This heating element control with up to 3 kW was obtained from an old American documentation. SCRs must be load-compliant, and transistors can be current equivalents.
This circuit can use 20 to 100 mA relays with a general-purpose transistor such as the BC538. The...
The tone control in the figure below can be implemented with almost any operational amplifier. The power...
This circuit was found in a 1967 General Electric documentation. The components are from that...