Hacking Torches

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Hacking LED-based Torchesto become infrared-sources


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This document describes experiments done and the procedure arrived at to convert an LED-based torch or flashlight into a powerfulinfrared-emitting source. The idea is that such an IR-torch will emit a bright cone of light, just like a regular flashlight, with theimportant difference that the emitted IR light is invisible to the human eye. The IR light can however be detected by infrared-cameras. Please see the related document on how to make an IR-camera out of an ordinary USB webcam.

The TorchesFor this experiment, I bought two different LED-based torches. These torches are approximately the same price (at MediaMarkt, inJuly 2009), and each has some advantages.

Philips SFL5561A simple torch, it is powered by two AA-size batteries (provided) and has 3 high-brightness white LEDs and a clear plastic front witha triple collimator lens that focuses the light from the 3 sources into a single bundle (at some distance from the torch). The housingof this torch is rubberized, which supposedly makes it splash-proof and somewhat impact-resistant. Its only other feature is the on/off switch.

Alecto ATL-120WThis torch comes with a set of 3 rechargeable AAA-size batteries, and a handy ring-shaped holder/charger which plugs into a wall-socket. Whenever the torch is taken out of its charger/holder or when there is a power-failure) it turns on automatically. It does havethe regular on/off button, of course, but also a smaller button which selects one of 3 intensity-levels. Holding the small buttonpressed for a while puts the torch in a special emergency mode where it starts blinking out the letters S O S in morse-code.The ATL-120W has 7 white LEDs, and a front cover with 7 (small) collimator lenses. It might be expected that this torchs 7 LEDs giveoff more light than the 3 LEDs of the Philips torch, but here it may not be a simple case of more is better, as we shall see.

The different intensity-levels and S O S-blinking function of this torch are not at all needed for our purpose, and so choosing thistorch might be overkill, but i do like the fact that its rechargeable, and the nifty charger/holder gadget.

But first we need to have a close look at the two rival torches, as they come, out-of-the-box. And have a look inside, of course.
http://www.alecto.info/TradePoint/Item_View?itemNo=ATL-120Whttp://www.alecto.info/TradePoint/Item_View?itemNo=ATL-120Whttp://www.p4c.philips.com/cgi-bin/dcbint/cpindex.pl?scy=GB&slg=ENG&sct=TORCHES_SU&cat=FLASHLIGHTS_CA&session=20090901120437_83.163.15.17&grp=PORTABLE_ENTERTAINMENT_GR&ctn=SFL5561/10&mid=Link_ProductInformation&hlt=Link_ProductInformationhttp://www.p4c.philips.com/cgi-bin/dcbint/cpindex.pl?scy=GB&slg=ENG&sct=TORCHES_SU&cat=FLASHLIGHTS_CA&session=20090901120437_83.163.15.17&grp=PORTABLE_ENTERTAINMENT_GR&ctn=SFL5561/10&mid=Link_ProductInformation&hlt=Link_ProductInformation


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3-LED torch to out-perform the 7-LEDtorch at distances which are relevant forour purpose.

Also, replacing 3 LEDs with IR-LEDs isgoing to require less work, (and less

LEDs!) than replacing 7 LEDs.

Torch Brightness Comparison.The brightness of the torches depends on these four factors:1. The type of the LEDs used2. The electric current flowing through the LEDs. (see page #) It is relatively safe to assume that

the designers of the electronics in the torches have optimized this factor for the LEDs used,so this factor is largely dependent on point 1, above.

3. The number of LEDs4. The effectiveness of the plastic lenses in front of the LEDs

It is therefore not immediately a given that a torch with 7 LEDs is brighter than one with 3 LEDs.

Furthermore, since the idea is to replace the white LEDs in the torches with infrared LEDs,factors 1 & 2 will be totally different from the current situation, in the end. We want to choosethe right torch based mostly on the number of LEDs and the quality of the optics, but since thetwo torches have different LEDs in them, a simple empirical comparison will be difficult.

Shining the torches at a white wall in a darkened room reveals some important differencebetween the two torches. Firstly, the colour of the light is different, proving that we are indeedlooking at different LEDs having been used in each torch.A close-range test, with the torches about 1m from the wall, shows the two torches shining withapproximately equal brightness, though the Philips torch (at left in the picture) has a muchsmaller central light-spot, which looked much brighter to the naked eye then in the photo here.

