A LED torch is one of the simplest projects you can build and it's very interesting as it uses a super-bright WHITE LED.
In the history of LED production, red LEDs were the first to be invented and their output was so dim you could barely see if they were illuminated. You needed a darkened room to see them at all.
Then came green, yellow and orange LEDs.
As time went by, the brightness improved and it came to a point where the output would shine into the surrounding air. These were called Super-bright LEDs.
Then came the blue LED. At first it was dull, but gradually the output increased to a dazzling glare.
With the combination of red, green and blue, manufacturers had the potential of producing a white LED.
This was the dream of all LED manufacturers.
Since the illumination produced by a LED comes from a crystal, it is not possible to produce white light from a single crystal or "chip." The only way is to combine red, green and blue. As soon as the output of blue came up to the quality of the other colors, a white LED was a marketable product.
White LEDs are now with us and their output makes them a viable alternative to the globe.
There is an enormous array of LED torches on the market, from $2.00 "give-aways" to $200 "rip-offs."
Although a LED torch is passable for illuminating an area, it certainly does not have the illuminating capability of a $10 lantern, using a 6v battery.
A LED torch is more of a "fun-thing" to see how far LEDs have come in the past few years and see what can be done with a single cell and an handful of components.

 




We decided to fit the circuit into a 2-cell torch but a white LED requires about 3.4v to operate, and two cells produce only 3v.
As you know, a LED will not operate on a voltage below its characteristic voltage. It simply will not operate AT ALL.
This characteristic voltage depends on the type of LED and is about 1.7v for a normal red LED, while a super-bright LED is about 3.1v - 4v.
The exact characteristic voltage varies with the colour, the intensity of the LED, the current flowing and the way it is manufactured. This feature cannot be altered after it is manufactured and the EXACT voltage must be delivered, otherwise the LED will be not work or if the voltage is higher, it will be destroyed.
This sounds a difficult thing to do, but a simple solution is to add a resistor in series and the voltage across the LED will sit at the exact value required by the LED. The extra voltage will appear across the resistor and according to Ohm's Law, a current will flow though the resistor and this will also flow though the LED.
All we have to do is create a voltage higher than 3.4v and we can use one of the latest SUPER-HIGH-BRIGHT white LEDs with a single cell.
To do this we need a voltage-incrementing circuit in the form of a blocking oscillator.
In this project we have opted to remove one of the cells of a 2-cell torch and place the electronics inside a dummy cell. This means the circuit needs to operate from a single cell. Fortunately ours will operate from 0.8v to 1.5v

 Caution: Do not allow more than 25mA to flow though a white LED (unless it is being pulsed) as it will be instantly DESTROYED. Other LEDs (such as low-brightness red LEDs) are much more tolerant - but white LEDs are easily damaged.

THREE CIRCUITS
This project has three different circuits:
1. Circuit-1 drives a single LED from one cell
2. Circuit-2 drives two LEDs from a single cell, and
3. Circuit-3 drives three LEDs from two cells.

This gives a choice to suit a variety of torches. The smallest penlight torch will only have enough room to drive a single LED while the larger "C" and "D" cell torches will drive two or three LEDs.
There are some slight differences between each of the circuits and you need to read the article if you want to deviate from any of the layouts we have given.
For instance, the 2SC 3279 transistor is capable of sinking 2 amps and this makes it a better driver for circuit-2 but its collector-emitter voltage is only 10v and it may zener in circuit 3, where the voltage is very near this value.


 

Circuit-1 drives one LED from a single cell

Circuit-2 drives two LEDs from a single cell

Circuit-3 drives three LEDs from two cells

THE TRANSFORMER
The secret of this circuit is the transformer.
We normally think of a transformer as a device with an input and output - the voltage on the input and output being connected by a term called "turns ratio."
If the output has more turns than the input, the output voltage will be higher.  This is called a step-up transformer. If the output has less turns than the input, the output voltage will be lower.
This applies to "normal" transformers where the voltage is rising and falling at a regular rate, commonly called a "sinewave."
But the transformer in this circuit is different.
The voltage applied to it is not rising and falling smoothly, and thus it does not work in normal "transformer mode."
Our transformer is really a coil in flyback mode with a feedback winding. It can also be called a choke or inductor with a feedback winding.
The feedback winding delivers a voltage to the transistor to turn it on HARDER. If the winding is connected around the wrong way, the circuit will not work.
Thus the feedback winding is very clever. It produces a high voltage and it is delivered in a particular direction - in other words it can be a positive or negative voltage.
These two features cannot be obtained by any other componentry.
By simply adding turns to the inductor we can get a voltage of any desired value and either positive or negative polarity. 
In our case this voltage is called POSITIVE FEEDBACK as it turns the transistor ON during the active part of the cycle.
Now we come to the MAIN, PRIMARY or FLYBACK winding.
This winding produces a high voltage during part of the cycle (the FLYBACK part of the cycle) and the voltage is passed to a reservoir capacitor, via a high-speed diode.
This voltage is produced when the transistor is switched off and the magnetic flux in the ferrite core collapses.
The speed of the collapse produces a very high voltage in the OPPOSITE DIRECTION. These two facts are important to remember.
The other important fact is called "transformer action." This is the action of magnetic flux.
When a voltage is applied to a winding of a transformer or a coil of wire, a current will flow and this will produce magnetic flux. If another winding is present, the magnetic flux will cut the turns of this extra coil and produce a voltage in it.
However, there is a very important point to remember. The magnetic flux can be: EXPANDING, STATIONARY or CONTRACTING.
When the magnetic flux is expanding, a voltage will appear in the second winding mentioned above.
When the magnetic flux is stationary, NO VOLTAGE will appear in the second winding.
When the magnetic flux is contracting a voltage will appear in the second winding with REVERSE POLARITY.
The size (the amplitude or "value") of the reverse voltage will depend on the speed of the collapsing magnetic flux. If the flux collapses quickly, the amplitude will be very high.

