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White LED Drive Circuit

How do you get 3.5 volts to drive a white LED when you only have a 1.5v battery?
(With special thanks to Dave Eselius for keeping this project alive until it was finished.)

New on this site since original publication of this page in 2002:

•  Simplest LED Flasher Circuit (in the world? You tell me.) (this page, Click here).
•  Rusty Nail Night Light demonstration added (this page, Click here).
•  Alternative_Types_of_Cores  (this page, Click here).
•  Toroid and Ferrite Bead Winding Notes (Click here).
•  New resistor-capacitor circuits to drive LEDs from 1.5 and 3 volts (Click here).
•  White LED Stroboscope (Click here).
•  Solar Powered Garden Light with PC layout (this page, Click here).
•  A Note About Transistor Selection (this page, Click here).
•  Common sources of ferrite beads (this page, Click here).
• A note of warning about LEDs and Eye safety (this page, Click here).
• Some tips on getting it running (this page, click here).
• A Utility Flashlight Using One Alkaline D Cell (This page, click here).
• Contributors to this page (this page, click here).

-  You may see copies of this page on other sites, but you will only see updates and improvements at  www.projects.cappels.org, the only site authorized to publish this material.
Be Careful About Peak Current

A note of caution: These LEDs are comparatively expensive, so I suggest putting a small resistor (1 to 10 Ohms) in series with the cathode of the LED and measuring the peak current as inferred from the IR drop using either a scope or a peak detection probe (as described elsewhere in these pages) while testing this circuit to be sure you don't exceed the Manufacturer's recommendations. In lieu of specific guidance from the manufacturer, for greater reliability try to keep the peak current to half of the manufacturer's recommended maximum.


A minimum number of parts yields a compact switching converter that can provide sufficient voltage to drive white LEDs. The resulting lamp is much more efficient, in terms of lumen hours per pound of battery, than incandescent bulbs, and because the color of the light is determined by fluorescence of phosphors within the LED assembly, the color of the lamp does not change perceptibly as the battery runs down. As a result, very long battery life is achievable. This circuit is suitable for powering flashlights, emergency lighting, and other applications in which it is desirable to power white LEDs from one or two primary cell batteries using a low cost circuit.

The Circuit

The circuit could not be simpler than this. The transistor, 1K resistor and the tapped inductor form a blocking oscillator. When the power button is pressed, the transistor is biased on through the 1 k resistor. Voltage that appears from the tap on the inductor to the collector causes the voltage on the 1K resistor to be even higher than the battery voltage, thereby providing positive feedback. Also because there is voltage across the inductance between the tap and the collector of the transistor, collector current increases with time (this is in addition to a starting value that relates to the current supplied to the base, but this part of the collector current is rather small. Because of the positive feedback the transistor stays saturated until something happens to change its base current.

At some point the IR drop across the inductor from the tap to the collector approaches the battery voltage (actually battery voltage - VCEsat). As this happens voltage is no longer induced in the winding from the tap to the 1k resistor and the base voltage starts to drop, and this forces the base voltage to go negative, thereby accelerating the switching off of the transistor. Now, the transistor is off, but the inductor continues to source current and the collector voltage rises.

Quickly, the collector voltage gets high enough for the LED to conduct current, and it does for a little while, until the inductor runs out of current, then the collector voltage starts to ring toward ground base voltage swings positive again, turning the transistor on again for another cycle.

The Inductor

If you aren't designing this as part of a commercial product, you have a lot of latitude in the design of the inductor. The size of the core, its permeability, and its saturation characteristic (Physical dimensions, u and Bs) determine how many amper-turns it can sustain before it saturates. If the core saturates before the IR drop from tap to collector approaches the battery voltage, the circuit will switch quickly anyway because saturating the core makes the coil look like a resistor and coupling between the collector half and the base half (the side with the 1k resistor) drops to very little, so the effect is the same as the IR drop approaching battery voltage. The wire size determines how many amps the circuit will dray (well, ok, milliamps) before the IR drop gets large enough for the circuit to switch. The inductance constant of the core (physical dimensions and u mostly) determine how man microseconds it takes the collector current to rise to the point the circuit switches off, and it also determines how long current will be delivered to the LED when the transistor is off. Nearly every inductor parameter affects the performance of this circuit.

