GENERATOR or CONSTANT CURRENT CIRCUIT.
This means any component placed on the output of the circuit will pass 7mA if the capacitor is 100n on a 240v supply or 4.7 x 7mA = 33mA if the capacitor is 470n.
This also applies to a short-circuit on the output.
If no load is connected, the output voltage will be 230v x 1.4 = 320v and if the voltage across the load is 100v, the output will be reduced to about 20mA. If the output voltage is 200v, the current will be 10mA and if the output voltage is 300v, the current will be 0mA. In our case the output voltage will be about 35v and the current will be 30mA.
This means you cannot add LEDs endlessly. A time will come when they will simply not illuminate.
FLASHING RAILROAD LIGHTS
This circuit flashes two red LEDs for a model railway crossing.
This circuit will adjust the brightness of one or more LEDs from 5% to 95%.
DRIVING A BI-COLOUR LED
Some 3-leaded LEDs produce red and green. This circuit alternately flashes a red/green bi-coloured LED:
BI-POLAR LED DRIVER
Some 2-leaded LEDs produce red and green. These are called Bi-polar LEDs. This circuit alternately flashes a red/green bi-polar LED:
RGB LED DRIVER
RGB LED FLASHER
This LED flashes at a fast rate then a slow rate. It only requires a current-limiting resistor of 100R for 4.5v to 6v supply or 470R for 7v to 12v supply.
This LED is available from: http://alan-parekh.vstore.ca/flashing-5000mcd-p-88.html for 80 cents plus postage.
There are two different types of RGB LEDs. The RGB LED Driver circuit above uses an RGB LED with 4 leads and has 3 coloured chips inside and NOTHING ELSE.
The LED described in the video has 2 leads and requires a dropper resistor so that about 20mA flows. The LED also contains a microcontroller producing PWM signals. If you cannot get the 2-leaded LED, you can use a 4-leaded LED plus the circuit below. It is an analogue version of the circuit inside the self-flashing LED, for the slow-rate:
As with everything Chinese, the self-flashing LED is too gimmicky.
It is better to produce your own colour-change via the circuit above. You can alter the rate by changing the value of the components and/or remove one or more of the 100u's. The circuit for a common cathode RGB LED is shown in the RGB LED Driver above.
In the Knight Rider circuit, the 555 is wired as an oscillator. It can be adjusted to give the desired speed for the display. The output of the 555 is directly connected to the input of a Johnson Counter (CD 4017). The input of the counter is called the CLOCK line.
The 10 outputs Q0 to Q9 become active, one at a time, on the rising edge of the waveform from the 555. Each output can deliver about 20mA but a LED should not be connected to the output without a current-limiting resistor (330R in the circuit above).
The first 6 outputs of the chip are connected directly to the 6 LEDs and these "move" across the display. The next 4 outputs move the effect in the opposite direction and the cycle repeats. The animation above shows how the effect appears on the display.
Using six 3mm LEDs, the display can be placed in the front of a model car to give a very realistic effect. The same outputs can be taken to driver transistors to produce a larger version of the display.
Knight Rider circuit is available as a kit for less than $15.00
plus postage as Kitt Scanner.
Here's a clever circuit using two 555's to produce a set of traffic lights for a model layout.
The animation shows the lighting sequence and this follows the Australian-standard. The red LED has an equal on-off period and when it is off, the first 555 delivers power to the second 555. This illuminates the Green LED and then the second 555 changes state to turn off the Green LED and turn on the Orange LED for a short period of time before the first 555 changes state to turn off the second 555 and turn on the red LED. A supply voltage of 9v to 12v is needed because the second 555 receives a supply of about 2v less than rail. This circuit also shows how to connect LEDs high and low to a 555 and also turn off the 555 by controlling the supply to pin 8. Connecting the LEDs high and low to pin 3 will not work and since pin 7 is in phase with pin 3, it can be used to advantage in this design.
4 WAY TRAFFIC LIGHTS
This circuit produces traffic lights for a "4-way" intersection. The seemingly complex wiring to illuminate the lights is shown to be very simple.
DRIVING MANY LEDS
The 555 is capable of sinking and sourcing up to 200mA, but it gets very hot when doing this on a 12v supply.
The following circuit shows the maximum number of white LEDs that can be realistically driven from a 555 and we have limited the total current to about 130mA as each LED is designed to pass about 17mA to 22mA maximum. A white LED drops a characteristic 3.2v to 3.6v and this means only 3 LEDs can be placed in series.
