How to Dove-Tail Joint a Laser Cut Enclosure + Lipo Charger

Laser cut boxes for project boxes are all over the place lately, but I've found that they often need a fair amount of hardware and complex designs to make them solid. This Instructable will show you how to make attractive and structurally sound project enclosures held together with dove-tail style joints and a just a few machine screws.

The trick to making these dove-tail joints is an emery board, the kind for filing your nails. The joints of the project box are laser cut with a "V" shape on one side and a smaller, rectangular hole on the other. The receiving rectangle cut is then sanded down to fit the V shape using the emery board. When assembled, the dove-tails hold the sides together tightly. Screws are added to hold the top and bottom in place, securing the enclosure. I used 4 (one for each corner,) but the box could easily be held together with just one.

The project that I'm making a case for is a modified Sparkfun single cell, lithium-polymer battery charger. The charger comes as a kit for charging li-po's at 100 or 500mA, but with a little bit of tweaking and a few components it can be made into a variable charger.

Materials
1/8th inch acrylic plastic. It will work with thicker plastic, you'll just need to do more sanding.
Machine Screws such as 1.25" Length 2-56 Thread from Ponolu
A project to put in the box

Tools
Laser Cutter
Emery Board. These can be found at drug stores and mini-marts.

First, find some machine screws of a suitable length, longer screws will allow for more room in the project box. I used 1 1/4" long, 2-56 thread screws from Polulo; my project box needed to be exactly 1 1/4" tall for the screws to reach from one side to the other.

Design the enclosure with rectangular finger joints as per usual. This project was designed with 123D and then cut into slices using 123D Make for laser cutting. Another option would be to use the web based BoxMaker tool by Rahul Bhargava to automatically create a finger joint box design, and then use a vector editor (Illustrator, Inkscape, Corel) to add the dove-tails.

Based on the length of your machine screws, choose two opposing sides (top-bottom, left-right, back-front) and add 70 degree angles to the joints by making the outer edge wider. Here I've made all the tabs on the top and bottom into dove-tails and left everything else as it was.

Bolts are categorized by their length and thread type. Luckily someone has already figured out what size holes you need for each type of bolt. Hurray for standards!

The bolts used here are 2-56 thread which, according to a common tap & drill guide, need .07" diameter receiving holes on the bottom of the enclosure to bite into ("Tap Drill") , and .096" holes on the top for the bolts to easily pass through ("Clearance Drill").

If we were working with metal we would need a tool known as a tap to cut the threads into the sides of the receiving hole. But with acrylic plastic, and bolts of this size, you can just force the bolts in and let them thread themselves.

Add holes for the bolts to any opposing sides on the case, they do not necessarily need to be on the sides with the "V" tabs. If you want the bolts to also hold the circuit board inside, you'll need to make sure the holes in the case line up with those in the PCB. most of the time you'll be able to find the needed dimensions in the board's documentation, otherwise you'll need a digital caliper.

Now is also a good time to add holes for any other hardware, such as switches and ports on the circuit board. Again, datasheets are your friend here and should provide all the measurements you need.

After you've designed and cut your enclosure, lay them out as the pieces as they will be assembled. This will help you keep track of which joints need sanding down. Using the emery board, do your best to sand the slots that will receive the dove-tail tabs at a matching angle. Sand a little bit, test the joint, and sand a bit more until the two pieces fit together.

If your emery board is too flimsy and you are getting round edges instead of straight, hold a thick piece of wood or metal behind the board as you sand. It's ok if the edges turn out a little round, as long as the pieces fit together well.

Lately I've been using a lot tiny lipos that can only be recharged at a very low current. I got turned onto these little fellas after finding them used in all the micro-helicopters that I've being destroying dismantling for scientific purposes. I started looking for a way to recharge these batteries so they could be swapped in and out of the aircraft and my own little inventions. I eventually found a lithium-polymer single cell battery charger from Sparkfun (PRT-10161) and followed some of their advice on how to modify it.

At the heart of the charger is a MCP73831T chip that features a built-in charge management controller. The value of a resistor connected to the chip's current regulation set pin (PROG) tells the chip what rate it should charge at. The original circuit for this kit has a 2k Ohm resistor for recharging at 500mA, and a 10k for 100mAh. In this project the pre-etched jumper on the circuit board is scratched away, and 25k and 67k resistors are added. A 2-pole 6 position switch and a few jumper wires allow the resistor of your choice to be connected to ground, and thus setting the charge rate.

If you would like to know how these values were calculated or more info on what these chips do, have a peek at the MCP73831T datasheet attached to this page. They are a cool component and are often used in phones, mp3 players and other small, USB rechargeable doodads that stay on while they are charging. That's one of the chips best features, you can leave your circuit connected to the battery while it is being charged. USB recharging is super convenient so I'm thinking about just integrating the circuit into my future portable projects.

1.Use a sharp blade to scratch away the jumper connection between the 2k and 10k surface mount resistors.

2. Solder wires to the ends of these two resistors, a wire to the PROG pin of the MCP73831T, and a final wire to ground. Once you have a wire soldered, use super glue to stick it to the circuit board, this will prevent random tugging from ripping the components (and traces) right off the board.