I have made this with ferrite beads a few millimeters in diameter and toroid cores up to a few centimeters across (take a look at the Rust Nail Inductor further down on this page). Here is the general relationship between core size and characteristics:

Large core: Easy to wind, lower frequency operation, higher power.

Small core: Harder to wind, higher frequency operation, lower power.

How to get started: Get a transformer core, preferably ferrite, and wind 20 turns on it. Tap by pulling a short loop of wire off to the side, then continue winding another 20 turns. Increase turns to lower frequency, decrease turns to increase frequency. I've used as little as 10 turns, center tapped (5+5) and operated this at 200 kHz. Experiment. Scroll down to the bottom of the page - the tiny ferrite core used in the #222 bulb base run at about 200 kHz.

A Circuit Enhancement

One transistor blocking oscillator with tapped inductor driving LED with rectifier and capacitor filter to provide DC to the LED

The neat thing about this circuit is that it has the bare minimum number of parts to do the job. The LED runs from pulsating DC and since its forward voltage is higher than the battery voltage, it does not interfere with the switching of the transistor. The drawback is that the peak-to-average current of the LED is pretty high, it could be 3:1 or 5:1, depending on the circuit (mostly on the inductor and battery voltage). If you want the LED to be brighter for a given peak current, you can add the diode and capacitor shown in the schematic below.

Something that one online critic has pointed out is that where space if is available, it is a good idea to add a bypass capacitor between the negative terminal of the battery and the center tap of the inductor. Some batteries have a high impedance, and the decoupling capacitor might serve to increase the output of the circuit. A value of 10 microfarads should be sufficient unless you are using a very high inductance inductor, in which case a larger one might be better. 

Where Can You Put the Power Supply?

 Small penlight operating with LED instead of #222 incandescent lamp

Since this circuit has so few parts, I was able to make a couple of them so small that I could fit the entire circuit, including the inductor, 1 k resistor, 2N4401 (TO-92 package by the way), rectifier, chip capacitor, and Nichea NSPW315BS along with a little glue to hold it in, into the base of a number 222 incandescent penlight bulb.  

Using the LED replacement for the number 222 lamp, allows operation in a compact penlight. It gives enough light to walk outside on a moonless night. Based on about 35 ma currant drain at 1.5V, I estimated operating time, I estimated that this flashlight will operate for at least 30 hours of continuous use. That's a long time if your holding down the button. Data sheets for several of Duracell's alkaline cells can be found here.

Even though the battery voltage drops, the color of the light is always a crisp bluish-white. If its taken care of, this light "bulb" should last longer than me. I've had this one for 18 months as the latest update to this page and use this flashlight every night. I only had to change the battery twice. If the battery contacts didn't start to get intermittent from the corrosion, I wouldn't have know it was time to change the battery since the light was working fine. 

LED in #222 bulb base along side #222 bulb base 

LED replacement for the 222 lamp is on the left. The entire power supply is contained within its base.

White LED and drive circuit about to be inserted into the #222 lamp base

The hard part was taking apart the #222 lamp so I could reuse the base. 

Rusty Nail Night Light

These blocking oscillator type power supplies work best with ferrite cores, and sometimes they can be hard to locate. Some readers have expressed anxiety over making inductors, and that is understandable since to many, inductors have an aura of mystery about them.

Just to prove that these inductors aren't magic, or even that critical for that matter, I wound one on a rusty nail that I noticed laying beside the road one day while waiting for a tow truck. It is a 2-1/2 inch (6.5 cm) long flooring nail, which serves as the inductor's core.

The wire is a twisted pair of #24 solid copper wire that I pulled from a length of CAT-5 (ethernet) cable, which is similar to the wire used to connect telephones inside buildings. I wound 60 turns of the twisted pair in about three layers around the flooring nail, then I connected the start end of one conductor to the finish end of the other conductor and that made it into a 120 turn center tapped inductor.

I connected it to a 2N2222, a 1K resistor, a 1.5 volt penlight cell, and a white LED. Nothing happened. Then, I put a .0027 uf capacitor across the 1 K resistor (it happened to be on the work bench) and the LED came on. Sometimes you need .001 uf or so. The LED glows nicely and the circuit draws 20 milliamps from the AA cell. The waveform on the oscilloscope looks terrible, but the point is that the circuit oscillated with even this rusty nail, and it boosted the output of the 1.5 volt AA cell to over 3 volts peak to drive the LED.