This circuit drives a 3x3x3 cube consisting of 27 white LEDs. The 4020 IC is a 14 stage binary counter and we have used 9 outputs. Each output drives 3 white LEDs in series and we have omitted a dropper resistor as the chip can only deliver a maximum of 15mA per output. The 4020 produces 512 different patterns before the sequence repeats and you have to build the project to see the effects it produces on the 3D cube.
UP/DOWN FADING LED
These two circuits make a LED fade on and off. The first circuit charges a 100u and the transistor amplifies the current entering the 100u and delivers 100 times this value to the LED via the collector-emitter pins. The circuit needs 9v for operation since pin 2 of the 555 detects 2/3Vcc before changing the state of the output so we only have a maximum of 5.5v via a 220R resistor to illuminate the LED. The second circuit requires a very high value electrolytic to produce the same effect.
UP/DOWN FADING LED-2
The circuit fades the LED ON and OFF at an equal rate. The 470k charging and 47k discharging resistors have been chosen to create equal on and off times.
BIKE TURNING SIGNAL
This circuit can be used to indicate left and right turn on a motor-bike. Two identical circuits will be needed, one for left and one for right.
These three circuits flash the left LEDs 3 times then the right LEDs 3 times, then repeats. The only difference is the choice of chips.
LED DICE with
This circuit produces a random number from 1 to 6 on LEDs that are similar to the pips on the side of a dice. When the two TOUCH WIRES are touched with a finger, the LEDs flash very quickly and when the finger is removed, they gradually slow down and come to a stop. LED Dice with Slow Down kit is available from Talking Electronics.
This circuit creates a rotating LED that starts very fast when a finger touches the TOUCH WIRES. When the finger is removed, the rotation slows down and finally stops.
LED FX TE555-5
SOLAR GARDEN LIGHT
This is the circuit in a $2.00 Solar Garden Light.
The circuit illuminates a white LED from a 1.2v rechargeable cell.
This circuit is a SOLAR TRACKER. It uses green LEDs to detect the sun and an H-Bridge to drive the motor. A green LED produces nearly 1v but only a fraction of a milliamp when sunlight is detected by the crystal inside the LED and this creates an imbalance in the circuit to drive the motor either clockwise or anticlockwise. The circuit will deliver about 300mA to the motor.
The circuit was designed by RedRok and kits for the Solar Tracker are available from: http://www.redrok.com/electron.htm#tracker This design is called: LED5S5V Simplified LED low power tracker.
|BATTERY MONITOR MkI
A very simple battery monitor can be made with a dual-colour LED and a few surrounding components. The LED produces orange when the red and green LEDs are illuminated.
The following circuit turns on the red LED below 10.5v
The orange LED illuminates between 10.5v and 11.6v.
The green LED illuminates above 11.6v
|BATTERY MONITOR MkII
This battery monitor circuit uses 3 separate LEDs.
The red LED turns on from 6v to below 11v.
It turns off above 11v and
The orange LED illuminates between 11v and 13v.
It turns off above 13v and
The green LED illuminates above 13v
|LOW FUEL INDICATOR
This circuit has been designed from a request by a reader. He wanted a low fuel indicator for his motorbike. The LED illuminates when the fuel gauge is 90 ohms. The tank is empty at 135 ohms and full at zero ohms. To adapt the circuit for an 80 ohm fuel sender, simply reduce the 330R to 150R. (The first thing you have to do is measure the resistance of the sender when the tank is amply.)
This circuit is a game of skill. See full article: LED Zeppelin. The kit is available from talking electronics for $15.50 plus postage. Email HERE for details.
The game consists of six LEDs and an indicator LED that flashes at a rate of about 2 cycles per second. A push button is the "Operations Control" and by carefully pushing the button in synchronisation with the flashing LED, the row of LEDs will gradually light up.
But the slightest mistake will immediately extinguish one, two or three LEDs. The aim of the game is to illuminate the 6 LEDs with the least number of pushes.
We have sold thousands of these kits. It's a great challenge.
|THE DOMINO EFFECT
|10 LED CHASER
Here's an interesting circuit that creates a clock pulse for a 4017 from a flashing LED. The flashing LED takes almost no current between flashes and thus the clock line is low via the 1k to 22k resistor. When the LED flashes, the voltage on the clock line is about 2v -3v below the rail voltage (depending on the value of the resistor) and this is sufficient for the chip to see a HIGH.
(circuit designed on 9-10-2010)
|Emergency PHONE-LINE LIGHT
Here's a project that uses the phone line to illuminate a set of white LEDs.
The circuit delivers a current of 4.5mA as any current above 10mA will be detected by the exchange as the hand-set off the hook.
Be warned: This type of circuit is not allowed as it uses the energy from the phone line (called "leeching") and may prevent the phone from working.
