This circuit is from a 1965 General Electric manual. It uses components from the time that admit equivalents. The SCR can be the TIC106 and the LDR of any type.
Found in a Popular Electronics from the 1990s, this circuit uses components from National...
This circuit was found in Motorola documentation from 1968. SCRs can be modern equivalents and the...
Suggested by the 90s ARRL Handbook edition, this circuit aims to reduce or cancel noise at the...