Those who are familiar with some aspects of coil core selection would quickly point out that the eddy currents would be huge since iron has a low resistance compared to ferrite, or air for that matter, and that there would also likely be other types of large losses.  The point here is not that you should run out and buy some flooring nails to make LED lamps, but that this circuit was not "designed", but was thrown together and worked quite readily. If a rusty nail and some telephone wire is enough to light up a white LED, then the inductor is not so critical. So, relax, go buy a ferrite core and get started on your project.

Photo of transistor blocking oscillator driving a white LED. The inductor is wound on a rusty flooring nail.

(Above) Rust Nail Night Light. This LED power supply was thrown together in a few minutes using scrap parts.
Well, ok the LED itself was not really scrap.

Common Sources for Ferrite Cores

Wolfgang Driehaus from Germany wrote to point out that ferrite cores are used in compact fluorescent lamps, and he further stated that he has had success in making the cores work in this LED power supply circuit. It was only a day after receiving his email that I looked up at the ceiling and saw some lamps that needed replacement. Here is what I found.

Photograhp of a complete compact florescent lamp next to the circuit removed from a similar lamp.

Some compact florescent lamps from Sylvania had failed in my home. After buying new Philips lamps to replace them, I ventured into the garage to take one of the Sylvania lamps apart. The first problem was getting to the electronics in the base of the lamps. In later correspondence, Wolfgang showed me that the base of the lamp can be pried open and the circuit board removed without having to break any glass.  Be careful not to break the glass tubes in the lamp, as they contain mercury, which is toxic.

Photograph showing the removed inductors.

Inside the base of the lamp, as Wolfgang Driehaus, I found three inductors with ferrite cores, as well as a pair of high voltage transistors, a high voltage capacitor, and some other potentially useful components. The inductors were wound on three types of cores, which are a bobbin core (left, covered in heat shrink tubing), a toroid core (center) and an E-E core, (right).

I wanted to confirm the usefulness of the cores for myself, so I removed the existing windings from the bobbin and toroid cores. I cracked the E-E core in several places during the process of separating it from the coilform, so I did not have a chance to try it in the power supply circuit.

On the bobbin core, I wound 50 turns of #32 magnet wire, pulled out a center tap, and then wound another 50 turns. I connected this to a 2N4401, a 330 Ohm base resistor, and a white LED, according to the circuit at the top of this page. When I connected a power supply set to 1.5 volts, the LED lit up brightly. Ok, that is solid confirmation that the bobbin core from this particular Sylvania light works in this application.

On the toroid core, I wound 10 turns of #26 wire wrapping wire, pulled out a center tap, and wound another 10 turns. Connecting it in the same circuit (2N4401, 330 Ohms, white LED) with a 1.5 volt power supply, I saw that the LED lit up, but not as bright as with the bobbin core, but then again, I had only put 20 turns on the toroid.

So now we have a very common source of toroids. Compact florescent lamps are available in places, and as Wolfgang pointed out, they eventually wear out and need replacement.

Another reader pointed out that another source of ferrite cores is the shielding beads used on computer peripheral cables.  Those plastic-encased lumps on monitor, keyboard, and some USB cables are actually ferrite cores. If you are about to toss an old keyboard into the recycling bin, why not cut off that ferrite bead first? Christian Daniel of Gernany wrote, noting that the shielding beads are not idea for this kind of use, so you might want to try this last.

Alternative Types of Cores

If you can't find a ferrite core, or even an old rusty nail, all is not lost. You can still make a pretty good white LED power supply by using a non-magnetic core. It sounds like an oxymoron, but a nonmagetic core has little effect on the magnetic flux from the windings, and therefore does not interact strongly with the circuit -it is there mainly to provide mechanical support. Two experimenters have provided reports of their experiences with nonmagnetic cores, each with its unique attributes.

The wood core inductor

Bill Levan in the United States came up with a wood core inductor. His circuit powers a white LED from a 1.2 volt 700 milliamper-hour cell. Mr. Levan reports that he used the rusty nail night light circuit, but found that he did not need the capacitor across the resistor.

The coin is a United States quarter dollar, 2.54 cm  (1 inch) in diameter, for size comparison.
Take a close look at the LED - it really is on.