Once you know these facts, you will be able to see how the transformer in this project works.

CURRENT REGULATION
The circuit includes a feature called "current regulation."
This consists of a BC 547 connected to the base of the main transistor.
When the voltage across the "detector resistor" rises above 0.7v, the BC 547 turns on and prevents the main transistor operating.
This allows the LED to produce a constant brightness over a wide voltage range. The circuit will theoretically work to 0.8v supply.
Do not remove the current regulating transistor as the circuit will over-drive the LED when the supply is 1.5v. The excess current will instantly destroy the LEDs.

The actual operation of the circuit can be explained in a little more detail.
When the circuit is turned on, the oscillator transistor produces a high voltage from the inductor and this is rectified by a diode to charge a 100u electrolytic.
When the voltage rises to slightly over 9v, the three LEDs turn on and current flows though the 39R "detector resistor."
The voltage across the 100u will continue to rise and since the characteristic voltage of the LEDs has been reached, any further voltage rise will appear across the resistor. As soon as this voltage reaches 0.7v, the feedback transistor begins to turn on. The feedback transistor acts like a variable resistor as shown in the diagram below and some of the current from the feedback winding is passed to the 0v rail. The oscillator transistor sees a reduced "turn-on" effect and the output of the stage is reduced.

In this way the brightness of the LEDs can be kept constant throughout the life of the battery.


The circuit is actually being "pulled back" when a fresh cell is connected, by the action of the feedback transistor. As the voltage from the cell reduces, the oscillator circuit will not be able to produce a high output and the action of the feedback section will be hardly needed. Eventually the voltage of the cell will be so low that the LED will start to dim. This is the end of the life of the cell.

CIRCUIT SIMULATION
A number of circuits similar to this project have been presented on the internet. One circuit had twice the number of components and used 4 transistors.
The art of designing a circuit is to make it as simple as possible, while providing all the needed features. It is pointless making a circuit complex, as it simply adds to the cost and makes fault-finding more difficult.
But a note near one of the circuits was really annoying. It said the circuit "had not been tested, only a simulation was run."  While these simulation programs work in a number of applications, they certainly cannot take into account the characteristics of an inductor. This is one item that no-one can predict. It's performance depends on so many variables.
If you think you can design a circuit such as this on a simulator, and it will work, you are kidding yourself.
Electronics is not that simple.
Transistors exhibit different characteristics according to the current flowing though them and a circuit such as ours requires the main transistor to pass a very high current for a short period of time.
Fortunately, Japanese transistors are capable of passing a high current while some Philips transistors will fail to pass the test. The gain of a transistor under these stressful conditions cannot be determined from a data-sheet.
Circuits should never be presented in an article unless they have been tried and tested.
A simulation program cannot possibly take into account the effectiveness of an inductor in any particular situation, even though the inductance is known.
There are hundreds of ways to produce a 10uH inductor, or any inductor for that matter.
It can be air-cored or ferrite cored. The windings can be thick or thin wire. The core can be made of several different materials. On top of this it will depend on the frequency of the circuit.
The output voltage of an inductor that has been specially designed for a particular circuit can be 100 times higher than an incorrectly designed item. That's why it takes a considerable amount of "trial-and-error" to produce an ideal inductor or transformer.
The output voltage has a lot to do with the "Q-factor" or quality factor and this is a value that is associated with the way the inductor or transformer has been designed. The "Q value" is basically  the ratio of the supply voltage compared to the output voltage.
No simulation program can "guess" the value of "Q" and since the operation of the circuit is entirely dependent on this value, it has to be constructed.
I would not even attempt to put this type of circuit on a simulator.

DESIGNING AN INDUCTOR
There are many ways to go about designing an inductor or transformer.
You can sit down and study the theory of inductance, the effectiveness of ferrite material at different frequencies, the effectiveness of different wire gauges and the associated inductance formulae.
If you think you will be able to produce an inductor for this circuit entirely from theory, (with the first prototype working perfectly), you are kidding yourself.
There are a number of parameters you cannot specify for the formulae.
Even if you did come up with an answer, no electronics-designer would be satisfied with the first result. He would need to see the prototype and add remove turns to see the effect. He would use thicker or thinner wire and note the effect. And carry out all sorts of other experimentation.
He would monitor the battery current while noting the current though the LEDs and work out the efficiency of the circuit.
It could take 50 or more prototypes to arrive at the best design.
So, where do you start when designing a transformer or inductor?
No-one really knows where to start.
It all comes from trial-and-error and guessing a starting-point.
The easiest way is to copy an existing design.
But if you don't have something to copy, you can begin with say 10 turns. Note the output voltage and current taken by the circuit.
Increase the winding to 20 turns. Again note all details. From the figures you can work out if you are going in the right direction.
Continue collecting data with both additional turns and reduced turns as, sometimes, an unusual feature suddenly arises.
Keep working until you are satisfied with the results.
Even if you have studied inductor theory, you will still have to carry out the practical side of things.
Nothing takes the place of actually "doing-it."

