We present a flexible acoustic sensor that has been designed to detect wheezing (a common symptom of asthma) while attached to the chest of a human. We adopted a parallel-plate capacitive structure using air as the dielectric material. The pressure (acoustic) waves from wheezing vibrate the top diaphragm of the structure, thereby changing the output capacitance. The sensor is designed in such a way that it resonates in the frequency range of wheezing (100-1000 Hz), which presents twofold benefits. The resonance results in large deflection of the diaphragm that eradicates the need for using signal amplifiers (used in microphones). Second, the design itself acts as a low-pass filter to reduce the effect of background noise, which mostly lies in the >1000-Hz frequency range. The resulting analog interface is minimal, and thus consumes less power and occupies less space. The sensor is made up of low-cost sustainable materials (aluminum foil) that greatly reduce the cost and complexity of manufacturing processes. A robust wheezing detection (matched filter) algorithm is used to identify different types of wheezing sounds among the noisy signals originating from the chest that lie in the same frequency range as wheezing. The sensor is connected to a smartphone via Bluetooth, enabling signal processing and further integration into digital medical electronic systems based on the Internet of Things (IoT). Bending, cyclic pressure, heat, and sweat tests are performed on the sensor to evaluate its performance in simulated real-life harsh conditions.