It wouldn’t be October without Halloween, and it wouldn’t be Halloween without some spooky music. There’s no instrument spookier than a Theremin, which also happens to be one of the world’s first electronic instruments.
You’ve no doubt heard the eerie, otherworldly tones of the Theremin in various 1950s sci-fi films, or heard the instrument’s one-of-a-kind cousin, the Electro-Theremin in “Good Vibrations” by the Beach Boys. The Theremin turns 100 years old this month, so we thought we’d take a look at this strange instrument.
One hundred years ago, a young Russian physicist named Lev Sergeyevich Termen, better known as Leon Theremin, was trying to invent a device to measure the density of various gases. In addition to the standard analog needle readout, he wanted another way to indicate the density, so he devised an oscillator whistle that would change pitch based on the density.
He discovered by accident that having his hand in the field of the antenna changed the pitch of the whistle, too. Then he did what any of us would do — played around until he made a melody, then called everyone else in the lab over to check it out.
Theremin soon showed his device to Lenin, who loved it so much that he sent Lev on a world tour to show it off. While in New York, he played it for Rachmaninoff and Toscanini. In fact you can see a video recording of Leon playing the instrument, a performance that’s more hauntingly beautiful than spooky. In 1928, he patented the Theremin in the United States and worked with RCA to produce them.
Bats use echolocation to see objects in front of them. They emit an ultrasonic pulse around 20 kHz (and up to 100 kHz) and then sense the pulses as they reflect off an object and back to the bat. It’s the same type of mechanism used by ultrasonic proximity sensors for object-avoidance. Humans (except perhaps the very young ones) can’t hear the ultrasonic pulses since the frequency is too high, but an inexpensive microphone in a simple bat detector could. As it turns out bat detectors are available off the shelf, but where’s the fun in that? So, like any good hacker, [WilkoL] decided to build his own.
[WilkoL’s] design is composed primarily of an electret microphone, microphone preamplifier, CD4040 binary counter, LM386 audio amplifier, and a speaker. Audio signals are analog and their amplitudes vary based on how close the sound is to the microphone. [WilkoL] wanted to pick up bat sounds as far away as possible, so he cranked up the gain of the microphone preamplifier by quite a bit, essentially railing the amplifiers. Since he mostly cares about the frequency of the sound and not the amplitude, he wasn’t concerned about saturating the transistor output.
The CD4040 then divides the signal by a factor of 16, generating an output signal within the audible frequency range of the human ear. A bat signal of 20 kHz divides down to 1.25 kHz and a bat signal of up to 100 kHz divides down to 6.25 kHz.
He was able to test his bat detector with an ultrasonic range finder and by the noise generated from jingling his keychain (apparently there are some pretty non-audible high-frequency components from jingling keys). He hasn’t yet been able to get a recording of his device picking up bats. It has detected bats on a number of occasions, but he was a bit too late to get it on video.
Bats are fascinating animals, and despite all the myth and creepiness surrounding them, they really remind one more of a drunk bird lost in the night sky than the blood-sucking creature they’re often made out to be. Of course, some really fall into that category, and unlike actual birds, bats don’t tend to grace us with their singsong — at least not in ways audible for us humans. But thanks to bat detectors, we can still pick up on it, and [Marcel] recently built a heterodyne bat detector himself.
The detector is made with a 555, an MCP6004 op amp, and a 4066 analog switch — along with a bunch of passives — and is neatly packed into a 3D-printed case with a potentiometer to set the volume and center frequency for the detection. The bat signal itself is picked up by a MEMS microphone with a frequency range [Marcel] found suitable for the task. His write-up also goes in all the mathematics details regarding heterodyning, and how each component plays into that. The resulting audio can be listened to through a headphone output, and after putting together an adapter, can also be recorded from his smartphone. A sample of how that sounds is added in his write-up, which you can also check out after the break.
[regveg] was looking for a way to receive signals outside the normal 64-1700MHz range his TV tuner software defined radio dongle can get. After finding a few $100+ upconverters on the Internet, he stumbled across a DIY project that greatly expands the frequencies his RTLSDR can receive.
[George]’s upconverter uses heterodyning to increase the frequencies received by a SDR dongle. The basic idea is mixing a signal from an antenna with a 100MHz frequency oscillator. The resulting output will be λ + 100MHz and λ – 100MHz, allowing for a wider range of frequencies that can be received by the SDR TV tuner dongle.
Now [regveg] has a board and schematic that makes it possible to receive just about anything with his TV tuner dongle. Interestingly, this upconverter contains less than $10 in parts and is easily etched at home thanks to a single-sided construction and through-hole parts.