The L1 coil is formed by 4 turns of wire 20 in the form of 0.5 to 0.6 cm without core. The circuit is from an 80's documentation. The range reaches a few tens of meters.
We found this circuit in the Linear Applications Handbook of the National Semiconductor of 1994....
This simple continuity tester, whose configuration can be found several times on this site, is...
This circuit is from old documentation. The operational amplifier is uncommon nowadays. The...