Using a Theremin for Medical Applications

theremin

[Eswar] is not an ordinary 16 years old boy. He figured out a noninvasive way to measure breathing in hospitals for less than $50. He is using a theremin to measure the rise and fall of a patient’s chest. For our curious readers, this touch-less instrument was invented back in 1929 by the Russian inventor [Leon Theremin]. It uses the heterodyne principle and two oscillators to generate an audio signal. One electronic oscillator creates an inaudible high pitch tone while another variable oscillator is changed by adding capacitance to an antenna.

As you can guess the space between the patient’s chest and the antennas placed around the bed forms a tiny capacitor which varies when exhaling. With three simple TTL chips and a little guessing [Eswar] had a working prototype ready to be implemented in the real world. If you’re interested in theremin, we invite you to see one of our previous articles on how to make one in a few minutes with a soda can.

Soda Can Theremin Made In Minutes

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Looking for a fun weekend project? How about building your very own sci-fi Theremin using just a soda can and a few simple components you probably already have.

Not familiar with Theremins? It’s an electronic instrument that can be played without any physical contact. Essentially, the antenna (soda can) acts as half of a capacitor — your hand acts as the other. By varying the distance of your hand to the soda can, you change the capacitance, which can then be used to modulate a sound. It was invented in 1928 by the Russian inventor [Leon Theremin], and you can actually see a video of him playing it on YouTube — we’ve never seen this before, but we must say it’s actually quite impressive!

Anyway, back to the hack. All you need are some op-amps, a few capacitors, some resistors, a diode, a breadboard, and of course — a speaker. [Will's] gone into great detail in how each circuit works with both schematics and diagrams of the breadboard configuration — it’s a great little project even kids can follow along.

Take a listen after the break.

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IR Theremin Speaks In Four Voices

infraredTheremin

At the end of every semester, we get a bunch of cool and well-documented student projects from Cornell’s ECE4760 class. [Scott] and [Alex]‘s infrared theremin is no exception.

The classic theremin design employs each of the player’s hands as the grounded plate of a variable capacitor in an LC circuit. For the pitch antenna, this circuit is part of the oscillator. For the volume antenna, the hand capacitor detunes another oscillator, changing the attenuation in the amplifier.

[Scott] and [Alex] put a twist on the theremin by using two IR sensors to control volume and pitch. The sensors compute the location of each hand and output a voltage inversely proportional to its distance from the hand. An ATMega1284P converts the signal to an 8-bit binary number for processing. They built four voices into it that are accessible through the push-button switch. The different voices are created with wave combinations and modulation effects. In addition to Classic Theremin, you can play in pure sine, sawtooth, and FM modulation.

If you’re just not that into microcontrollers, you could build this digital IR theremin instead. If you find IR theremins soulless or plebeian, try this theremincello.

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The Ondes Martenot; Better than a Theremin

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[Ville] loves the sound of an ondes martenot and decided to build his own. No, it’s not made of vacuum tubes like a 1920s original, this one is made out of a cheap, off-the-shelf analog synth and just a few extra parts.

The ondes martenot is a theremin-like musical instrument; instead of waving your hands around aerials on the theremin, the ondes uses a small ring attached to the player’s finger on a wire loop and a volume lever. The ondes isn’t a common instrument by any means, but Radiohead uses one several in any event.

[Ville] began his build by taking a small, cheap, and new Korg Monotron analog ribbon synth, cracking it open, and reading the schematics. A 100k multiturn pot was wired into the monotron and fastened to a printed paper keyboard with a system of pulleys and a small metal ring. With the multiturn pot wired into the pitch input on the monotron, [Ville] had a semi-accurate and very functional ondes martenot replica.

You can hear [Ville]‘s ondes in action after the break. It’s a little rough starting out, but by the time he’s looping multiple phrases it really does sound wonderful.

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Steampunk theremin goggles

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Lots of people build custom steampunk goggles, but most don’t implement any interactivity – they’re just an aesthetic accessory. [Sarah] recently decided to built a pair that, besides looking cool, would engage the wearer in creating sound. She accomplished this by integrating an optical theremin into their design.

To keep the build both affordable and wearable she researched simplified theremins, and eventually settled on creating a basic model that uses only a handful of components and two 555 timers. The main body of the goggles was constructed using mostly random mismatched pieces of metal and leather.  Mounted on the outer edge of each lens, there is a photo sensor and a corresponding slider control. Adjusting the slider alters the level of resistance, therein changing the pitch of the sound. The theremin will produce different pitches and octaves depending on how much light the sensors receive. So, the wearer, or a nearby friend, waves their hands around the wearers head to control it.

The speaker and volume knob are cleverly disguised as the two ‘lenses’. Rotating the volume knob lens adjusts an internal potentiometer that’s held in place by a custom laser etched piece of acrylic. To top it all off, she even designed her own PCB using Eagle.

Check out a video demonstration after the break.

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Building a Theremincello

building-a-theremin-cello

We totally missed the ball on this project. It should have been run on April Fools’ day and you would have no idea if it were real or a hoax. That’s because the very serious performance given after the break is hard to watch without a least a bit of a chuckle. The instrument shown above is a Theremincello. It’s an instrument in the shape of a cello which functions in a similar way to a Theremin.

The instrument being played in that video clip is the first generation and the one pictured above is its successor. The creator wanted to refine the electronics so that the resulting sound wasn’t so ‘flutey’. The result can be heard on the video embedded in this Theremin World article and we think they’ve accomplished the goal; it sounds much better! In the clip [Thierry Frenkel] demonstrates changing notes on the fingerboard with the left hand. The right hand which would normally bow the strings operates the lever to adjust the volume of the note being played back.

If a single fingerboard isn’t enough for your needs you may consider building this four-track design instead.

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Digital IR Theremin

Digital IR Theremin

This Digital IR Theremin creates tones based on the distance of an object from its IR sensor. There’s no microcontroller here, since the project is part of an Introduction to Digital Electronics course. Instead, it uses a handful of comparators, transistors, AND gates, and a 555 timer to make noise.

The comparators are connected to create window comparators. This configuration will output a digital 1 if the input is between two reference voltages, and 0 if it is not. Using this, the analog output of the IR range sensor can be converted to digital values.

The 555 timer takes care of creating the output waveform. A specific resistor is switched in to the timer’s RC circuit depending on which window comparator is active. This allows for a different tone to be played depending on the distance from the IR sensor.

The result is a square wave, which has a frequency dependant on how close an object is to the IR sensor. By selecting the right resistances for each distance, the theremin can be tuned to play a specific scale.

This is a neat project for people looking to learn digital electronics, and the write up does a great job of explaining the theory. After the break, check out a video of the theremin generating some tones.

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