A 2-leaded dual colour LED can be connected to the outputs of a microcontroller and the brightness can be equalized by using the circuits shown.
A Flickering LED is available from eBay and some electronics shops.
It can be connected to a supply from 2v to 6v and needs an external resistor when the supply is above 3v. The LED has an internal circuit to create the flickering effect and limit the current. We suggest adding a 150R resistor when the supply is above 3v and up to 6v. Above 6v, the current-limit resistor should be increased to 220R for 9v and 330R for 12v.
You can connect the flickering LED to an ordinary LED and both will flicker. Here are some arrangements:
The Pulse-Width Modulation to activate the flickering can be observed on an oscilloscope by connecting the probe across the LED. It is a very complex waveform. It is approx 1v in amplitude and approx 15 x 1kHz pulses to create each portion of the on-time, something like this:
The pulses vary in width to create a brighter illumination.
|CONSTANT-CURRENT 7805 DRIVES 1 WATT
The circuit can be reduced to 2 components:
The 7805 can be converted into a content-current device by connecting a
resistor as shown above.
Since the LED and regulator are in series, the LED can be placed before the regulator:
Driving a single 1watt LED from 12v is very inefficient.
1-WATT LED - very good design
This turns off the BD139 a little more and the current through the inductor reduces.
This creates a collapsing flux that produces a voltage across the coil in the opposite direction. This voltage passes via the 1n to turn the BC547 ON and the BD139 is fully turned OFF.
The inductor effectively becomes a miniature battery with negative on the lower LED and positive at the anode of the Ultra Fast diode. The voltage produced by the inductor flows through the UF diode and both 1-watt LEDs to give them a spike of high current. The circuit operates at approx 500kHz and this will depend on the inductance of the inductor.
The circuit has about 85% efficiency due to the absence of a current-limiting resistor, and shuts off at 4v, thus preventing deep-discharge of the rechargeable cells or 6v battery.
The clever part of the circuit is the white LED and two diodes. These form a zener reference to turn the circuit off at 4v. The 10k resistor helps too.
The circuit takes 70mA on low brightness and 120mA on HIGH brightness via the brightness-switch.
The LEDs actually get 200mA pulses of current and this produces the high brightness.
The coil or inductor is not critical. You can use a broken antenna rod from an AM radio (or a flat antenna slab) or an inductor from a computer power supply. Look for an inductor with a few turns of thick wire (at least 30) and you won't have to re-wind it.
Here are two inductors from surplus outlets:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G16521B - 50 cents
Here are the surplus inductors:
The cost of surplus is from 10 cents to 50 cents, but you are
sure to find something from a computer power supply.
By using the following idea, the current reduces to 90mA and 70mA and the illumination over a workbench is much better than a single high-power LED. It is much brighter and much nicer to work under.
Connect fifteen 5mm LEDs in parallel (I used 20,000mcd LEDs) by soldering them to a double-sided strip of PC board, 10mm wide and 300mm long. Space them at about 20mm. I know you shouldn't connect LEDs in parallel, but the concept works very well in this case. If some of the LEDs have a characteristic high voltage and do not illuminate very brightly, simply replace them and use them later for another strip.
You can replace one or both the 1-watt LEDs with a LED Strip, as shown below:
No current-limit resistor. . . why isn't the LED damaged?
Here's why the LED isn't damaged:
When the BD139 transistor turns ON, current flows through the LEDs and the inductor. This current gradually increases due to the gradual turning-on of the transistor and it is also increasing through the inductor. The inductor also has an effect of slowing-down the "in-rush" of current due to the expanding flux cutting the turns of the coil, so there is a "double-effect" on avoiding a high initial current. That's why there is little chance of damaging the LEDs.
When it reaches 65mA, it produces a voltage of .065 x 10 = 650mV across the 10R resistor, but the 1n is pushing against this increase and it may have to rise to 150mA to turn on the BC547. LEDs can withstand 4 times the normal current for very short periods of time and that's what happens in this case. The BD139 is then turned off by the voltage produced by the inductor due to the collapsing magnetic flux and a spike of high current is passed to the LEDs via the high speed diode. During each cycle, the LEDs receive two pulses of high current and this produces a very high brightness with the least amount of energy from the supply. All the components run "cold" and even the 1-watt LEDs are hardly warm.
Charging and Discharging
This project is designed to use all your old NiCad cells and mobile phone batteries.
It doesn't matter if you mix up sizes and type as the circuit takes a low current and shuts off when the voltage is approx 4v for a 6v pack.
If you mix up 600mA-Hr cells with 1650mA-Hr, 2,000mA-Hr and 2,400mA-Hr, the lowest capacity cell will determine the operating time.
