We found this circuit in the Electronics Experimenter Handbook magazine from 1993. L1 has 2 = 2 turns of 28 or 30 wire in the shape of 0.5 cm without core. The range reaches a few tens of meters.
This circuit was suggested by Texas Instruments in a manual of operational amplifiers. Other...
This circuit for vehicular use is from an 80's American publication. The circuit has a light...
An interesting point of this project is that it is used not only for a robotics project but also...