This unusual circuit from a Popular Electronics of the 90s uses LED indicators, operating as a bridge. The components used are common. It is based on the tuning of the signal by a PLL.
Frequency Indicator (CB6342E) This simple circuit was found in an Exar publication from the 1970s....
This circuit with TTL compatible output was found in a 1968 US documentation. The circuit is...
This is a circuit that appears several times in this section and in articles. It is a preamplifier for...