This crystal or galena radio was found in documentation from the 1990s. The earpiece must be a high impedance crystal or magnetic earpiece and D1 can be any germanium diode.
This circuit is from a Popular Electronics of the 90's. Any power MOSFET can be used. The circuit...
The critical component of this circuit is the transformer. T1 and T2 can be BD135 and T3 / T4 can...
This circuit generates 500 Hz rectangular signals. The circuit is from old American documentation. Neon...