Curious about how respiratory belt transducers work—or how to design one yourself? This quick guide walks you through the essentials, from sensing principles to circuit basics. Whether you are a hobbyist, student, or engineer exploring wearable health technology, you will find practical insights to kickstart your own design.
Belly breathing, also known as diaphragmatic or abdominal breathing, involves deep inhalation that expands the stomach and allows the lungs to fully inflate. This technique engages the diaphragm—a dome-shaped muscle at the base of the lungs—which contracts downward during inhalation to create space for lung expansion and relaxes upward during exhalation to push air out.
In contrast, chest breathing (also called thoracic or shallow breathing) relies on upper chest muscles and produces shorter, less efficient breaths, limiting oxygen intake and often contributing to stress and tension. Belly breathing has been shown to lower heart rate and blood pressure, promote relaxation, and improve overall respiratory efficiency.
What if you could measure your breathing motion, capture it in real time, and receive meaningful feedback? A respiratory belt transducer offers a simple and effective solution. It detects changes in chest or abdominal diameter during breathing and converts that movement into a voltage signal, which can be recorded and analyzed to assess breathing patterns, rate, and depth.
First off, note that while piezoelectric, inductive, capacitive, and strain gauge sensors are commonly used in respiratory monitoring, this post highlights more accessible alternatives, namely conductive rubber cords and stretch sensors. These materials offer a low-cost, flexible solution for detecting abdominal or chest expansion, making them ideal for DIY builds, classroom experiments, and basic biofeedback systems.
Figure 1 A generic 2-mm diameter conductive rubber cord stretch sensor kit that makes breathing belt assembly easier. Source: Author
As observed, the standard 2-mm conductive rubber cord commonly available in the hobby electronics market exhibits a resistance of approximately 140 to 160 ohms per centimeter. This capability makes it suitable for constructing a respiratory belt that generates a voltage in response to changes in thoracic or abdominal circumference during breathing.
Next, fabricate the transducer by securely bonding the flexible sensing element—the conductive rubber cord—to the inner surface of a suitably sized fabric belt. It should then be placed around the body at the level of maximum respiratory expansion.
A quick hint on design math: in its relaxed state, the conductive rubber cord (carbon-black impregnated) exhibits a resistance of approximately 140 ohms per centimeter. When stretched, the conductive particles disperse, increasing the resistance proportionally.
Once the force is removed, the rubber gradually returns to its original length, but not instantly. Full recovery may take a minute or two, depending on the material and conditions. You can typically stretch the cord to about 50–70% beyond its original length, but it must stay within that range to avoid damage. For example, a 15-cm piece should not be stretched beyond 25–26 cm.
Keep in mind, this conductive rubber cord stretch sensor does not behave in a perfectly linear way. Its resistance can change from one batch to another, so it’s best used to sense stretching motion in a general way, not for exact measurements.
To ensure accurate signal interpretation, a custom electronic circuitry with a sensible response to changes in cord length is essential; otherwise, the data will not hold water. The output connector on the adapter electronics should provide a directly proportional voltage to the extent of stretch in the sensing element.
Frankly, this post doesn’t delve into the mechanical construction of the respiratory belt transducer, although conductive rubber cords are relatively easy to use in a circuit. However, they can be a bit tricky to attach to things, both mechanically and electrically.
The following diagram illustrates the proposed front-end electronics for the resistive stretch sensor (definitely not the final look). Optimized through voltage scaling and linearization, the setup yields an analog output suitable for most microcontroller ADCs.
Figure 2 The proposed sensor front-end circuitry reveals a simplistic analog approach. Source: Author
So, now you have the blueprint for a respiratory belt transducer, commonly known as a breathing belt. It incorporates a resistive stretch sensor to detect changes in chest or abdominal expansion during breathing. As the belt stretches, the system produces an analog output voltage that varies within a defined range. This voltage is approximately proportional to the amount of stretch, providing a continuous signal that mirrors the breathing pattern.
Quick detour: A ratiometric output refers to a sensor output voltage that varies in proportion to its supply voltage. In other words, the output signal scales with the supply itself, so any change in supply voltage results in a corresponding change in output. This behavior is common in unamplified sensors, where the output is typically expressed as a percentage of the supply voltage.
Before wrapping up, I just came across another resistive change type strain sensor worth mentioning: GummiStra from Yamaha. It’s a rubber-like, stretchable sensor capable of detecting a wide range of small to large strains (up to twice in length), both statically and dynamically. You can explore its capabilities in detail through Yamaha’s technology page.
Figure 3 GummiStra unlocks new use cases for resistive stretch sensing across wearables, robotics, and structural health monitoring. Source: Yamaha
We will leave it there for the moment. Got your own twist on respiratory belt transducer design? Share your ideas or questions in the comments.
T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.
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