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If you are like me, you have been to hobby stores on occasion and you see this and that, but never really buy anything.  I worry about spending money on something I can't use and frankly some of that hobby stuff is expensive!  One thing, or one COMPANY I have seen many times, but never purchased is Tamiya.  I have seen their kits around for years, they have been around for years (1980's I believe). They are hard to miss, big red and blue star with Tamiya written underneath, white box with an 1980's looking picture printed on it with a variety of English and Asian writing on it.  If you see one, you will probably recognize it.  Regardless if you have or haven't seen them, they are quite cheap and an excellent base for robotics as they are easy to work with, most of their parts fit interchangeably from one kit to another and their products are fairly readily available.

What made me go and purchase one of these Tamiya kits was Maker Faire YYC.  While wandering around I saw a few kits that had been used for robotics and seen what others had done with them, making more complicated robots so I decided to go out and get one myself.  Boy, had I been missing out!

For this Instructable, I will be hacking a Tamiya model kit to make a simple, but effective robot.

ALSO!  I decided to make this now because I wouldn't mind winning a Raspberry Pi Model B from the Hack It! Contest, so vote for me if you like 'Ible!  :)
 

Step 1: Tamiya Educational Kits

I spent almost all of the Sunday after Maker Faire YYC researching Tamiya kits, what all they sell, how much, that sort of thing.  The kit I used and other people use are the Tamiya Educational Model Kits.  They range in price from $10 up to about $80.  3 in particular stood out to me though, the Bulldozer kit, simply a 2 track system, the Shovel/Dozer kit, 2 tracks and the buckets works, and the Rescue Vehicle kit, 6 tracks and the front ones can be manipulated.  Respectively in that order, they increase in price from $40, $60 and $80.  A fair price for what they are in my mind.

Besides the fact that they are great for hacking, they are tons of fun to put together, and after a day of researching their website, their R/C kits seem pretty cool too.

I built the model kit as per the instructions first so that I could test everything, make sure there were no defects before hacking.  That and it was a ton of fun to drive around with the controller and pick stuff up with the bucket.

I suggest that you buy some 5 min Epoxy though.  Tamiya gives you everything in the kit, including glue, however it was dried up in my kit.  You will also need an Exacto Knife (Hobby Knife), Side Cutters and a medium Phillips Screwdriver.

Step 2: The Circuit

For the circuit, you will need:

2x 2n4403 PNP transistor (Or other general purpose PNP transistor, like 2n3906)
2x 2n4401 NPN transistor (Or other general purpose NPN transistor, like 2n3904)
1x 10 ohm resistor (color code, brown, black, black)
2x resistor with a value between 1k and 1.5k ohm (code should start with brown and end with red, the middle band will vary between black and green)
2x ceramic capacitor (small ones, identical, I don't really know how the code works on some of them.  I believe they are 100pf)
1x 3v battery pack (2 AAA's, 2 AA's, 2 D-cells)

Optional: (I didn't use these yet myself, I thought of other thinks to modify the circuit with, so I left these out, for now!)

2x DPDT relay (I have never had good luck with H-Bridges, DPDT relays make it very simple, also, the lower the coil voltage the better)
3x mini lever switches/bump switches
2x diodes (identical, 1n4001 or similar)
1x 7805 5v regulator (rather then buying these, I suggest looking at a few old circuit boards, I have dozens of these things from electronics from the dump, it looks like a transistor and says 7805 on it somewhere.  If it says 7812 its a 12v regulator)
1x 12v battery pack (if you are using relays)
Assorted plugs/connectors (If you think you will want to modify and try other things with the kit, add connectors to the motors so that you can disconnect this circuit and build your own circuit or program an Arduino and let it control the kit.  With connectors you can easily remove one board and add in something else)

The circuit will make the robot follow or flee from light (if you switch the light from one side to the other will make the robot go to or flee light).  

The relays are there so if it bumps into an object, depending on what sensor gets the bump, it will turn or reverse.  I have never been able to build an H-Bridge with Transistors that worked very well, so I decided to use relays.  Work fantastically in tests on my breadboard, but I have not implemented them yet, I want to modify the light sensing circuit more before adding them.  For me, this is something I am going to continually build up as I think of things, and I would like to use it to cover Braitenberg Vehicles in another Instructable.

Step 3: How It Works

The circuit is fairly simple, the photocells vary resistance based on the amount of exposure to light, this controls the first transistors on each side (When I say each side, I mean the NPN transistors as one side, PNP transistors as the other side).  The second transistor is like that in a Darlington pair, they amplify the signal from the first transistor even more.  The 10 ohm resistor between the 2 photocells along with the outputs of the first transistors being connected together, makes sure that both motors turn the same amount if the photocells are near the same resistance.  Without that it would turn one way or the other a little bit because of one photocell may turn the transistor on more or less then the other, or if you have only a slight difference in amount of light on each photocell.  With a larger difference in the amount of light on the 2 photocells, one side or the other turns on more then the other and then causes it to turn.  The 1k to 1.5k ohm resistors give the motors a little bit more juice, I found without them the motors ran a bit slow and wouldn't turn as well.  The ceramic capacitors go across the leads of the motors, I have always seen this done and been told to do this, it absorbs spikes and bumps from the motors if I am correct, but its not something ever really explained well, if at all to me so I can't give a concrete explanation of that.

Step 4: Going Further

I made this as something I hope will be simple enough that beginners will be able to make and learn from, gain some understanding from, as well as help finding a small, cheap, low power platform for robots.  I never had any instruction as far as electronics, I learnt through dismantling, and de-soldering parts from my mums VCR and trying this and that.  When I started, it didn't seem like there was that much in the way of instructions for learning electronics, there is a lot more now-a-days.  I hope this helps get someone started, and they take it much further, and this can be taken much further.  Add Arduino to control it, add a motion sensor to it and a electric automatic BB gun internals to it with servos, make it shoot anything that moves?  You could even add R/C Circuitry to it and make it remote control.  Its up to whatever you can think of and that whatever you use for a platform can handle the weight of what you put on it.  Start thinking and get out there and build it!
great work! so the robot always moves away from light? could you wire it so that it was attracted to light?
You don't even have to change the wiring actually! Since the sensor controls one side only regardless of what side they are on, switch the sensors from one side to the other and it will then follow light! That is why I didn't glue mine down, I can bend the wires so the sensors are reversed, and it will follow light! I should add a video of that actually, start it fleeing and then, move the sensors, then show it following light!

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