If the wood is dry, the material itself will not have a significant effect on circuit operation. Wood that has a lot of water in it might lower the efficiency of the circuit a little bit, but most likely, you would not be able to tell.

Mr. Levan's wood core is 5.08 cm x 12.7 mm x3.18 mm  (2 inches x 1/2 inch x 1/8 inch). The wire is 30 gauge solid conductor insulated wire wrapping wire, Radio Shack #278-501.

Wind 100 turns, pull out the center tap, then wind 100 turns more. In total, there are 200 turns.

You can contact Mr. Levan with questions about his wood core inductor and the circuit at the email address below.
  (The email address is an image.)

Air Core Inductor

Antonis Chaniotis in Greece converted his children's incandescent night light to use two LEDs in parallel, and increased battery life from one night to about 30 hours of light over three nights.

The LEDs, while emitting green light, are electrically similar to the white LEDs in the other circuits because, similar to most white LEDs, the die emits ultraviolet light, which excites green phosphors. Of course in the white LEDs, the phosphors emit white light. The large capacitor in the picture is 100 uf 25 volts, connected from the emitter of the transistor to the tap on the inductor, and it acts as a bypass capacitor to insure that the circuit sees a low impedance from the battery and switch. It may increase efficiency, especially as the batteries runs down and the batteries' internal resistance increases.

The base resistor is 10k, and the batteries is made of two 1.2 volt rechargeable cells in series.

Mr. Chaniotis' analyzed the circuit with SPICE and confirmed his findings by experiment. Interestingly, his analysis showed that for his circuit the optimum location for the tap is not in the center.

The coil is a total of 35 turns 80 mm (3.2 inches) in diameter with no core.  Start by winding 14 turns, the start is the collector winding. Pull out the tap for the battery connection, and then wind an additional 21 turns for the base winding.

Once, I made a similar coil for a different kind of application. I used a plastic food storage container from the kitchen as a coil form, then after winding, carefully slipped the coil off the container and held the wires together with tape. From the photograph, Mr. Chaniotis wrapped some wire around the bundle to hold it together.

You can contact Mr. Chaniotis with questions about his air core inductor and the circuit at the email address below.
(The email address is an image.)

Solar Powered Garden Light

This simple flyback/blocking oscillator LED power supply was adapted to and built into solar powered garden light by a talented experimenter in the United States known as "mrpiggss". The method of switching the power supply off during daylight hours, thus allowing the rechargeable cell to recharge, was taken from a circuit design by Nick Baroni, of Willetton, Washington, and published on the siliconchip.com.au website.  Here is the design from mrpiggss' workbench.

Photograph of 1.5 volt battery, solar garden light circuit board and solar battery.

The 1.5 volt cell used in this picture was replaced with a 1.25 volt nickel-cadmium cell.

Schemaitc of solar powered garden lamp.

Actual values used in mrpiggss' circuit.

Mr. Baroni's original circuit used a BC547, but mrpiggss found that a BC547C (the version he bought) would work as Q1 but not Q2 (see "A Note On Transistor Selection, below). In his version, mrpiggss used 2N4401 transistors for both Q1 and Q2, to keep the bill of material as simple as possible.  He also noted that if R1 was 15k, the sunlight gating function would be more sensitive, thus keeping the LED power supply off until it was darker than when R1 is 22k.

The core for L1 came from ebay with no part number or supplier but it is the size of a penny and about 3mm thick. The wire came in a 3 pack of magnet wire from Radio Shack and and are pure copper.  Being green in color and 30 gauge, the wire is easy to handle and wind. The cores have 40 turns, 20+20 (wind 20 turns, pull out the center tap, then wind 20 turns more). and removing the coating is easier than with really thin wire. The insulation on this wire can just be burnt off with a lighter, and it comes off just like magic. No scraping or sanding, although he use a small emery board to give it some tooth for soldering. 

Another very nice method of removing the insulation from magnet wire came from Christian Daniel of Germany: To quote his email:
     "Scratching away the insulation is difficult with thin wires, it's too easy to cut or weaken them. I prefer fine corund sanding paper. Or - for very thin wires - I use an Asperin(R) pill and press the wire with the tinned hot soldering tip on it : opacht ! Put your eyes and nose away ! The hot organic acid destroys the insulation and it can be tinne nicely."
The solar panel is a standard one-battery panel. it has no info on it but puts out 1.5 volts in full sun. No idea about the current rating.

