Vampire Flashlight

A toroid, in an electronics context, is a ring-shaped “bead” made of some material with a high magnetic permeability. Sometimes they are made of epoxy or another plastic filled with powdered iron, and sometimes they are made of ferrite, a dark gray or black ceramic material consisting mostly of iron oxide. They are typically wound with coils of wire to make toroidal inductors and/or toroidal transformers, which are very widely used in alternating-current (AC) power supplies. The “donut” shape is useful because it shapes the magnetic field in a way that reduces the inductive noise created in nearby components.

We’re going to start with a bare toroid and wind our own toroidal transformer. If you want to scavenge a toroid, find a junk PC motherboard, inline laptop or printer power supply, or compact fluorescent (CFL) light bulb and open up the case. (If you’re working with a CFL bulb, take care not to breach the fluorescent tube, as it contains trace amounts of mercury.) If you’d rather just buy a toroid than have to hunt one down, I recommend Fair-Rite #5943004901, which is the part shown in the title photo and through the rest of this build.

Once you have the case open, look for a little donut-shaped bit wrapped with coils of apparently bare copper wire. I found two inside the AC adapter (ViewSonic model HASU12FB60) for a long-dead LCD monitor. Either the big one or the small one would work with the Joule Thief circuit, but the small one is more practical for a compact build. You can try to desolder the toroid from the printed wiring board if you want to, but we’re going to be stripping the coils off, anyway, so the easiest thing to do is just snip the windings on the component side of the board.

Once you have the toroidal inductor loose, unwind and/or or cut away the wire until you’re left with just the bare magnetic core.

We’re going to be soldering directly to the battery case’s built-in terminals. These are mounted in relatively thin plastic and it is easy to melt them loose and ruin the battery pack if you try soldering to them while they’re in place. It can be done with skill, speed, and luck, but you’ll likely ruin one or more cases getting the knack of it. A better way is to demount the terminals from the plastic, tin them, and then put them back in place. Start by prying up the folded-over metal flaps securing each terminal in place with a hobby knife and/or small flat screwdriver. Once you’ve got all four flaps on each terminal pried out straight, pop the terminals out of the ends of the battery case.

Clamp the terminals in a vise or other work-holder by their wire leads. Apply heat with your soldering iron and then “tin” the inner, rounded sides of the terminals with solder. If you accidentally solder the flaps together, don’t worry — just remelt the solder with your iron and shake or blow it away (in a safe direction), then try again. Remember that solder will flow towards the heat, so apply your iron to the rounded side (which is where you want the solder to go), and not to the flaps.

Once you’ve got the solder where you want it, clip the terminal leads off right at the ring, then carefully reinstall the terminals in the battery holder and secure them in place by first folding the metal flaps back down with the tip of a small screwdriver, then squeezing with a pair of small pliers.

We use a normally open SPST momentary switch (aka “a button”) to turn the flashlight on. The switch is mounted directly to the negative terminal of the battery holder, and activates the circuit by completing the connection to ground while the button is depressed.

Position the switch so that two diagonally opposite leads fit into the negative battery terminal and the ready-made hole in the battery holder where the black lead used to come out. Mark the location where the other two leads contact the battery holder, and drill clearance holes for them using a small (about 0.04″) twist bit in a pin vise. Fit the switch on the battery case and make sure the leads protruding inside are not shorted against the negative contact spring. The bottom-center switch lead is the one to watch out for; if it does contact the spring and short the switch, just clip it off long enough to penetrate the case wall from the outside but short enough that it does not protrude inside.

Mount the switch in a vise or other workholder and generously tin the lead that will connect to the battery holder’s negative terminal. Then strip one end of the black wire you cut off the battery holder earlier and solder it to the lead diagonally opposite. Make sure your solder joint here doesn’t add too much width — you’ll need to be able to pass it through the hole in the corner of the case.

Thread the black wire through the corner hole and fit the switch in place on the end of the battery case. Touch the tip of your soldering iron to the switch lead at the battery negative terminal just long enough to reflow the solder on the lead and on the terminal and get a good joint, then immediately remove the iron and blow to cool. Again, the thing you want to avoid here is overheating the case terminal and melting it loose from the surrounding plastic. Fold the unconnected switch leads over, inside the case, using the tip of a small screwdriver. This will help hold the switch in place.

