Making a big robot
Small robots like the Lego EV3 can teach you a lot about programming and about robotics but they're not really big enough to impress your girlfriend. Yes, size matters. I intend to make a robot one day that is about as big as C3PO (from Star Wars) or Wall-E. My big robot will be controlled by a Raspberry Pi, using Python. It certainly won't be a humanoid robot - it's extremely difficult to get a robot to move around on two legs and the obstinacy of many roboticists to focus on this formidable challenge is, in my opinion, the biggest reason why robots can do so little. The very best humanoid robots can be interesting or amusing, but have you ever seen any that can do anything useful? The famous humanoid robots of Boston Dynamics (Atlas, Pegman etc) are so expensive and fragile that you will always see them working with a cable above them ready to catch them if they fall over (i.e. useless in the real world) and Honda's Asimo robot seems incapable of doing anything more useful than carrying drinks on a tray! The wheel was one of the greatest inventions of mankind and it's hard to understand why so many roboticists don't want their robots to have wheels. At the very least, they could at least give them four legs instead of two - still interesting and much more stable than a biped (look how impressive Big Dog and Spot have become). This video neatly shows the superiority of tracks over bipedal motion:
So my big robot will run on wheels (tracks would be even better for outside use, but I'll aim for an indoor robot), or possibly the neat hybrid wheels/legs shown in one of the videos below. I'd like my robot to be useful, so I want it to use the Raspberry Pi camera and have a robotic arm to grab and carry things. It may end up looking like one of these:
Note that the surveillance robot on the right above retails for more than 20 thousand Euros (25 thousand USD) so making something comparable for less than 200 Euros (250 USD) will be a challenge...
What do I hope to learn from building a big robot (apart from impressing my girlfriend)? Building a big robot will teach me a lot about motors and relays. A relay is a magnetically operated switch that allows a small current (such as that from the Lego EV3 intelligent brick) to control a much larger current (such as that needed to run a big motor). I note that relay boards are easily available for the Raspberry Pi credit card sized computer and for the Lego EV3.
For the moment, all I have done is start collecting some inspirational videos:
Here is a crawling robot that has an impressive ability to move around on rough ground but uses a very simple mechanism:
HERE is a Wikipedia list of open source robotics projects.
Some large robots are already available to buy. The Kuratas, for example, would probably have a good chance of impressing your girlfriend, if you can afford the 1,353,500 USD price tag:
Having said that I think it is a mistake to focus on humanoid robots, here are a couple of open source projects for building large humanoid robots using parts that can be printed on a standard 3D printer like the one on your desk (what do you mean, you haven't bought one yet??).
Inmoov is a lifesize robot. Sensibly, the creator has not given the robot any legs. According to this article, you can build the two adult size robot arms, with individually-actuated fingers, for around US$900 dollars (excluding the cost of the 3D printer itself). Note the presence of a Microsoft Kinect depth camera beneath the robot's chest.
Poppy is a child-size robot with legs. Surprise, surprise, she seems to be unable to walk without assistance. I think Poppy is programmed in Python - this would be a plus. Don't get too excited though - it's estimated that the set of parts needed to make Poppy would cost around 7500 Euros.