I have been triangulating on a final PCB design for an internal WiReach card. I am not an EE, so the design is hobbiest-grade at best. Here is the thinking behind some of what you see in the picture above:
1. Board size is less than 5cm x 5cm in order to print it at iTead Studio at a reasonable cost. 10 of them on 0.8mm board should be around $15.
2. Hole locations were determined based on trial and error. I have a 3D printer, so I printed 6 or so iterations of the board and then checked the fit and hole locations. If someone has better dimensions for the location of the mounting points, please let me know.
Screw size seems to be #1-72. Length is 3/8″. Flat head. Phillips. Shannon from NewtonTalk was selling an original SER-001 on eBay. I have a SER-001 too, but I’ve lost the screws. Shannon was nice enough to measure the screws for me, and confirm that the head diameter is ~0.136. This seems consistant with McMaster part number 91771A168, but I would love to find an flat undercut version of this screw. Unfortunately, it doesn’t seem to be available in this length. At least now I can get some screws for both my SER-001 and this project!
Eagle PCB tends to want to route things across the mounting holes as plated vias. Since these holes will need to be countersunk, I didn’t want to rely on these holes. Drilling the countersink will break the plated thru hole. Hence a couple of extra vias are nearby to make sure there isn’t an issue.
3. I trimed down one side of the board to give a little more clearance for the microphone and power switch wires. This isn’t strictly necessary, since the wires have their own recess in the stylus assembly. Since I don’t know how/where I’m going to route the antenna yet, but I figured a little extra room wouldn’t hurt.
4. I added a 500mA PTC Fuse, since the battery leads are unfused. While there’s not going to be any external connector to short out, I figured this was a safe play.
5. I chose the Texas Instruments REG103 regulator, mainly because I was able to get a handful cheaply on eBay and it has a shutdown input. Also its pretty low profile and won’t interfere with mainboard components. The shutdown signal will will be driven by SerPortSel3. SerPortSel3 is normally low, keeping the regulator off. When toggled high in software the regulator will come online and power the WiReach. This way, the module will only be powered when in use. This regulator can handle 500mA. At peak, the WiReach should only use 60% of this amount.
6. The large copper planes on the top side are for battery power, ground and 3.3v. The intent to help cool the regulator. I’m not sure what the thermal characteristics will be in practice. Power and ground traces are 30 mils wide, which is more than double what the online trace width calulators say you need for 500mA.
7. Headers for debugging give access to the power lines on one side and the communication lines on the other. In practice, I intend to tape over the pads so that there’s no possibility of a short. (I may go a little crazy with Kapton tape just to make sure the board doens’t flex into something and cause a short to the mainboard.)
8. The ground plane on the backside is only broken by a few signal jumpers where routing couldn’t be done on the top side.
My next steps are to verify the connections one more time to make sure everything is correct, and then send the board off to be made. In the interem, I’m going to build a reflow toaster oven.