Intro: A Protective Case for the Atmel AVR Dragon
This instructable will show you how to easily modify a readily available plastic case to hold and protect your AVR Dragon PCB.
Atmel promotes their AVR Dragon as a low cost development product tool for use with their AVR microcrontollers. While the product does come in a fancy red cardboard box, unlike all the other AVR hardware debug/emulation development tools, the Dragon is sold as a single PCB with no cables or even a protective case.
Atmel does not even offer the option of buying the product with a protective case.
The customer is on his own as to how to best do this.
Before going any further, I must say that the Dragon has a tremendous amount flexibility and huge number of features and options - almost too many in my opinion.
As a result, there is no practical way to build a case that could provide easy access to every single combination of features and capabilities.
This instructable will focus on the creation of a protective case that wraps the Dragon in as small of a package as possible. As a result, when the enclosure is closed, functionality is primarily limited to that available from ISP or JTAG connectors, essentially giving you similar functionality to that of the JTAG ICE MKII, but only for the 8 bit microntrollers, and at a fraction of the cost.
Step 1: Different Dragons and Different Cases
When I first got my Dragon, I noticed that it was a new revision of the product. All the versions of the Dragon board that I had seen had no mounting holes in the PCB. The version of the board that I received had 4 mounting holes - which I thought would make the task of putting the Dragon in a protective a case easier.
That said, I was also looking for a case, that could still work with the older version of the board using a hole-less attachment option such as double stick tape.
The biggest problem in locating a case for the Dragon is the size of PCB. Many of the off the shelf cases are very similar in dimensions to each other and none of them that I was able to locate (and I looked at MANY) were an exact fit - especially for the mounting holes. I even asked Atmel directly on their support site if they designed the Dragon for a specific case. - I got nothing back from them. Nearly all of the cases were either too large for what I wanted or just slightly too small in one dimension. Also complicating matters is that since the Dragon has status LEDs on the PCB I was looking for a case that had a translucent plastic option so that the LEDs could be viewed through the case rather than have to deal with any sort of light pipe.
I did manage to find two cases that could be coaxed into working.
Serpac H-Series H-65: http://www.serpac.com/Products/Hseries/h-65.aspx
Dimension Drawing: http://www.serpac.com/Products/Hseries/drawings/h-65.pdf
Hammond 1553 (1553BTBUBK) 4.615x3.109x0.945 : http://www.hammondmfg.com/1553colors.htm
3D Dimension Drawing: http://www.hammondmfg.com/pdf/1553B.pdf
These cases are readily available including places like Digikey, and Mouser who also sell the Dragon.
At the time I bought them, the Hammond is about half the cost of the Serpac.
Serpac runs about $13 and the Hammond about $7.
For Translucent Colors, the Hammond only offers blue, while the Serpac offers translucent gray, red, green, or blue. The Hammond has rubber along each side. The Hammond case is also a bit thicker and sturdier plastic than the Serpac.
Both come with several battery & door options (which were not used for this project) and the Hammond 1553 comes in two different sizes. Also some of the pictures in the online catalogs have photos of the wrong colored case (the text description is correct). So be very careful when ordering.
Now all that said about the translucent case, there is an alternative if you really want another color or don't like the translucent look. Since these cases have replaceable face plates and the Dragon LEDs are near the edge of the PCB, you could replace the face plate with a clear plate which still allows you to see the LEDs. That way if you want black, or that bright red or safety yellow Hammond case, you can get it and still see the LEDs.
NOTE: The clear panels are harder to find as they are often not carried by online merchants.
Step 2: Which Case?
While either the Hammond or the Serpac case should work for either PCB version of the Dragon, there are a few differences beyond the obvious color options and costs that are worth noting.
While I have both cases and have examined and measured both very carefully, I will point out that, at this point I have only done the modification on the Serpac case; however, having completed this, I can comment on the effort for the Hammond. (I will update this instructable, once the Hammond is completed)
The Serpac might be the better option for the older board without the mounting holes. I say this only because the Serpac is a tighter fit and has screw posts at either end that could be used to brace the board to keep it from moving forward/backwards when the USB cable is inserted.
I only mention this because I really don't know how strong double stick tape is.
I believe that the Hammond case will be easier to modify. This is because on the Serpac the 4 posts used to hold the screws that hold the case shut are very close to the PCB mounting posts which have to be shaved down. It is difficult to get the dremel cutting tool into the corner by the front faceplate. The Hammond only uses 2 screws to secure the case so those posts are in the middle of the case and out of the way of the PCB posts which have to be shaved down or removed..
Because the Hammond case has only 2 case posts instead of 4, you will be able to mount the PCB all the way up to the face plate. This is not possible on the Serpac.
On the Hammond case, the distance between the Dragon PCB and the metal screws is VERY tiny like 1mm or less. In fact if the Dragon PCB is not precisely placed, the case screws might rub against the Dragon. I don't think that this should be an issue.
The Serpac has 6 PCB mounting posts. If you plan on adding a Dragon Hyde "daughter board" to your dragon, the Serpace has two PCB posts already at the far end of the case that could be used to secure the daughter card.
Bottom line, is either case will work. And some of it comes down to preference.
Step 3: Additional Supplies
In addition to one of the cases you will need:
- Safety Glasses
- Dremel Tool
- cutoff wheel
- Fine/small grinding bit
- 5/64" drill bit
If using the the older board with no mounting holes
- Double stick tape
If using the newer board with mounting holes:
- Four #2 x 1/2" screws with binder/pan head
- Four hex nuts
- Four lock nuts (regular hex nuts could be substituted)
I used Nylon screws and highly recommend using nylon over metal because, if you don't get your holes in the case exactly correct, the nylon has a bit of give that will make mounting possible and prevent cracking the case.
Step 4: Modify the PCB Mounting Posts
Since the distance between the outer edges of the PCB mounting posts is narrower than the width of the Dragon PCB, the PCB mounting posts will have to be shaved down a bit.
NOTE/WARNING: I can't stress this enough. Make sure to wear the safety glasses for this as plastic shards will go all over the place as you use a cutting wheel to cut/shave the plastic. In fact make sure that the glasses are very snug on your face as it is very easy for tiny remnants to sneak around the glasses or fall from your hair into your eyes.
This is where there is a difference between the old vs new Dragon.
If you have a new Dragon, you can shave down the posts or even completely remove them as the new Dragon will be using mounting posts to sure the PCB.
If you leave part of the PCB posts, as I have shown, you must shave them down from the top
to the bottom of the post as the Dragon will have to slide down vertically and cannot be rotated
when placing the board over the mounting screws.
On the older Dragons, there are no mounting holes. So if possible it may be desirable to leave as much of the PCB mounting posts as possible to keep the board from moving back and forth.
Also, you will have to shave the post from top to bottom on one side. On the other you can angle cut the post. Doing it this way will actually secure the PCB and keep it from moving or rotating.
NOTE/WARNING: on the Hammond case, the center posts are for securing the case not for mounting the PCB so do not remove these posts. They must be shaved down just about to the point of breaking through to the hole.
If it breaks through, it shouldn't be a problem as the posts are reinforced.
The 4 PCB mounting posts will also have to be shaved down. The comments above about them apply here as well.
Step 5: Add Mounting Posts for New Dragon
If using mounting posts with the new Dragon, insert the Dragon board so that holes can be marked and drilled.
NOTE: The holes on the Dragon are not symmetric front to back. This means that the board must be placed in the case facing the correct direction before marking the holes.
On the Serpac case, due to the design of the case, the USB connector must go through the face plate. This means that when inserting the Dragon make sure that the USB connector on the Dragon is facing the face plate opening - See the photo.
On the Hammond, The Dragon board can be mounted either way.
If mounting with the USB connector going out of the face plate, make sure that when marking the holes that the Dragon is slid up all the way such that the USB connector is flush against the face plate when the faceplate is fully installed.
