For the BigTrak conversion project, I plan to retrofit the lunar rover toy with a modern small board computer like a Raspberry Pi or something similar, my plans to start with are to install the new hardware and get it to function like the original device and then add other hardware/software components at a later time.
Today I took apart the bigtrak lunar rover toy for some preliminary investigations, I discovered that the drive trains feedback system uses optical encoders rather than hall effect sensors which are described in the wikipedia article.
There are magnets attached to the motors but these form a rudimentary synch lock mechanism, while the bigtrak is driving in a straight line the magnets pull each other forward or drag backwards to correct for slight drift in the motors.
I had a quick look at the main circuit board too, its pretty crusty and the ic's are black blobs but interestingly not formed on the main circuit board, they've been blob formed onto a separate circuit bard which has been cut out and then soldered onto the main circuit board.
Finally a quick look at the sounds the bigtrak makes using an oscilloscope. It'd be great to keep the original sounds which are quite basic tones so I plan to find their frequencies to replicate them.