Method required to enable a toy "vehicle" to follow a "target" wirelessly?
An ultrasonic ping mechanism using 3 ultrasonic receiver transducers on the vehicle, and a single omnidirectional ultrasonic TX transducer (a piezo speaker or metal-cone ultra-tweeter would work) on the device to be followed, would yield direction and distance information, so long as something approximating line of sight (sound) could be maintained. A simplistic illustration:







simulate this circuit &ndash; Schematic created using CircuitLab

The follow-me unit generates an ultrasonic pulse at a calibrated time interval. The difference in time-of-flight of the pulse on the three RX units allows determination of relative distance from each RX, and thus spatial location via triangulation. 

If there is a wall blocking one of the sound paths but not the others, this would add some uncertainty to the process, but so long as the follow-me is moving, this too can be worked around using a bit of signal processing and interpolation between known-good pulses.

If highly directional RX units are used, direction can be determined with just two units sufficiently separated from each other, but this will not allow estimation of distance, only angular direction.