Found in a 1948 MIT documentation, this circuit uses a 0 to 100 uA microammeter as an indicator. The power supply is 150 V.
This alarm circuit was found in a 1982 publication. It uses components that can still be found...
With this circuit, signals in the range of 10 kHz to 150 kHz can be received at close to 4 MHz...
Found in American documentation from the 1970s, this circuit is intended for amplifying analog TV...