D1 can be nearly any silicon or germanium diode, provided it is rated to handle the solar panel's output current into a short circuit. For most of the panels used in garden lights, this means basically, any diode you can buy. A low power Schottky or germanium diode would have a lower forward voltage drop than a small signal silicon PN diode. The 1N4001 series, as used by mrpiggss is a good choice as well because its large junction area results in a relatively low forward voltage drop.

Photograph of PC board layout of solar garden light, including indication of parts placement.

This is the "circuit side" of the printed circuit board.
The components are mounted on the opposite side. It should be noted that
the basing for the transistors corresponds to the 2N4401, and NOT the BC547.

mrpiggss' email address is "thetraindork (at) gmail.com" -please note that this email address needs to be retyped with "@" in place of (at). This is meant to stump spam robots, not people.

 A Note About Transistor Selection

The transistor used in this circuit can be any one of a wide variety of transistors. I recommend trying the 2N4401, 2N3904, and 2N2222, listed in no particular order. Dariusz Flaga in Poland, noted that the BC338 is popular in Europe and that its specifications, including voltage and current ranting, and saturation characateristics, suggest that it is a good fit for this application. Mr. Piggs in the U.S.A. (see the Solar Powered Garden Light, above), successfully used the BC547 on several prototypes.

You can even use PNP transistors, but if you do, remember to reverse the battery and LED connections. I have received email from a couple of project builders who used very high gain transistors and had trouble with their circuits. Transistors with high DC current gain (hfe) tend to switch slowly and may operate inefficiently or not at all.  Stay away from  the very high-gain audio transistors in particular.  The small capacitor across the base resistor, as shown in the Rusty Nail Night Light can speed up the transistor and make oscillation possible when you have low a inductance inductor, or if the transistors are a little slow. Speedup capacitors beyond  .0033 uf (33000 pf) are probably excessive and may actually slow down the circuit by causing the base to be over-driven.  If your circuit doesn't work, consider using a faster transistor.

Some tips on getting it running

Bob Parrott sent in the following helpful hints:

I had great fun playing around with your blocking oscillator circuit and thought your readers might like to try this variation if they have difficulty getting the thing to start oscillating.

Initially, I tried a BC107 transistor (because it was to hand) but it would only oscillate if I lowered the base resistor to 220 ohms and when I measured the current taken from the battery it was about 90mA - even before the LED was connected.

A BC108 oscillated easily as did a H945 removed from a dead switcher - also a good source of coils/ferrites.

But I was intrigued with getting the BC107 to run and added a small capacitor (22nF) across the base resistor to 'kick-start' the oscillations. It worked so well - with various transistors and coils that I was further intrigued to see how much I could increase the resistor value - hence the 20k trimpot. (The 22nF also got over the problem of the oscillator failing when I tried to add an ammeter in the battery circuit).

I found it would continue to oscillate right up to 20k ohms and this also had the effect of reducing the supply current (osc only) from 90mA to 800microamps - very important if using batteries.

Bob Parrott's email address is:  beep1952@hotmail.co.uk

Utility Flashlight Using One Alkaline D Cell

By: John S Rohrer  

This design is for a small flashlight that is reliable, manufacturable on a small scale, and which provides more than 100 hours of light from a single alkaline D cell. Such a light would be suitable for emergency situations. The D cell is chosen because it is available worldwide. Alkaline D cell voltage starts at 1.6 volts (V), with most of the energy expended at 1.2 V, and virtually no energy left at 1.0 V. For maximum life, this design will function down to 1.0 V.

White LEDs are more efficient than incandescent bulbs, more vibration resistant, and last much longer. A downside is the loss of the pleasing visual spectrum of an incandescent bulb. LEDs are also chosen because the 5 millimeter (mM) or T-1 3/4 package is available with a narrow (15 degree) viewing angle, eliminating the need for focusing reflectors or lens. A 15 degree viewing angle illuminates a 1 foot diameter area 6 feet away from the LED. This may seem small, but a beam covering a 2 foot area would be only 1/4 as bright. So the choice is concentrated brightness.

