For hams who build their own radios, mastering the black art of radio frequency electronics is a necessary first step to getting on the air. But if voice transmissions are a goal, some level of mastery of the audio frequency side of the equation is needed as well. If your signal is clipped and distorted, the ham on the other side will have trouble hearing you, and if your receive audio is poor, good luck digging a weak signal out of the weeds.
Hams often give short shrift to the audio in their homebrew transceivers, and [Vasily Ivanenko] wants to change that with this comprehensive guide to audio amplifiers for the ham. He knows whereof he speaks; one of his other hobbies is jazz guitar and amplifiers, and it really shows in the variety of amps he discusses and the theory behind them. He describes a number of amps that perform well and are easy to build. Most of them are based on discrete transistors — many, many transistors — but he does provide some op amp designs and even a design for the venerable LM386, which he generally decries as the easy way out unless it’s optimized. He also goes into a great deal of detail on building AF oscillators and good filters with low harmonics for testing amps. We especially like the tip about using the FFT function of an oscilloscope and a signal generator to estimate total harmonic distortion.
The whole article is really worth a read, and applying some of these tips will help everyone do a better job designing audio amps, not just the hams. And if building amps from discrete transistors has you baffled, start with the basics: [Jenny]’s excellent Biasing That Transistor series.
[via Dangerous Prototypes]
Perhaps unsurprisingly given the prevalence and cost of these devices, the answer is quite simple. The key interaction is between a permanent magnet mounted to the end of the waving arm/pendulum and a many-turn wire coil attached to the body. As the magnet swings over the coil, its movement induces a voltage. A small blob of analog circuitry reacts by running current through the coil. The end effect is that it “senses” the magnet passing by and gives it a little push to keep things moving. As long as there is light the circuit can keep pushing and the pendulum swings forever. If it happens to stop a jolt from the coil starts the pendulum swinging and the rest of the circuit takes over again. [Josh] points to a similar circuit with a 
