Aquadoodle car/train follows line, how?

Tonight I saw a new toy for the kids. On a regular Aquadoodle mat a child can draw any line and a car will follow the line. How does THIS work?

I can’t say specifically how this toy works, as there are several possible methods. It’s a trivial problem, however. One of the simplest techniques uses two LED-photodiode pairs, spaced a small distance apart to detect the line by reflection much like supermarket bar-code scanner, and it is programmed to simply keep the line in between them. When one detector “sees” the line, the toy turns toward it gradually until the other detector picks up the line, whereupon it begins to turn back the other way; this process repeats the length of the line.

All I know is that I want one, or maybe two!

Based on the description, I think it has to be based on optical sensors, like QED says. I used to have a toy like that, it had two rear wheels with a separate motor on each wheel. When the right sensor sees a darker area (i.e. encounters the line), it turns off the right rear motor, causing the toy to turn to the right (because the left rear wheel is still turning). Similarly the left sensor turns off the left rear motor.

My husband built something similar using the Lego MindStorm kits a few years ago (believe it or not, it was for a university class!) It worked the same way as described above. He says it can be done with only one sensor, but it requires more computer logic, and a wider line. The sensor would be programmed to keep the vehicle centered on the line, rather than with a sensor to either side.

This isn’t exactly a new idea, either. I remember when I was a kid there was a toy dog that would follow a line you drew in crayon. This was in the early 80s, if I recall.

We had a mail robot in Chattanooga when I worked with TVA. It followed a scent trail, that could be layed down by spraying a colorless line on the carpet. Maybe it works like that? I would imagine scent trail following robots have improved in the last 15 years or so.

I had one of the crayon line following ones (a truck, I think). That one worked because the crayon line had more friction then the mat. This newer one looks to be a different system though.

There is a product similar to the Aquadoodle that seems to work on this principle, or along the same lines if you prefer.

Enter product code 6853 here and you get: