This Texas Instruments circuit is from the 80's. However, it can be implemented with modern operating systems. The power supply must be symmetrical.
We found this logic level indicator in a 1976 amateur radio documentation. The circuit is powered by...
This 6-transistor transistor radio is a RCA product from the 1960s. The circuit is powered by a 9...
The operational amplifier is old. The circuit is from an 80's documentation but can be designed...