We found this circuit in a 1976 publication. The transistors can be BC547 or 2N2222. Crystals up to a few tens of Megahertz can be tested. The LED can be of any type. The capacitors must all be ceramic.
This circuit that appears in similar versions on this site was found in a Popular Electronics from the...
This circuit is based on the bistable configuration, but with the difference that the values of...
This circuit was found in a Popular Electronics from the 90's. The circuit has gained 10 to 20 dB...