So This is SMT
Up until this project, I've always done through-hole soldering. But for this project, I knew the chip was surface mount, so I figured, why not make the whole circuit surface mount.
So what is the project? Well, over the holidays I played with this little tank-like robot. My wife calls it "tank", I call it "battery-sucker 4000" (now sucking more than ever!)
Each of those IR leds is running at 60mA, and they are not configured in series at all. Overall, there are 10 IR LEDs on this guy, all running all the time. So for one thing, I need to rebuild the IR bars. To run this guy, I have 4x AA's to run the motor, and a 9V to run the Arduino and logic circuits. The Arduino has a motor shield on top, that has been slightly customized to eliminate the extra features (all jumpers are cut) and make it stack tighter (TinkerKit connections are gone). I also cut the traces between the TK analog inputs and the analog pins to ensure no interference. On top of the motor shield is a custom cap-shield that I made to distribute power and sensor headers.
The red/yellow wires run off to a switch. The thin wires run off to a bluetooth serial adapter, which was added as an afterthought of course. The double sided tape is where it was "attached" in the picture above. If you look, you'll see that originally I designed it to take in 6V + 3V to supply 9V to the Arduino's Vin pin. Because the motors killed the 6V source, I changed it to where they share GND and the Arduino gets a fixed 9V battery instead.
And that's where the problem lies. My Arduino setup is pulling 600mA constant draw off that 9V battery. Going thru the 7805 on the Arduino, that means I am wasting a constant 2.4 watts of energy thru the shunt regulator. The 9V was falling rapidly. In fact, within 10 minutes of playing the 9V cell was under 7V.
Time for a new cap, this time with a switching regulator instead of a linear regulator. In my (admittedly not very thorough) search, I found the MAX1674 boost regulator. In theory, capable of boosting 0.7V to 5V. So I layed out a double sided board to handle the surface mount 1674 and components along with the through-hole headers.
The transfers were beautiful. And my three alignment holes made sure I had both sides relatively in line. So I transferred to both sides and took it to the etchant. One thing I did do was fill in the large planes with a permanent marker on top of the toner. After etching, I think the alignment holes and permanent marker both had a good effect.
Damn, I screwed up. All of the header solder points should be on the other side. Other than that, this is damned near perfect alignment and very little pitting in the copper. Time for the fun part, putting it together.
Since I am limited to two hands, and at times I wasn't sure that was enough, there are no pictures during the assembly. So here's some post assembly pictures.
Yes, I am that lame. I called it the "Robo-Cap Shield". For the most part, the SMT parts were easy to work with. The 0402 caps and resistors were a little tricky at first and are so freaking tiny the hardest part was handling them, but I think I worked them out fairly well. The 0402's at the top of the first picture were the early ones, and tend to have more solder and are sloppier. The 0805's are nice and easy. The large caps were easier still. Even the chip was easy. The inductor wasn't hard, but once the solder bonded, the inductor core started acting as a heat sink. Not wanting to damage the wire on the inductor, I didn't want to heat it enough to remelt the solder to straighten it out. Luckily it doesn't need to be perfectly straight to work.
Half a dozen tests later, here is the finished product. Yes, I cut too close to the edges and nearly took off my 6V sense wire on the top edge. The bridge wire connects the "dirty" groundplane on the bottom with the "clean" groundplane on top. The Arduino fills this role, but for testing I wanted the board to work on it's own (and I have corrected my drawings to include this connection). I also removed the LED header in favor of some SMT LEDs on the right side. Finally, I had to cut a chunk out of the corner so I could get to the screw terminals on the motor shield.
I definitely like SMT, but I think once I use the last of these 0402's, I probably will pick up 0805's or 0603's instead.
So far, this shield works good. It powers the 5V Arduino from 2x AAA batteries and runs somewhere (theoretically) around 80-90% efficiency. Now I just need to remake the IR bars. One major change on this shield from the last one besides the regulator circuit: I included transistors to toggle power to the IR bars on the undercarriage header. I also removed the track sensors. Hopefully, when I finish the IR bars and put it back together, this robot may suck less than ever.
