All kinds of simple AM radio circuits exist, but FM simple radio circuits have been limited during the transition from vacuum tubes to transistors. A simple one-transistor FM radio is the focal point of this article. It is based on some earlier designs of super-regenerative FM radios from the 1950s and 1960s. It is also available in a kit form with parts sources provided in the Reference section. A summary of low technology used on simpler FM radios is also covered.
Building a One-Transistor FM Radio
This simple FM is sensitive, selective and powerful enough to operate an earphone. It is similar to a kit, but you obtain parts from three independent companies. Schematics for several alternative designs to increase audio gain are provided. You may even add a small speaker. A detailed parts list is included. which may allow some components to be used from your junk box. You will have to wind L1 (a small coil of wire) that sets the frequency, serves as an antenna and is a primary adjustment for super-regeneration. If you like to tinker with your radio, this is a perfect design. Start with the recommended printed circuit board and build the radio in its simplest form. After you get the bugs out and feel its performance is in line with the Reference, try some or all of the modifications recommended. You can even convert it into a simple CB radio receiver.
A Variety of Simpler FM Radios
The "Low Technology ...Theory" (See Reference) is a bit complex in the few pages allotted for a lead into some truly unique "Simpler" FM radios. The introduction was excellent, with a historical perspective on AM (Amplitude Modulation) and the advantages of FM (Frequency Modulation). Many FM detection systems are briefly described, including:
1. Slope detection and the FM radio Crystal set
2. An oscillator is an FM detector
3. Regenerative detectors
4. Super-regenerative detectors
5. The Hazeltine-Fremodyne super-regenerative detector
6. Frequency to voltage converters
7. Phased FM detection
Projects included were crystal radio and tuners; resonant cavity designs; tuned diode VHF receivers; a one-tube FM tuner; the one-transistor FM radio (featured above); a two-transistor Radio Shack Science Fair FM radio kit (circa 1972); a homemade Hazeltine-Fremodyne example; commercial examples; others.
Some of these simpler designs are classical museum pieces and others are kits that a child can build for a science fair project.
- Photo Credit George Doyle & Ciaran Griffin/Stockbyte/Getty Images
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