Aiming both torches on the same spot on the wall from about 3m away shows how the Philipstorch has a much smaller, sharper defined light-spot, which easily outshines the wider wash oflight from the Alecto torch. Apparently the SFL5561 has much better optics, and this allows the

Both torches The Philips SFL5561 The Alecto ATL-120W


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Opening-up the TorchesSo far, it looks like the Philips SFL5561 is going to be the flashlight ofchoice for hacking. It had better optics, and with 3 LEDs it is going to beeasier & cheaper to modify. Lets have a look inside the torches and seeif we can find any compelling arguments to the contrary.

The ATL-120Wcomes apart quite easily. The end-cap unscrews to reveal a cylindricalbatter-holder with the three rechargeable AAA-batteries.

The plastic ring at the top also unscrews, allowing us to remove theplastic lens and the shiny plastic thing underneath, which houses theLEDs. In fact, the LEDs are soldered onto a circuit-board, which in turnis screwed to the underside of the shiny plastic thing.

Deeper inside the housing we can see several cables running from thecircuit-board to the two pushbuttons (on/off & mode/brightness) and,presumably, to the battery-pack and the coil for the inductive couplingwith the charging ring/holder.

It is important to note how the bottom side of the circuit-board is quitefull of electronics. Tiny surface-mounted parts at that.

Unscrewing the four little Phillips-head screws that keep the circuit-board attached to the shiny plastic thing allows us to separate the twoand get a closer look at the LEDs on the top-side of the circuit-board.The 7 white LEDs are clearly visible. Also there is a smaller 8th LED (atthe top edge of the circuit-board in the last picture below). This is ablue LED which lights up when the torch is charging.


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The SFL5561Is a much simpler beast to tame. The head of the torch unscrews to allowaccess to the battery-compartment (with two AA-size batteries).On the inside of the head, we find a black plastic cap with a brass + contact,held in place by 3 little Phillips-head screws. Unscrewing these makes the cappop off (there are springs underneath!), revealing a small circular circuit-board.

It is noteworthy that this circuit-board has no components on its bottom side,no surface-mounted components at all. Aside from the 3 LEDs and the two

contact-springs, theres only 6 other components involved. This is evidently amuch simpler circuit that the one in the Alecto torch.

And, hopefully, simpler means easier to understand, and easier to hack.


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Different LEDs, Different VoltagesA brief explanation about LEDs, their colours, operating voltages and drive currents.LEDs are not at all like regular light-bulbs. They dont have a glowing metal filament. Instead, the light-sources in LEDs are made from semi-conducting materials, which has all sorts of important implications.LEDs behave like diodes, electrically (in fact, the acronym LED stands for Light Emitting Diode. they arediodes.)In general, a diode can only conduct electricity in one direction but, more importantly, it requires acertain voltage to be applied before they even start working. This voltage is called the forwardvoltage (written as Vf). The current flowing through a diode (Id) has a highly non-linear relationship to

the voltage that is present on the diode. In other words, a diode is not an Ohmic resistor.Depending on the voltage across the diode, we can distinguish four cases:1. The voltage is reversed (i.e. the wrong way round); no current can flow

through the diode, Id = 0, an LED will not light up.2. The voltage is lower than Vf; the current that can flow is negligible, Id 0, an

LED will not light up, or maybe very very dimly.3. The voltage is approximately equal to Vf; a normal diode-current can now flow.

Id is now effectively determined by the rest of the circuit containing the diode.An LED will emit light, and the brightness of the light is in turn determined by Id

4. The voltage is higher than Vf; this will cause a really large current to flowthrough the diode, with Id being many times larger than the rated maximum.The diode (or LED) will quickly overheat and burn itself out. Obviously, this

situation must be avoided!

Different Colour LEDs have Different VfsThe Vf of a given LED is obviously an important parameter. The V f is also directlyrelated to the colour of the light emitted by the LED. In fact, both the V f and thewavelength of the emitted light are determined by the same property of the LED,namely the semiconducting materials used in manufacturing it. This Wikipediapage has a table of various colours, common wavelengths, Vf ranges (denoted as

V this time) and the semiconducting materials commonly used.

Voltage RegulationSince a small variation in LED-voltage has big consequences for the brightness, normally the circuits that power LEDs employ some

sort of voltage regulation. Just connecting an LED to a battery might work for a while, if the battery-voltage is close enough to theLEDs Vf, but as the battery is gradually depleted, its voltage will drop, and the LED will grow dim very fast, long before the battery isreally empty. More often than not, the battery voltage does not match the Vf, and some sort of voltage-regulating circuit is necessary.