Getting the holes marked properly is very important so make sure to steady the board so that it does not move when marking the holes. One thing that helps on the Serpac is to push the board either all the way forward or all the way back until it rests against the case securing posts.
Use a fine tipped sharpie to mark the holes. After you have marked the holes, remove the Dragon and flip over the case. Make sure the marks on the case align with the holes in the Dragon. If they don't, use rubbing alcohol to remove the marks and remark them.
On the Serpac case, there is a decorative line/groove in the plastic that is very very close to the center spacing between Dragon holes on each side.
Drill them with the 5/64 bit. Go slowly and try not let the bit wander off the original mark. Be careful when drilling not to go too fast which can cause the plastic to melt.
Once you have the holes drilled, insert the #2 screws and secure them with a nut that not only will secure the screw to act as mounting post but will also act as a spacer to lift the board a bit to properly align it with the face plate.
NOTE: as I haven't done this yet on the Hammond I'm not sure of the vertical spacing needed. If pushing the USB connection out the front plate, the vertical spacing is very critical on this case. It looks like it may require slightly more height than the Serpac and so a small washer may be needed for each screw.
Step 6: Cut Case for USB Cable
It is now time to cut the case for the USB cable.
This is probably one of the most visible operations in the entire project that can make or break the finished look of the case.
NOTE: The USB connector on the Dragon is not centered on the board. Until I modified a case for the Dragon, I did not notice that the USB connector is NOT centered on the PCB.
On the Serpac case there will be a large gap between the USB connector and the face plate. Because of this, the entire USB cable including the insulator will have to be inserted through the face plate.
The USB cable I purchased had a housing that was just a tiny bit smaller than the size of outer edge of the USB connector on the Dragon. This allowed me to use the connector itself along with a proto board I had handy as guide to mark the face plate.
First mark the right and left sides then mark the top. After removing the face plate measure the size of the Dragons USB connector and mark the bottom of the face plate.
Then use a dremel tool to slowly carve out the opening. Slowly carve until there is no more red visible. I then sanded the edged with some 320 sandpaper to smooth out the edges.
Step 7: Room for Dragonhide
For all of you that are interested in making their Dragon tougher and would like to add the Dragonhide, note that there is room for it.
A dragonhide daughter board could be made that would mount to the existing Dragons ISP/JTAG and power headers. All the components would face down towards the Dragon.
It is even possible to mount a boxed 90 degree 10 pin connector so that the cable could be disconnected from the case.
Even if the Dragonhide circuitry is not desired, a daughter board could still be used to allow a boxed header to exit the case which would allow the cable to be removed from the case or easily replaced should there be a problem.
The Serpac has 2 additional PCB posts that could be used to secure the board.
Because the Hammond uses only 2 center posts to secure the case, it has more room and could allow multiple connectors.
With a daughter board, it would even be possible to add a jumper to provide a way to allow configuring whether the Dragon supplied power to the target. The daughter board could even have a fuse on the power line to protect the USB bus from excesses loads from the target.
Step 8: Can Still See Dragon Markings
With the Serpac translucent gray clear case, if you shine a bight light on the case, you can see through the case well enough to still make out the all the informative text on the bottom of the Dragon.
I'm not sure if this will be the case on the Hammond.
Step 9: Finishing Up
On the Serpac case, in order to close the case when using a cable attached to the ISP or JTAG header, the strain relief on the cable will have to be removed.
I'm sure that this will also be the case on the Hammond.
The Serpac case actually already has tiny indentations in the case along the back edge that are almost the correct size for a cable. So until you finally decide which cable or whether you intend to use a daughter board with a boxed header, you can simply close up the case with the screws without any further modifications and it will clamp down on the cable.
On the Hammond, you will definitely have to cut a notch the case to get it to close.
The only final thing needed are a set of 4 tiny stick-on rubber feet to keep the case from sliding around as well as protect the nylon screw heads on the bottom of the case.