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 using an operational amplifier was found in old American documentation. It can be...
This is a traditional comparator configuration to obtain an under and over voltage indicator. In...
This circuit has an accuracy of 1%. The components used must be precision as well. The input...