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.
The Buck-type DC-DC converter (voltage lowerer) shown in figure 11 is suggested by Analog Devices...
This circuit can be used to directly control direct current loads or higher alternating current loads...
This circuit can turn-on (only) a load (relay or lamp) when S1 is pressed. The circuit can be used as...