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.
The operational amplifier used in this application is not the most common today. The circuit is...
This basic blocking oscillator configuration was obtained from a 1954 transistor book. The...
This circuit appears elsewhere on this site, but from different sources. Using the LM3900, we find this...