Given that LEDs become less efficient at higher currents, the best way to power an LED would be continuous direct current at about 60% of the LED maximum current. But the conversion from 1.5 V to 3.5 V requires oscillation or pulsing. The pulsed drive should have as large a duty cycle as possible to reduce the peak currents versus the average current in the LED, and thus maintain LED efficiency.

There should be no perceptible flickering in the light output. So the pulse rate should be well above visual sensitivity, which is about 60 times per second.
Some transformers and ceramic capacitors are microphonic, converting electrical signals into audible ones. Since noise is not desirable, the pulsing frequency also should be above the human audible range, which is roughly 20,000 times per second (Hz).

An easily buildable flashlight implies common mechanical parts, with no machining.


Altho’ white LEDs require about 3.5 V to turn on, they are easily destroyed if the voltage is forced higher. In the simplest circuits that can do the job, a single inductor stores energy in a magnetic field. The inductor energy is released as current to drive the LED, with the voltage determined by the LED forward drop. The time required to store the energy at the lower cell voltage is greater than the time to release it at the higher LED voltage; specifically, the storage time divided by discharge time is proportional to the LED voltage divided by input (cell) voltage. This 3:1 to 4:1 ratio can be reduced by placing the inductor step-up voltage on top of the cell voltage, dropping the ratio to 2:1 to 3:1, but even this is less than 50% duty cycle for the LED.  The problem can be eased by a tapped inductor (transformer), which decreases the time required to store the magnetic energy. Such designs are common, yielding a simple circuit that supplies the needed current during each charge-discharge cycle.

The tapped inductor can also be employed as a transformer to provide voltage to drive an LED while magnetic energy is being stored in the transformer magnetic field. Thus LEDs can be driven during energy storage and during energy release. But the transformer output voltage is of opposite polarity for energy storage versus energy release. Using a single LED implies diodes or transistors to steer the alternating current to it, but these would waste power. Two LEDs paralleled in opposite directions across the transformer output is more efficient. One LED would be on while the other is off.

If the transformer turns ratio is 4 to 1, then less than 1 volt can produce enough voltage to drive an LED during energy storage. This means that the LED powered by transformer action during energy storage will produce light for cell voltage down below 1 V.

The following schematic is the result of several "Not that way!” experiences. For instance, transformer T1 is relatively expensive; however, when the assembly time and effort to produce a replacement are considered, it is a bargain.

Circuit Operation
When switch SW1 is turned on, DC current flows thru T1 winding 2-1-4-6 and R3 to the parallel combination of DZ1 and Q1 base-R1. In other designs, the current increases until limited by transformer saturation. This design has a specific current limit implemented by zener diode DZ1, transistor Q1, and the paralleled emitter resistance R1-R2. Forward-biased DZ1 has a higher voltage drop than the Vbe of Q1 because the junction doping for zener diodes raises the forward voltage drop over that of a comparable transistor base-emitter junction. This voltage difference can be increased if a small area DZ1 is used with a large area Q1. The difference can be 40 to 60 millivolts (mV). This difference causes most of the R3 current to flow into the base of Q1, turning it on.

As Q1 turns on, the voltage across winding 2-3 of T1 increases. Transformer action produces four times that voltage across winding 3-1-4-6 and thus the LEDs. This drives Q1 on even harder thru R3 (and C2, which speeds the transition), until LED1 is turned on. Now there is a constant voltage momentarily across winding 2-3 while Q1 is saturated, and the current thru 2-3 increases at a rate determined by T1's inductance. As the current increases, the voltage drop across R1-R2 increases until that voltage plus Q1 base-emitter voltage begins to equal the drop across DZ1. This diverts more current into DZ1, limiting Q2 current. Measurements with an FMMT617 transistor, an R1-R2 emitter resistance of 0.5 ohm, and a FLZ5V1A zener diode yield a Q2 current limit of about 90 mA. This implies that the peak voltage across DZ1 minus the voltage across Vbe1 is about 90 mV / 0.5 ohm = 45 mV. 

With a 1 to 4 reduction in current due to transformer action, the LED1 peak current is 23 mA. Each LED is on for only part of a cycle, with the current ramping approximately linearly (constant voltage) from zero to peak and back to zero during the "on" time. So a 23 mA peak current with linear current ramping averaged over a full cycle (0-23-0 mA) corresponds to an average current of about 6 mA per LED. The typical operating current of a 5 mM white LED is 10 mA.

