There are no microcontrollers in this project. In fact you wont find a single transistor. This classic regenerative tube radio, modeled after an early 20th century homebrew is complete with schematic and additional photos. For those who are not familiar with tube designs and for simplicity, the regeneration circuit can be thought of as feedback though this relation may be argued. Read the rest after the break which includes a crash course in tube operation.
A basic vacuum tube generally consists of a heater, grid, and plate. A current is passed through the heater which as the name suggest, creates heat. This allows electrons to “boil off” under proper conditions. That is the hot heater emits electrons when the grid and plate are positive potential. Opposite charges attract so the electron is attracted towards the grid and plate. The grid has holes very much like a screen door does. Some of the electrons pass through these holes missing the grid and strike the plate. The greater the electrical potential the greater the attractive force in the grid and plate. When an electron is permitted to move from one potential to another an electrical circuit is created. This circuit forms the basics behind vacuum tube operation.
With that said, the project uses a heater powered from two D size batteries. There doesn’t seem to mention of the plate source, though we suspect a few 9 volt batteries wired in series does the trick. Some of the newer commercialized radios (World War II era) operated from considerably higher heater and plate voltage potentials, rendering the common shock hazards associated with tube designs. Tuning is accomplished with an air cap and home made variocoupler. We covered a VFD based regenerative reciever earlier this year.