Motorcycle headlight monitor (CB9117E)

This simple circuit was found in Popular Electronics from the 1980s. The circuit uses an LED as an indicator.

Resistance to voltage converter (CB9597E)
This circuit allows you to read resistors up to 20 M ohms, converting it into voltage. The circuit is...
Phase shifter (CB21955E)
This circuit for low frequency signals was found in a publication by Gernsback from 1960. The...
Speaker Balance Indicator (CB6316E)
This 1969 Mullar circuit appears elsewhere on this site. The circuit uses germanium diodes and the...