CR2450 Coin Cell Charger

This project was mainly a way for me to learn Altium Designer. It started out with me just messing around, but slowly a nice looking board started to emerge. So, I sent the design off to Gold Phoenix and got back 90 boards. It’s a charger for a single CR2450 (LIR2450 to be exact) coin cell. CR normally applies to primary cells (non-rechargeable). I’m guessing LIR stands for Lithium Ion Rechargeable. I bought a few of these cells from SparkFun a while back, but never really used them because I didn’t have a convenient way to charge them. Sparkfun sells a single-cell lithium charger based of the MAX1555 IC, but that chip has been hard to source lately. Regardless, its charging current is too high for this 110 mAH cell, and their board requires another daughterboard to hold the coin cell.

SchematicThis board is based around the MCP73832, which contains all the logic and active devices to charge a single lithium ion/polymer cell. It has multiple charge modes, including constant current which does the bulk of charging. R3 sets the constant current. I chose a resistance of 20k ohms, to get a constant current of 50 mA; that’s about the max charging current one should charge a 110 mAH battery at.

Purchase

Right now I’m only selling bare-boards, $5 each including shipping in North America. If there’s enough interest, I’ll sell fully populated boards. Email me to purchase one.

Downloads 

• Altium Files
• Schematic (PDF)
• Bill of Materials (Google Doc)

Update on Saturday, March 31, 2012 at 10:43 by Jay Kickliter

Post edited. Someone reminded me to post the Bill of Materials (BOM).

Touch Enabled Business Card

My last business card was too expensive to give out, so I designed this one using a Texas Instruments MSP430. Combined with the chip’s Pin Oscillator hardware, and TI’s Capsense Library, it wasn’t too difficult enabling capacitive touch. The great thing is that in software I can have it go into a low power mode and check the button every second or so. When it senses a a touch, it checks more frequently and only leaves low power mode to toggle an LED. I guess when you’re driving LED’s, low power sleep isn’t going to help much, but there are times when no LED’s are lit, even in the middle of a display routine.

I rushed to have these cards in hand before a job at my school. It was about two weeks between blank slate and getting the boards back from Gold Phoenix in China. I made a few dumb choices. Particularly, when laying out the board I have one of the LED’s on a different IO port than the rest. That makes it pain to write good code, since I have to do an if..then to determine which port to write to for a particular LED. Also, I didn’t lay out the LED’s in a sequential manner. Ideally LED1 should be attached to port 1.0, LED2 to 1.1, etc. I’m sure I had a reason at the time, most likely for cleaner routing, but that just made for uglier coding. Nevertheless, it works fine. Especially considering the prototype consisted of nothing more than one LED, and a penny with tape over it to substitute for the touch pad. 

Even using much cheaper parts and smaller batteries, I’d say it’s still too expensive. The parts probably come to $2, but the board is almost $8. I remember Gold Phoenix being cheaper, but I may be wrong. $110 for 155 in2 seems pretty good, until you start throwing in rounded corners, black soldermask, and very thin FR4. I think it’s about time to start looking for a new Chinese PCB manufacturer. Am I the only one receiving emails from Peak PCB? Has anyone else tried them? 

Downloads

• Source Code (mspgcc)
• Design Files (Altium)
• Schematic (PDF)

EM Fields Reference

Here’s a reference I made for an engineering electromagnetics course I’m taking. I hope it can be of use to someone else. Both the PDF and LaTeX source file are here for download.

PDF

LaTeX