The DZ1 current limit stabilizes the light output versus cell voltage. However, the time to store this energy each cycle varies with the cell voltage, so there is some brightness variation versus voltage due to this. But there is less variation than with no current limit. The current limit also prevents transformer saturation, which would lower efficiency.

As the current limit is reached, the "rate of current increase" thru T1 becomes zero, so the voltage across pins 3-6 drops to zero, as expressed in the equation dI/dT = V/L. This shuts Q1 off, and the magnetic energy stored in the transformer produces a negative output voltage across pins 3-6, which powers LED2 until the energy stored in T1 inductance is depleted. Then R1 supplies base current to Q1 again, and the cycle starts over.  

When the cell voltage drops below 3.6 V / 4 = 0.9 volts, there is insufficient voltage to drive LED1, but the transformer still delivers energy to LED2 on the second part of the cycle during the magnetic field discharge. This provides an end-of-battery-life indication as LED1 goes out but LED1 stays on.
If Q1 is oscillating, it will continue to do so until the cell voltage drops below 0.2 volt because transformer T1 steps that up to 0.8 volt, which is enough to turn Q1 on to start a new cycle. And current is delivered to LED2 each cycle, altho' it dims considerably as the battery voltage drops. And because the cell will usually recover from 0.2 volt to above 0.7 volt with a little rest, it will produce some light from a cell that would be totally dead in any other device.

Note: A transformer's ability to hold a pulsed output voltage constant versus time is measured by its "volt-microsecond" capability. A transformer with a 50 V-uS rating can hold 4 volts for: 50 V-uS / 4 V = 12.5 uS, which is the half-period of the ~ 40 KHz operating frequency.
C1 provides a low impedance across the D cell at higher frequencies.

Prototypes have been built using a commonly available (Eagle Industries) plastic D cell battery holder with the printed circuit board mounted to the battery holder. The shape of this assembly allows for convenient positioning of the flashlight when it is set down, and it fits nicely in the hand. The assembly does not break despite multiple 3 foot drops onto concrete.

The circuit draws about 80 mA at 1.6 V and 35 mA at 1.0 V. A D cell typically stores 12 ampere-hours (AH), yielding about 200 hours of life. Eight hours use a day means 25 days of life.

The brightness at 1.6 V decreases to about one-half at 1.2 V.

In quantities of 100, the circuit board runs about $4.00, the switch $3.00, the transformer $2.70, the battery holder $1.00, with miscellaneous parts adding another $3.00, for a parts total of $13.70.

With this, all design goals have been met.

John S Rohrer  
original 1Jun2011   last revision 19Feb2018 q

Mr. Rohrer's email address is jsmarohrer@fastmail.com

Danger of Eye Damage From Visible Light Emitting Diodes

Contributors to This Page

What started out as a simple web page to share a simple circuit that allows white LEDs to be driven by a single 1.5 volt cell has grown bit-by-bit over the years to make it even more helpful to experimenters.  The contributions of the individuals listed below, in the order of the appearance of their contributions on this page, is noted with appreciation.

Wolfgang Driehaus             - Ferrite cores from compact florescent lamps.

mrpiggss                               - Garden Light.

Bob Parrott                          - Tips on Getting It Running.

Geoff Davies                         - LED Safety

Bill Levan                              - Wooden Core

Antonis Chaniotis                 - Air Core

Christian Daniels                  - EMI Cores, removing insulation from wires.

John S Rohrer                         - Utility Flashlight Using One Alkaline D Cell

And many others who provided suggestions and feedback to improve the page, but who were not directly quoted.

If you have questions or comments about this project, please send email to me at projects (at) cappels.org.

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Contents ©2002,2004,2005, 2006, 2007, 2012 and 2008, 2018 Richard Cappels All Rights Reserved. Some images copyright mrpiggss, 2005. Some Tips on Getting it Running and accompanying illustration are copyright Bob Parrott 2007. Photos by Bill Levan  and Antonis Chaniotis copyright 2008. Article and schematic "Utility Flashlingt Using A Single Alkaline D Cell" copyright 2018 by John S. RohrerCopying for commercial purposes is not permitted without permission of the authors.   Dick Cappels' project pages http://www.projects.cappels.org

Last updated March, 2018.

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