3. Solder a 25k and 67k ohm resistor to the ground wire. Or rather, solder two sets of resistors until one set equals 25k and the other adds up to 67k. I didn't have those values laying around, so I used 22k + 3.3k, and 56k+10k+1k to get the total value. Use heat shrink tubing to protect all the leads and prevent shorts.

4. Test your rotary switch with a multimeter to determine where your wires need to connect. The Radioshack switch used here had two center poles and when the switch was turned these poles would connect to the outer leads. The IC's PROG pin wire goes to the center pole and the sets of resistors get soldered to separate terminals along the outside. Make sure to keep track of which dial position connects to which resistor set, I needed to draw a little reminder for myself. I also planned this out ahead early enough to laser etched these positions on the outside of the enclosure.

5. Advanced "Caution-to-the-Wind" Bonus Step (Optional)
These particular JST connectors have little hooks that make them near impossible to dislodge from each other. Since I plan on constantly plugging and unplugging batteries to it, I decided make the batteries easy to disconnect by snipping the locking slots from the female connector  Only do this if you have extreme confidence in yourself to NEVER accidentally pull the battery out while it is charging!  That would be bad.

How bad?
More on that later.

Why all this concern over charging rates? Well, lithium-poly batteries have limits on how fast they can be recharged. Going over this limit results in fire and explosions that are not only dangerous, but could totally destroy whatever project the battery is in. Even if it doesn't blow up, improper charging will reduce the life of battery.

For most lithium-polymer batteries, it is safe to assume that they should be recharged at a current no greater than their capacity (or 1C.)  For example, the tiny battery I'm using here has a capacity of 40mAh (milliamp hours) and as such can be recharged at 40 milliamps or lower. A 100mAh lipo can be recharged at 100mA, a 500mAh at 500mA, and so on, you get the picture.

IMPORTANT INFO ABOUT BATTERY SELECTION!
This charger is designed to work with batteries that have their own built in protection circuitry. These circuits prevent the battery from being overcharged, or over-discharged. Surface mount chips near the terminals of the battery usually indicate protection circuitry, but it's always best practice to check the datasheet. I get these batteries from Sparkfun as well, and I believe that all of their single cell lipolys have these built in safeguards.
Do not attempt to charge the battery unless you are certain that your battery has overcharge protection!

Assemble the enclosure, cram the electronics inside, and bolt it all together; the lipo-charger is now ready for action! By pure chance, mine fits snugly inside one of my organizers.

Using the Charger
1. BEFORE connecting the charger to a USB port, select the charge setting for your battery.
2. Plug the battery into the charger.
3. Connect the charger to a computer USB port (you'll need a cable with a micro-USB connector) The charging LED will turn on.
4. Wait until the light is off, then disconnect the battery. DO NOT REMOVE THE BATTERY WHILE THE CHARGE LED IS LIT! Doing so will burn the MCP73831T, releasing all of its life giving smoke and ruining the charger kit.

A Note on Safety
It is incredibly important that you use the correct charge current for your battery. Always be absolutely sure that you know your battery's capacity and maximum charge rate. Be absolutely super positive that you select the right setting before adding USB power.

DO NOT
Charge a lithium-polymer battery at a rate higher than its capacity.
Remove the battery from the charger while the charge LED is lit.

So yeah, that part about not removing the battery while it is being charged, I learned that the hard way...

Within an hour of putting the kit together and closing it up in its new cube of solitude I, for reasons I do not remember, yanked the battery out of the charger while it was being charged. I noticed the smoke right away. The box filled with it and, suprisingly, when I plugged the lipo back in the charge light did not turn back on.

I unscrewed the case to get a closer look at the PCB and the first thing I saw was a big bulge on the top of the MCP73831. Just being curious, I plugged the charger back into the USB port and the chip started to sizzle immediately. That's when I started to suspect that I may have ruined some part of the circuit.

MCP73831Ts are only 68 cents on Digikey, and I figured that was cheap enough to get a few and see if I could repair the $15 kit.

First I removed the burnt out chip and cleaned the traces as best I could with a soldering iron and a dedicated electronics toothbrush. The copper traces where a disaster, but a salvageable one. I repositioned the traces that had lifted with tweezers and added a few dabs of solder to the pads. A new chip was carefully positioned on top of the pads, and a quick touch with the iron soldered its leads down to what was left of the footprint, all except for one. I bent the PROG pin up away from the board to make attaching a new, thinner wire easier.

Hey, sometimes you get lucky. I set a meter up to test the current and plugged the charger back into the USB. The charge LED turned on and I measured a charge current of 17.15mA (it should be 15, but eh, whatever.) I left the battery plugged in this time, and about an hour later the light turned off. The battery was fully charged, the fix had worked!

Thank you for taking the time to check out my project. I'm happy that the enclosure came out looking clean and that nothing exploded (as of yet.) I do like using these miniature lipo batteries, they really pack a lasting punch for their size and now recharging them is both convenient and stylish.

Do feel free to ask any questions, give any suggestions, or just say hi. Hi.