The characteristic graph of a diodes voltage-to-currentrelationship. In this graph Vf is called Vd.
http://en.wikipedia.org/wiki/Light-emitting_diode%23Colors_and_materialshttp://en.wikipedia.org/wiki/Light-emitting_diode%23Colors_and_materialshttp://en.wikipedia.org/wiki/Light-emitting_diode%23Colors_and_materialshttp://en.wikipedia.org/wiki/Light-emitting_diode%23Colors_and_materialshttp://en.wikipedia.org/wiki/Light-emitting_diode%23Colors_and_materialshttp://en.wikipedia.org/wiki/Light-emitting_diode%23Colors_and_materialshttp://en.wikipedia.org/wiki/Light-emitting_diode%23Colors_and_materialshttp://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Light-emitting_diodehttp://en.wikipedia.org/wiki/Light-emitting_diode


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Testing the Torches in ActionBy using some electronic measurement-equipment (a multimeter and anoscilloscope) we can get a pretty good idea of how the LEDs are beingdriven, and thus make an educated guess as to how the circuit works.

The ATL-120WWhen measuring the LED voltage of this torch, we note that it averages3.172V, just a tiny bit below the expected Vf of 3.2V for white LEDs.


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Type to enter text


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Hacking the Philips SFL5561To modify the the SFL5561 with IR-LEDs, we cannot use the existing 3.2V step-up regulator circuit, and it will have to be completelyreplaced with our own 1.5V step-down circuit based on the LM3671MF-1.5 chip.

Steps:1. Open-up the torch, unscrew the + cap and remove the circuit-board from inside.2. Desolder all components on the circuit-board, except fro the two contact-springs.3. Solder in the IR-LEDs4. Solder the LM3671 and the 2 capacitors on the bottom of the board5. Solder in the 2.2H inductor6. Temporarily solder on 2 wires for testing prposes7. Test the new circuit8. Desolder the test-wires again9. Put the circuit-board back in place and screw on the + cap.

Pictures of each of these steps, with more detailed explanations followon the next pages. (step 1 was already covered on page 5)

DesolderingUse desoldering-wick and/or a solder-sucker (not pictured) to removeas much solder as possible from the existing solder-points. When allthe solder is removed, the components should drop right off the circuit-

board, or else come loose with a minimal amount of wriggling.Preparing to desolder. A spool of desoldering-wick nearby

The wick up close It works by absorbing the solder from thejoints into the wick. Some patience is required.

The empty board and the desoldered components.
http://www.national.com/pf/LM/LM3671.htmlhttp://www.national.com/pf/LM/LM3671.html


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Soldering in the IR-ledsSince the LEDs are diodes, their orientation matters. They will not work if solderedinto the circuit the wrong way round. The fresh IR-LEDs have a longer and a shorterleg. The longer leg is the Anode or the + side of the LED.Printed in white on the top side of the circuit-board, inside the circled where the LEDswere soldered, there is a little arrow. This arrow is pointing from the anode (+, thelonger leg) to the cathode (-, the shorter leg).One LED sits right next to the contact-spring pointing upward from the board. ThisLED should have its shorter leg in the hole closest to this spring. The other two LEDshave the same orientation when the board is rotated over 120, so that the LED in

question ends up in the same place as the previous LED.

It is also important that the LEDs are soldered in sitting flush on the circuit-board.This is tricky, because the spring sticking out does not allow for the circuit-board tobe placed horizontally with the LEDs resting on the table. One way to deal with this isto solder the LED in place with one joint at first, at whatever angle it happens tohave.Next, hold the the circuit-board between thumb and fingers, exerting a little force onthe LED while briefly re-melting the solder-joint holding it in place. The LED will snapinto position, landing flush on the board. Hold it there until the solder-point hassolidified again. Now solder the second point.

Repeat this procedure for all 3 LEDs, taking care to put them in the correctorientation.

When this is don, carefully snip off the excess leads.


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Soldering the Surface-Mounted partsBefore soldering the tiny LM3671 chip, some preparation is required. Since there is no place on the boardwhere this chip can be placed with its 5 legs all on different copper pads, we will have to improvise.Firstly we can make a place on the bottom of the board where the chip can be soldered with 3 of its 5 legs onthe right copper traces. Secondly well have to bend the two remaining legs up so that they do not touch theboard. We can later solder in place a floating bridge made from a piece of snipped-off LED-lead.

