Design Diary: Varactor-Tuned Regen Receiver

[QRP Gaijin] likes to build regenerative receivers. If you’ve ever built a serious one, you know there are (at least) two problems: One is you need a variable capacitor (hard to find these days). The other annoyance is that if you cover a wide frequency range, you probably need more than one coil.

[QRP Gaijin’s] latest radio design doesn’t have either of these problems. He uses a coil with a single pole double throw switch to bandswitch a single coil. There is no traditional main tuning capacitor. Instead a 1SV149 varactor provides the radio’s main tuning capacity (the diode tunes between 35 to 500 pF).

The post provides a nice design and a neat looking build in a Tupperware container (well… the outside is neat, at least; the inside is… best left inside the Tupperware). Better still, [QRP Gaijin’s] post details how he got to the final design, starting with the idea, and detailing the original design and the changes he made along the way. He also used data from an earlier build to limit how much the regeneration control has to be changed over wide frequencies and details how that changed the design. The prototype actually lacks the planned bandswitch, but will cover 3 to 30 MHz with the right coil.

There are certainly simpler regenerative receivers out there. However, the sophistication of this design along with the details of the designer’s thought processes makes this an interesting intermediate weekend project.

Continue reading “Design Diary: Varactor-Tuned Regen Receiver”

Simple One-Chip Regenerative Receiver

Crystal radios may be the simplest kind to make, but regenerative receivers are more practical and only a little more complicated. A recent design by [Selenium] is super simple because it uses a single LM386 audio amplifier IC.

You might be surprised that you can convert an audio amplifier to a receiver using just a handful of components (a variable capacitor, a coil, a handful of capacitors, and a speaker). However, [Selenium] realized he could subvert the gain and bypass pins to cause regeneration and wound up with a very simple receiver.

If you haven’t looked at regenerative receivers before, the principle is simple (and dates back to 1912). An oscillator is an amplifier that gets (theoretically) an infinite amount of gain at one particular frequency. A regenerative receiver is just an amplifier that is almost (but not quite) at the point of oscillation. This gives it very high frequency-specific gain and a measure of selectivity. You can also nudge the receiver just into oscillation to receive CW or SSB signals.

[Selenium] built his prototype on an old receiver chassis because it had the IC and the variable capacitor already in place. However, others have built successful copies on breadboards ([Austin Heller] created several good looking breadboard versions) and on PCB material. [Selenium] also released some other unique LM386-based designs that use more parts (and, probably, have better performance). Looks like a simple way to build a practical receiver.

Homemade regenerative tube radio

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. Continue reading “Homemade regenerative tube radio”

Regenerative bike brakes

For their senior design project at Arizona State University, these guys built a regenerative braking system for their bike. As they brake, the system spools up to give them a push back up to speed.  They achieved 25% efficiency on one run, which isn’t too shabby.

The site has all kinds of information. You can check out their different plans and ideas from before they decided on this specific layout as well as all their data from the test runs. Be sure to look at the piece by piece breakdown of what changes were made from the initial design.

[thanks Mario Gomes]