A few months before Maker Faire KC a friend started working on a Mini Sumo Bot and in passing I said I could probably get a bot ready for that...well those months slipped away and two weeks prior to the faire I had no bot (and had been constantly hounded by said friend). So I did a grand total of 30 minutes of research on the internets and ordered some parts (not recommended).
I ordered everything from Canada (Fingertech and Solarbotics) so I had to wait almost another week to get started.
Here's the parts list I ended up with:
- Chassis/Motors/Tires/Line Sensors: FingerTech "Cobra" Mini-Sumo Robot 4WD Kit
- Motor Controller: 2x Sparkfun's Motor Driver 1A Dual TB6612FNG
- Micro-controller: Arduino Pro Mini 328 - 5V
- Proximity Sensors: BittyBot 3-Way Object Sensor
- Battery: Venom 10C 2S 1250mAh 7.4V LiPO
- Line Sensors (Included w/ chassis kit): Phototransistor - QRD1114
- More Stats below...
The first thing I did was construct the BittyBoty 3-Way Object Sensor which is used to find the other robot. This kit is a nice compact proximity sensor, but it's a bit tough to mount (double sided tape...). Building the kit was a challenge because it's a very small board with several tiny surface mount parts to solder...and I didn't have the right kind of tweezers. After melting one of the indicator LEDs and testing the rest of it I deemed it passable and moved on.
My first attempt at getting everything working consisted of using solid core and wire-wrap wire to do point-to-point connections. That was a mistake.
But it did work! I rigged it up to work with a 9 volt battery and let it tear around my desk for a few days like that.
After a few days fighting that solid core wire jumble I got fed up and designed a single sided PCB that I could etch. Unlike my previous etchings, this time I went shopping for some Hydrochloric Acid and Peroxide. This worked AMAZINGLY well compared to Ferric Chloride. I didn't want a top layer or vias so I intentionally left out some of the complex traces so that I could do them with wire-wrap wire instead. This took some time and patience but worked okay in the end... A hint if you do this, use Eagle to keep track of the wires you patched on to the board by adding traces to a layer. Maybe next year I'll mod it by getting the whole board fabbed. In the end I managed to get the board made and only fried one line sensor on power up...crossed the streams...
After all the solid core wire was replace with either traces on the helper board or stranded wire (stolen from various old USB cables) this setup was MUCH easier to work with. I went to the local hobby shop (which has no website or I'd link it) and picked up the LiPo battery and charger. The battery fits beneath the helper PCB, keeping the center of gravity low.
- Total Estimated Cost: $285.69
- Chassis: $119
- Proximity Sensor: $30
- Motor Driver(s): 2 x $8.95 = $17.90
- Arduino Pro Mini: $18.95
- Battery & Connector: $16.99 + $3.25 = 20.24
- Shipping: $17.98 + $61.62 = $79.60 (ouch!)
- Total Estimated Hours: 30
- Final Weight: ~450 grams (I still have room for better sensors :) )
- Width: 9.9 cm
- Length: 9.75 cm
- Height: 7.3 cm
- Code is on github
- Don't use solid core wire in tight spaces...I already knew this, I just wasn't clever enough to chop up some useless USB cables for multi colored stranded wire until the second attempt.
- Buy some good tweezers and fine solder tips for surface mount.
- Find better paper for toner transfer. I used plain paper this time, but I need to find something better.
- Procrastinating and then having to pay for 3 day shipping from Canada really raised the price on this thing. It's quite reasonable if you remove shipping from the final cost...
- I'll just have to get a lot of use out of this bot to recoup the cost.
- You could also add in $110 for the battery charger...but I got a really good one so I can use it on more projects in the future. You could also throw in $70 for the Dremel drill press, Dremel bit chuck and Dremel drill bits...but I'll used the crap out of those for boards from now on...
- I don't recommend the drill bits though...get them online, get several of whatever size you need.
- Cut down copper-clad PCB board to size before running it through the lamitator for toner transfer. I tried this this time and it worked better than sending the whole board through (less jams).
- Hydrochloric Acid is better and faster than Ferric Chloride for etching PCBs.