Take a small, sharp tool and carefully scratch away the green paint covering the copper traces, exposing thecopper layer underneath the paint in the two places indicated with arrows in this picture:

Then, very carefully using tweezers, bend leg 1 & 3 of the tiny chip upward so that theirends rise above the chips top surface. leg 1 & 3 are the two outer legs of the row of 3 legs.Take care that leg 2 (the middle one) stays where it is.

Solder the chip onto the board with leg 2 in the position indicated in the middle picturebelow. It is probably easiest to first put a small drop of folder on the solder-pad first, thenmaneuver the chip into position with the tweezers and re-melting the solder whilst holdingthe chip in place.

Once the chip is fixed in place by the solder-joint at pin 2, its easy to solder pin 4 & 5 ontothe board, in the places where the copper traces were exposed by scratching off the greenpaint. Take care not to accidentally close up the hole near pin 5 (indicated in the last picture),because well need that hole later.

The LM3671 with leg 1 & 3 bent up


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The two surface-mounted capacitors are relatively easy to solder in place. However,care must be taken not to confuse the two, since the circuit requires two capacitorsof different values. Theres a 10F output-capacitor (connected in parallel with theLEDs) and a 4.7F input-capacitor (connected in parallel with the batteries). Thesetwo capacitors look almost exactly the same, so its probably wisest to take onefrom the strip inside the bag it comes in, and solder it onto the board, before takingthe other one from the strip inside the bag that that one comes in.

In the picture at left you can see the 2.2uH inductor (at top), the 4.7uF capacitor(bottom left) and the 10F capacitor (bottom right). A keen observer might notice a

slight colour-difference between the two capacitors, but its not recommended torely on that, since lighting-conditions may vary.

Solder the 10F capacitor in the position show in the leftmost picture below, being careful not to blob solder across to pin 5 of thechip. If you follow the copper traces across the board, you will see that one side of this capacitor is connected to one leg of each LED,and to pin 4 of the chip. The other side of the capacitor is connected to the other legs of the LEDs, pin 2 of the chip and the upward-pointing contact-spring.The 4.7F capacitor gets soldered in place as shown in the second picture below. One end of the capacitor gets soldered to a solder-pad where an LED-leg is already present. It is easiest to solder this side of the capacitor first, by simply re-melting the existing solder-joint and then sliding the capacitor in place with the tweezers. The solder the other side of the capacitor to the empty pad. Followingthe traces you will see that the capacitor is now connected between the two contact-springs.

Bend over one leg of the 2.2H inductor, and insert the legs in the two empty holes in the middle of the board, exactly as shown in

the last two pictures below. Soldering this component in place and trimming the legs should be easy.

The 10F capacitor The 4.7F capacitor


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For a final trick, we need to place the floating bridge connection mentioned on page11. The idea is to connect the two upward-pointing legs of the chip with the + sideof the 4.7F capacitor.

Take a bit of snipped-off LED-leg and bend it into a tiny L shape as shown here.

Pre-solder the short leg of the L, and very carefully hold it in place over the chip,preferably touching both pin 1 & 3 and the nearest end of the 4.7F capacitor. Seethe two pictures below.

Soldering this bridge in place is very difficult. It requires steady hand and a certaindeftness with the soldering-iron. An iron with a very fine tip is also a must-have.

If the bridge is soldered to pin 1 and/or 3, but not to the capacitor, do not try to forceit in place. This will break off the chips pins. Instead, de-solder and try again.

4 mm

2 mm


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The IR-flashlight experiments and this document were made for the projectThe City Is Creative by Michelle Teran. The project is a co-production between Flux-S and Baltan Laboratories.See also the Baltan blog

Torch hacking and documentation by Stock.

2009, Mister Stock Interfaces.

Find related documents on Mister Stocks Wiki
http://www.mrstockinterfaces.com/groups/wiki/wiki/bb069/The_City_Is_Creative.htmlhttp://www.mrstockinterfaces.com/groups/wiki/wiki/bb069/The_City_Is_Creative.htmlhttp://www.mrstockinterfaces.com/http://www.mrstockinterfaces.com/http://www.baltanlaboratories.org/?cat=39http://www.baltanlaboratories.org/?cat=39http://www.baltanlaboratories.org/http://www.baltanlaboratories.org/http://www.flux-s.nl/http://www.flux-s.nl/http://www.ubermatic.org/http://www.ubermatic.org/http://www.thecityiscreative.net/http://www.thecityiscreative.net/

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