A friend of mine contacted me asking if it were possible to create LED batons that flashed red and blue like a police light. His plan was to use these in his upcoming marching band show.
Upon initial inspection this seemed like an easy task, stick some LED tape onto a couple of poles and make it flash. Easy, right? After I foolishly agreed to take on this project I realized it wasn't going to be as simple as I initially thought. These poles are going to be subject to repeated shock and abuse. All the electronics must be fit inside the aluminum poles including a battery and a charger. The poles must also remained balanced so that they spin in a predictable manner when tossed into the air.
Step-Up Converter https://amzn.com/B00W94H41I
Battery Holder https://amzn.com/B00FGX8NVO
Red LED Strip https://amzn.com/B01ELDOXOK
Blue LED Strip https://amzn.com/B01ELDOSZY
Clear Heat Shrink https://amzn.com/B003ICY2HO
I did use some 3D printed parts which I printed on the M3 mini, but with some ingenuity you could create these parts the old fashioned way.
Step 1: Make It Flash
In order to create the flashing, I implemented an astable multi-vibrator circuit. Sounds complicated, but this is a simple circuit consisting of two PNP transistors, a few resistors, and a pair of capacitors. This circuit turns one side on and the other off continually. The timing of the flash is changed by altering the value of the resistors. I found that 20K ohms was the ideal value for the desired rate of blinking. Since I didn't have a 20K ohm resistor I used two 10K ohm resistors in series.
Sure I could have stuffed an arduino in this and made it programmable, but that is outside the scope of the desired operation. Also, that would have been more complicated and expensive.
To keep things simple I used some strip board to build the circuit on. The strip board was cut to size using a Dremel and a cut off wheel. Follow the schematic and solder the components on the board. Be sure to clip the legs of the components as close to the board as you can. It is important that the strip board lays flat so that it will fit in the baton.
Step 2: Boost!
Most LED strip lights run on 12V. To keep things light, I only used one 18650 lithium ion battery (3.7 V). In order to get the required voltage I used a boost converter. This device is capable of taking 3-32V input and outputting 5-35V making it perfect to boost the 3.7V from the battery to the voltage required to power the LED's.
While the step-up converter is relatively small, it is slightly too large to fit into the baton tube. To make it fit into the tube I had to move the potentiometer and the inductor. The potentiometer (blue plastic box) was de-soldered from the top of the board and soldered on face underneath the board. Since this is a through hole component no additional wires are required, just fold over the leads and solder back on.
Once the potentiometer is moved to the bottom of the board, the inductor coil can be moved over. This is a surface mount component, but don't let that frighten you. There are metal pads on the left and the right of the part. Apply heat to one side at a time alternating back and forth. Gently pry back the part as you are applying heat and it should pop off with ease. Once the coil is off remove the left over solder on the board and replace with fresh solder. This will make it easier to re attach the inductor. Solder the inductor back onto the board and shift it over as far as the solder pads allow. The solder pads are slightly over sized so there is some wiggle room.
With the components in their new homes temporarily connect a battery and set the output voltage. To change the voltage use a flat head screw driver to turn the potentiometer. While the LED's are wired for 12v they will light up at lower voltages. In order to extend the battery life as much as possible I set the step up converter to 9.75V. These LED's are very bright so 9.75v was enough voltage to make them visible. On a side note blue LED's typically require a higher voltage to turn on whereas red LED's glow at a lower voltage.
Step 3: Battery and Charger Housing
In order to keep the electronics suspended in the baton, I modeled some parts to print. The 18650 battery is situated between the USB charger and the step up converter. The battery holder I was hoping to use to, you know, hold the battery, didn't fit in the tube. So instead I cut out the spring and contact pad and glued it into my 3D printed part.
With the battery contacts glued into place, it is time to install the 18650 lithium ion battery.
Once the battery is installed, the wires are live - i.e. turned on. Be careful not to let the battery lead wires short out
Insert the battery into the casing and apply some super glue to the two parts and clamp together. Take your time and ensure that the two parts are lined up all the way around. If they are misaligned it won't fit into the baton.