DESIGNING OUR TRANSFORMER
In our 3 circuits, there are many different combinations of windings that will work.
The reason is the circuit is non-critical.
You have to understand the operation of an inductor in an entirely different way to the theoretical model to see how it operates.
This is called a "loose" circuit and a wide range of primary windings will produce the same result.
For example, a primary winding of 35 turns will produce the same LED brightness as 55 turns and the current from the supply will be the same.

The output of the transformer (on no-load) will be more than 200v and thus the circuit must not be operated on no-load as the voltage may damage the transistor.
If the LEDs are removed, the circuit will charge the capacitor to more than 45v and this is above the operating voltage for a 100u/25v electrolytic.
If you remove the LEDs and turn the circuit on, then re-solder the LEDs, they will be damaged. This is because the electrolytic will have charged to 45v.
Thus it is very difficult to experiment with the circuit to see how the transformer charges the electrolytic.
You will have to follow our explanation:

HOW DOES THE ELECTROLYTIC CHARGE?
The electrolytic is charged by pulses from the inductor.
In circuit-3 the voltage across the electrolytic is 10v and it is delivering current to the three LEDs at a constant rate of 17mA.

CRO waveform - output of inductor

In the CRO diagram above, the pulses (or spikes) occupy about 10% of the total time.
The area under the graph (under each spike) is shown in orange and this represents the energy supplied to the electrolytic.
The inductor is capable of producing a very high spike when in flyback and this voltage allows a burst of current to pass though the diode and charge the electrolytic.
When the inductor is operating under no-load, it is capable of producing a spike of more than 200v, but this voltage is not allowed to be produced when the load is connected. The voltage-spike is limited to the characteristic voltage across the LED or LEDs, plus the voltage drop across the diode and minus the battery voltage. The voltage will be about 9v.
If we are drawing 17mA for 100% of the time, we must deliver 10 times 17mA for 10% of the time to keep the electrolytic charged. Thus a current of about 17 x 10 = 170mA is needed to pass through the diode to charge the electrolytic.
The other feature of the diode is it prevents the voltage on the electrolytic being discharged to the 0v rail via the transistor when it is turned on.

The frequency at which the circuit operates is determined by the inductance of the inductor. The cycle start when the power is applied and the transistor turns on to allow current to flow though the main winding. This produces magnetic flux in the feedback winding to turn the transistor on harder. This continues until the transistor is turned on fully and maximum flux is produced.
But the flux is not expanding flux and thus it does not cut the turns of the feedback winding and the transistor does not get the full turn on current into the base.
The transistor is turned off and this causes the magnetic flux to collapse. This flux is in the opposite direction and it produces a reverse voltage in the feedback winding to keep the transistor fully turned off.
The main winding also produces a voltage in the opposite direction and it delivers a pulse of energy to the electrolytic via the high-speed diode.
As soon as the magnetic flux is spent, (converted to electrical energy) the cycle starts again.
The combination of these two operations creates the length of time for one cycle.
In our case the circuit operates at approx 90kHz.



THE OSCILLATOR TRANSISTOR
The oscillator transistor needs to sink a very high current for a very short period of time (as mentioned above) and thus it must be a "high-current" type. A "high-current" type improves the efficiency of the circuit. The efficiency improves because the circuit is capable of "topping-up" the electrolytic to a higher voltage and this allows the LED to take a higher current and thus produce a higher brightness.




LED OUTPUT
There is a lot of hype and confusion about the light output of some super-bright LEDs.
Sometimes there is very little difference when you compare the output of 1cd, 3cd and 6cd (6,000mcd) LEDs, when supplied from wholesalers.
One of the reasons is the difficulty in identifying each LED. They have no markings and if they are not kept in their correct bag, they can get mixed up! There are literally dozens of different types.
Secondly, the difference in brightness is due to the angle at which the light-beam emerges from the LED. This is due to the lens inside the LED and/or the way the LED is potted, producing a divergent beam or a narrow beam.
Almost all LEDs have a different illumination intensity, color and spot-size, depending on the manufacturer, beam angle and quality of the chip producing each color (efficiency).
Some have a blue appearance in the centre of the spot white light while others have a noticeable green fringe.
This project is an ideal way to test 2 or 3 LEDs at the same time. Since they are in series, they pass the same current and the intensity control will allow you to vary the brightness and compare the outputs.
When experimenting, keep a record of the type of LED by paining it with red or while nail polish. Keep the same reference on the bag from which they came. This will prevent them getting mixed up.