Picture of Switch mode LED torch
I modify a cheap chinese rechargeable torch with a switch mode LED drive circuit.

 The advantage over the original circuit is that it has constant brightness even when the battery voltage drops as it discharges, down up to the very end when its voltage goes below 2.7 volts.

The torch had seven LEDs in an aluminized plastic reflector, and a three position switch lighting zero, three or seven LEDs on successive positions. The LEDs were all connected to the 4 volt sealed lead acid rechargeable battery with independent resistors.

When the battery died, I replaced it with a 3.6 volt nicad pack from a defunct mobile phone I had lying around, and it worked, but the brightness had dropped. Replacing all resistors to raise the current through the LEDs (all seven of them) was thought to be too much of a chore, so I went ahead and wired them all in series. Now I had seven LEDs in series, requiring about twentyfour volts to light them, at around twenty milliamps.

Then I used an integrated circuit driver to raise my battery voltage to the value required.
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Step 1: The schematic

Picture of The schematic
This is the circuit I intend to use: The texas instruments TPS61041 LED driver, wired up according to the circuit in the datasheet, reproduced below.

TI make the circuit, so they are the people to tell me how to use their chip. It looks simple: Two capacitors to store energy and decouple the supply voltage and output voltage, a rectifier, an inductor, and finally a resistor to set the LED current.

According to the datasheet, as long as the other components are correctly chosen and wired up right, the LED current depends only upon the value of  the resistor Rs in the figure below. It doesn't matter how many LEDs there are, and what voltage the battery has.

 This particular integrated circuit can drive LEDs in series up to 28 volts, and the input voltage has to be in the range 2.7 volts to 6 volts. Since a white LED requires around 3.5 volts, upto eight of them in series may be used. Since the minimum voltage has to be 2.7 volts, at least two NiCd cells in series would be required to drive it - or one Li-ion.
hobbyman1 year ago
a nice handbuilt pcb, liked the idea. good job.
This is a great solution! I have been looking for something like this. I am glad I came across your instructable. Thank you for sharing.

Now I just have to figure out how to incorporate it into my existing LED fade-in/fade-out circuit which is a little bit of a challenge. Here is my schematic:
http://i232.photobucket.com/albums/ee38/__darkside__/Misc images/LEDfadein-fadeoutcircuitw-sound.jpg

I am thinking I should hook up your ic to the collector of T1 (the power transistor). Although, I am not sure if my circuit will work if T1 is hooked up to what is basically a different power source than the rest of the circuit. Maybe it would be better to use your ic hooked up to the emitter of T1 but I don't know if this ic will work properly if it is not constantly connected to the power source. The other concern with using these circuits together is whether or not the ic will interfere with the fading functions of my circuit.

On my circuit R2 is a bypass for the switch and allows the LEDs to receive a lower current so that they are dimly lit. When you activate the reed switch power will travel through the circuit and fade in the LED (time depending on value of R4) and when the switch is disconnected then C1 will drain (time depending on value of R8, slowly fading out the LEDs.

For more info on my circuit check this link:

Thanks for any help you may be able to offer.
qwerty1563 years ago
wow, amazing job, im going to build this myself now :)

You seem to like TI alot ! Tell me more .
bhvm5 years ago
Was there any Improvement in the brightness mate? How much lumens does this give? This may be of your interest- http://www.instructables.com/id/Make-yourself-Powerful-LED-Off-road-lights-headli/
neelandan (author)  bhvm5 years ago
This mod was to use a different type of batteries, and to have an acceptable performance while doing so. Three ni-cads, while fully charged, measure about 4.2 volts. The voltage quickly falls to around 3.6 volts on load, where it stays for most of the discharge. With the original circuit, individual resistors set the LED current. They will have to be dimensioned to be within the safe current for the LEDs at the maximum battery voltage. Therefore, during most of the discharge of the battery, the LEDs will be running at less than maximum current and therefore the light output will be less than the maximum possible. This circuit allows me to set the LED current at near maximum and this current is maintained until the battery is almost completely discharged.
bhvm neelandan5 years ago
Excellent thinking, Being an LED freak myself, Its well known that drving LEDs at slightly below their maximum rated power will work wonders for long life, at negligible drop in performance. .. Have you worked with Seoul p4 or Luxeon Stars neel?
newzy5 years ago
hello neel, thanx 4 the ckt.. what is the value of zener diode used and how to calculate the output voltage and current of this ckt theoretically, ???thanx
neelandan (author)  newzy5 years ago
If you go to the website of Texas Instruments, ti.com they have the datasheet for this chip as well as useful info on driving LEDs and more theory than you would ever wish to wade through. Happy hunting!
.Unknown.5 years ago
lol....replacing all the resistors is a chore, so you put them in series.
neelandan (author)  .Unknown.5 years ago
Three nicads in series will have voltage in between 3.6 volts and 4 volts or so depending on the state of their charge, so calculating the new values to put in was something of a riddle. Much easier to connect them in series, so that their currents will all be equal, and then use active electronics to hold that current near the maximum rated current for the LEDs.
yoyology5 years ago
I'm always impressed by your builds.  They seem so straightforward, and then we see them in context and realize how tiny and intricate they are.  Great instructable. Thank you!