The capacity of a cells is called "C."
Normally, a cell is charged at the 14 hour-rate.
The charging current is 10% of the capacity. For a 600mA-Hr cell, this is 60mA. In 10 hours it will be fully charged, but charging is not 100% efficient and so we allow another 2 to 4 hours.
For a 2,400mA-Hr cell, it is 240mA. If you charge them faster than 14-hr rate, they will get HOT and if they get very hot, they may leak or even explode. But this project is designed to be charged via a solar panel using 100mA to 200mA cells, so nothing will be damaged.
Ideally a battery is discharged at C/10 rate. This means the battery will last 10 hours and for a 600mA-Hr cell, this is 60mA. If you discharge it at the "C-rate," it will theoretically last 1 hour and the current will be 600mA. But at 600mA, the cells may only last 45 minutes. If you discharge is at C/5 rate, it will last 5 hours.
Our project takes 120mA so no cell will be too-stressed. A 600mA-Hr cell will last about 4-5 hours, while the other cells will last up to 24 hours. Try to keep the capacity of each cell in a "battery-pack" equal.
BUCK CONVERTER for 3watt LED
This circuit drives a 3watt LED. You have to be careful not to damage the LED when setting up the circuit. Add a 10R to the supply rail and hold it in your fingers. Make sure it does not get too hot and monitor the voltage across the resistor. Each 1v represents 100mA. The circuit will work and nothing will be damaged. If the resistor "burns your fingers" you have a short circuit.
The BC557 multivibrator has a "mark-to-space ratio" determined by the 22n and 33k, compared to the 100n and 47k, producing about 3:1 The BD679 is turned ON for about 30% of the time. This produces a very bright output, and takes about 170mA for 30% of the time. You cannot measure this current with a meter as it reads the peak value and the reading will be totally false. The only way to view the waveform is on a CRO, and calculate the current.
The 100-turn inductor allows the BD679 turn turn ON fully and "separates" the voltage on the emitter of the BC679 from the voltage on the top of the 3watt LED.
When the BD679 turns ON, the emitter rises to about 10v. But the top of the LED NEVER rises above 3.6v. The inductor "buffers" or "separates" these two voltages by producing a voltage across the winding equal to 6.4v and that's why the LED is not damaged.
When the transistor turns off (for 60% of the time), the magnetic flux produced by the current in the inductor collapses and produces a voltage in the opposite direction. This means the inductor now becomes a miniature battery and for a very short period of time it produces energy to illuminate the LED. The top of the inductor becomes negative and the bottom is positive. The current flows through the LED and through the Ultra High-Speed 1N4004 diode to complete the circuit. Thus the circuit takes advantage of the energy in the inductor.
A 500R pot is placed across the LED and a voltage is picked off the pot to turn on a BC547 transistor. This transistor "robs" some of the "turn-on" for the BD679 transistor to reduce the brightness of the LED.
Because the circuit is driving the LED with pulses, very high brightness is obtained with a low current.
Our eyes detect peak brightness and you can compare the performance of this circuit with a DC driven LED.
CONSTANT CURRENT DRIVES TWO 3WATT LEDs
This constant current circuit is designed to drive two 3-watt Luxeon LEDs. The LEDs require 1,000mA (1Amp) and have a characteristic voltage-drop across them of about 3.8v. Approximately 4v is dropped across the LM317T regulator and 1.25v across the current-limiting resistors, so the input voltage (supply) has to be 12.85v. A 12v battery generally delivers 12.6v.
The LM 317T 3-terminal regulator will need to be heatsinked.
This circuit is designed for the LM series of regulator as they have a voltage differential of 1.25v between "adj" and "out" terminals.
|DIMMING A 10WATT LED
The following circuit is a request from a reader. He wanted to dim a 10 watt LED from a 4.2v Lithium Ion cell.
The current will be about 3 amps and a power MOSFET is needed to deliver this current.
The characteristic voltage across the LED is about 3.3v to 3.6v and this leaves very little voltage for the control circuit. The resistance of the MOSFET is about 0.05 ohms and very little voltage is lost here.
The 0.22 ohm (1 watt) resistor will drop about 650millivolts.
The LED will not be overloaded or damaged by this circuit. When the pot is adjusted from full brightness, the MOSFET will dissipate a lot of energy and will get very hot if not properly heatsinked.
You can buy two 3200mAhr Li-ion cells for $4.00 (posted) on eBay. Some suppliers want $15.00 per cell.
You will need 4 cells in parallel to keep the current from each cell below 1 amp and this will allow the circuit to operate for about 2 hours.