Solder the battery terminals to the charger board and the step-up converter board. Again, use caution, the wires are live. With the wires soldered, tuck them into the trough and glue into place with some hot glue. It is important that the wires are below the surface of the 3D printed part. If they aren't, they may short out against the metal tube as the assembly is inserted into the baton.
Add some hot glue to all solder connections to help prevent things from moving around as the baton is inevitably dropped and tossed around. Don't go too crazy with the hot glue though.
Step 4: Light It Up
The LED strips I used are solid color and not RGB. Since the goal was to have a red and blue flashing baton, RGB strips would have been a waste. I linked to the LED strips I used in the first step, but should warn you that these are not the best quality. Sure they light up, but the strips are spliced together and are not continuous lengths. I purchased two rolls of red and two rolls of blue; all four of the 16ft spools had 3-4 splices per roll. Since I am coating these in heat shrink tube, I'm not concerned about the splices; but they certainly are not waterproof like the packaging says.
I applied the LED strips to the batons 180 degrees apart from each other. The strips are adhesive backed, but it is relatively weak adhesive. Once both strips of LEDs were on the baton, I temporarily wrapped them with some electrical tape. This helped hold the LEDs on through the rest of the assembly.
Once the strips are on, mark and drill a 1/4" hole for the LED wires to pass through. I recommend pre-drilling with an 1/8" bit first to prevent the larger bit from walking around. The hole should be roughly 4-5" from the end of the tube and centered between the two LED strips.
Step 5: Last Minute Cramming
Solder on wires the length of the baton to the output of the step-up converter. -WARNING- nothing has changed, these are still live! Insulate the wires with tape or a dab of hot glue to prevent them from shorting out as you pass them through the baton - which brings us to the next step. Pass the wires through the baton and insert the battery module into the tube. If everything goes right this should be a snug, but not tight fit. Do not force it in - if it is stuck, you will most likely break it and have to start over. I had to sand my housing a bit in order to get a proper fit. The sanding was done mainly around the glue seam which was bulging slightly from the glue squeeze out. Once installed, glue into place.
Solder on wires to extend the power from the flasher circuit as well as power for the LEDs (refer to the earlier pseudo schematic). These wires should be longer than the inset of the hole we just drilled (if the hole was 4" from the end of the baton, the wires should be 5-6" long). If the wires aren't long enough you will have a very hard time trying to fish them through the hole. I taped the LED wires to a zip tie to help pull them through the hole, but use whatever you have lying around.
Solder the power wires from the battery to the power wires of the flash circuit and insulate with heat shrink. Insert the flasher circuit into the baton and glue into place.
Step 6: Shrink It
Connect the LED power wires to the LED strips and hot glue into place. These solder pads are fragile and will separate from the strip easily. Once both LED strips are wired, add a wrap of electrical tape over the connections. This will indicate to the user which end the switch is on, as well as hold the wires in place.
Unlike the LED strips I purchased, this clear heat shrink tube is awesome. It has an excellent shrink ratio, is nice and thick, but most importantly is crystal clear. Once shrunk it is hard to tell it is even there when observing the baton from a foot or two away.
Roll out a length of clear heat shrink tubing and cut about an inch longer than the baton. Sleeve the heat shrink over the baton removing the temporary electrical tape wraps from step 4 as you go. Grab your favorite heat gun and start shrinking. Note that this is relatively large heat shrink and takes some time to shrink. Take your time and don't get it too hot, you risk damaging the electronics inside. Take special care not to over heat the end containing the battery. I found that holding the heat gun at an upward facing angle and rotating the baton provided the best results.
Step 7: All Done!
If you made it this far hopefully you have a functional light up baton. Be sure to check that each part as working throughout assembly, this isn't an easy device to disassemble if something isn't working.
Even if you aren't in the color guard this is a fun project. As you saw in the video, the guys were enjoying them as light sabers.