Temporarily install an AA battery in the case and connect a voltmeter across the end of the black wire and the positive battery case terminal. The reading should be 0V, or something very close to it, until you press the button, at which point you should see it climb to about 1.5V.

Reel out and cut 16″ each of green- and clear-enameled magnet wire, and twist the two wires together securely over a length of about 1″ at each end.

Wind the pair 10 times around the toroid. Take your time and make sure the coils are evenly spaced, and that the wires do not cross each other at any point on the core. When you’re done, one pair of wires should protrude from the top of the coil, and one from the bottom.

Clamp one end of the wire pair in your vise and seal the whole assembly with two or three coats of nail varnish.

After the last coat of varnish is dry (usually about an hour after application), release the toroid from the vise, untwist the two wires on each side, swap out the green wires between the pairs, and twist them lightly together again so you don’t get them mixed up. This step is critically important. The circuit will not operate unless current is flowing in opposite directions along the two wires wrapped around the toroid.

It is a very good idea to connect transformer, transistor, resistor, LED, and battery pack together on a breadboard and verify that the entire circuit is working before you start soldering. Use the actual components that will go into your finished flashlight and, once you’ve proved they’re all working, take them directly off the breadboard for assembly.

Wrap one leg of a 1/8W 1K resistor around a scrap of wire or cut-off component leg to form a small loop right next to the resistor body. Then bend the middle (base) leg of the NPN transistor out 90 degrees away from the flat side of the transistor case and solder it to the resistors’s other leg, as shown. You want the body of the resistor right up against the transistor’s rounded face.

Cut a 3/16″ length of 1/4″ diameter heat shrink tubing, slip it over the resistor/transistor pair, and apply heat to shrink and secure. Snip off the excess transistor base lead, and the excess resistor leads on both sides of the resistor body (leaving the loop attached).

Butt the flat side of the transistor against the back side of the LED — it should just fit between the LED pins. With the LED facing down and the legs of the transistor protruding toward you, make sure the “clipped” corner of the LED case is in the near-right position, as shown. Wrap the outer transistor legs (the collector and emitter) around the outside of the two LED pins on each side and solder them in place. Snip off any excess lead from the LED pins and the transistor legs.

Cut the wire pair on either side of the toroid to about 1″, then carefully and thoroughly clean the enamel off each wire with a piece of fine sandpaper. Twist the wires together firmly, and evenly, then solder them together. Be generous with the solder, but don’t add too much width.

Slip the soldered wires through the hole in the center of the positive battery case terminal. Make sure the toroid is centered and firmly seated against the end of the battery case, then briefly touch your soldering iron to the positive terminal, inside the case, to reflow the solder there and on the toroid windings and make the joint. As before, be careful not to overheat the terminal and possibly melt it loose from the surrounding plastic. When the joint is secure, trim the protruding wire pair flush with the battery terminal.

Cut the remaining wire pair protruding from the toroid to about 1″, then carefully and thoroughly clean the enamel off the entire length of the protruding clear-coated wire with fine sandpaper. Also clean the enamel off the green-coated wire over a length of about 10mm right at the end. Slip a 1/4″ length of 1/16″ diameter heat shrink tubing over the green wire, then hook the stripped section through the resistor loop and secure by soldering. Slide the tubing up over the solder joint and apply heat to shrink.

Strip the end of the black wire that runs through the battery case to the power switch, and hook it around the bent transistor lead on the emitter/negative side of the LED case. Hook the bare copper wire from the toroid around the bent transistor lead on the collector/positive side of the case. Don’t solder anything yet. Insert a battery in the case and press the power switch to confirm that the LED illuminates before proceeding.

Carefully cinch the LED/transistor/resistor bundle down along the toroid and ground wires until it is seated against the toroid face with the transistor body nestled inside the toroid ring, folding the green wire in underneath as you go. Once everything’s in place, bend the toroid and ground leads out to either side as shown. Confirm that the circuit is still working properly, then solder the leads in place on either side of the LED.

Trim the excess ground and bare copper wires away on either side of the LED, reinstall the battery, and you’re done!