Project Overview
This case study focuses on how I used the limitations of successive prototypes to define requirements for further hardware development.
Capabilities Demonstrated
[1]
Translating prototype test results and observed limitations into requirements for subsequent design iterations.
[2]
Coordinating sensors, electronics and mechanical interfaces within a compact product design.
[3]
Designing component supports, fasteners and interfaces to improve assembly and serviceability.
[4]
Designing a custom PCB that integrates sensing, processing, wireless communication and power.
[5]
Building and iterating physical prototypes to evaluate component fit, packaging and assembly.
PROJECT CONTEXT
How It Works
The device is a wrist-worn prototype designed to estimate changes in hydration for outdoor workers. It combines capacitive skin-contact sensing, UV monitoring and real-time processing. The sensing approach uses changes in skin capacitance as an indicator of changing water content, comparing readings against a calibrated baseline for each user. UV measurements are used to weight the capacitance signal, and a companion interface displays the interpreted hydration level and notifies the user when it falls significantly below their baseline.
Project Evolution
The project began as a six-person capstone, during which the team developed the functional prototype shown as Gen 1. This stage included four physical prototype iterations. After the capstone, I continued independently through Gen 2 and Gen 3, first addressing mechanical integration and serviceability, then consolidating the electronics onto a custom PCB and redesigning the enclosure around its smaller footprint.
Gen 1
Capstone team.
Gen 2
Independent development.
Gen 3
Independent development.
PART I
Functional Prototype
This section introduces the capstone prototype and the testing results that established priorities for further development.
Table 1.1: Gen 1 Functional Requirements
1.1 Capstone Development
During the capstone, the team built and tested four physical prototype iterations, reducing the device envelope by 45% across that stage of development. The resulting prototype integrated UV and capacitive skin-contact sensing with real-time processing and a functional companion interface. This established the initial system, while testing identified limitations that would inform my independent continuation.
Figure 1.1: Gen 1 functional prototype
1.2 Prototype Limitations
Sensor readings were inconsistent during testing. UV readings averaged 35.7% error, while capacitance readings varied significantly over time.
Figure 1.2: Measured vs. Expected UVI
Figure 1.3: Capacitance Reading
Figure 1.4: Gen 1 internal assembly
The enclosure also lacked dedicated internal supports, leaving the controller, battery and sensors without defined mounting features. These limitations established a need to revisit both the sensor interface and the internal assembly.
Figure 1.5: Gen 1 prototype beside a WHOOP on the same wrist
The device was also substantially larger than a commercial wrist wearable, as shown in the comparison with a WHOOP on the same wrist. Although the capstone iterations had reduced its size, the remaining enclosure volume and internal arrangement provided further opportunities for improvement.
PART II
Mechanical Redesign
This section explains how I translated the prototype’s limitations into mechanical requirements and redesigned the enclosure, component supports and skin-contact interface.
2.1 Requirements for Independent Development
For Gen 2, I retained the existing off-the-shelf electronics and focused on improving their mechanical integration.
Table 2.1: Gen 2 Mechanical Redesign Requirements
2.2 Enclosure and Sensor Interface Redesign
I worked through several CAD iterations to refine the component layout, internal supports and enclosure geometry as I developed the Gen 2 prototype.
Internal support geometry
Component packaging
Figure 2.1: Early Gen 2 CAD iteration showing internal supports and component packaging
I redesigned the enclosure with dedicated press-fit supports for the controller, battery and sensors. These features established defined positions for the components and removed the need for internal glue. I also replaced the previous assembly approach with an M4 screw-fastened case so the device could be opened, serviced and reassembled.
Figure 2.2: Gen 2 exploded assembly
2.3 Integrated Prototype and Remaining Limitations
The Gen 2 prototype incorporated the revised internal supports, fastening and electrode interface.
Figure 2.3: Completed Gen 2 prototype, top and underside
For the sensor interface, I replaced the square, flush-set electrode with a circular electrode measuring 20 mm in diameter and protruding 0.4 mm from the underside of the enclosure. This change was intended to increase skin-contact area and maintain more repeatable contact with the wrist, addressing one of the sources of inconsistency identified during capacitance testing.
Its size remained constrained by the off-the-shelf development boards, sensors and interconnects. Further reduction would require changes to the electronics themselves.
The assembly also retained prototype-specific hardware, variable wall thicknesses and loosely defined part interfaces that limited its suitability for repeatable manufacturing. These limitations informed the next stage of development: consolidating the electronics onto a custom PCB and standardizing the enclosure around that hardware.
PART III
Custom PCB and Enclosure Redesign
This section explains how I used a custom PCB design to reduce the electronics footprint and redesigned the enclosure to support more repeatable assembly.
Table 3.1: Gen 3 PCB and Enclosure Requirements
3.1 Custom PCB Development
For Gen 3, I designed a custom PCB that integrated sensing, wireless processing, power and USB-C charging onto one board. Consolidating the previously distributed electronics reduced the footprint needed inside the enclosure and allowed its internal layout to be designed around a defined board geometry.
Figure 3.1: Design renders of the custom PCB
I checked trace, spacing, drill and edge clearances against the manufacturer’s design rules. I then produced the fabrication package, including Gerber layers, drill files, a bill of materials and component-position files. This completed the design work needed to fabricate the board, with electrical bring-up and integrated testing to follow after delivery.
Figure 3.2a: PCB routing view 1
Figure 3.2b: PCB routing view 2
3.2 Enclosure Redesign
Figure 3.3: Gen 3 enclosure
I redesigned the Gen 3 enclosure and internal layout around the smaller footprint enabled by the custom PCB. This included standardizing screw sizes, wall thicknesses, component clearances and part interfaces to support repeatable assembly. I also reduced the total part count to simplify the assembly and support further durability testing.
Figure 3.4: Protective case with carabiner attachment
The design includes a protective charging case with a carabiner attachment so the device can be carried on a backpack when not in use. This provides a way to store and charge the wearable while protecting it between uses.
3.3 Envelope Reduction
The Gen 3 redesign reduced the device envelope from 57 × 51 × 13 mm to 39 × 33 × 9.7 mm. This represents a 56% reduction in footprint area and a 67% reduction in bounding-box volume relative to Gen 2. The comparison below shows the change between the two prototypes alongside a WHOOP, with Gen 3 approaching the general form-factor range of commercial wrist-worn devices.
Top: Gen 2, 57 × 51 × 13 mm
Middle: Gen 3, 39 × 33 × 9.7 mm
Bottom: WHOOP
Gen 3 on the wrist
Figure 3.5: Prototype size comparison and Gen 3 wrist fit
Current Status and Planned Testing
I am currently awaiting delivery of the custom PCB. Once it arrives, I will bring up and validate the board, integrate it into the Gen 3 enclosure and test the complete wearable. The results will inform further revisions before a pilot manufacturing run.
Planned mechanical testing includes strap durability, drop resistance and compression. These tests will evaluate strap retention, enclosure damage, component displacement and deformation under controlled conditions. The findings will help determine whether the attachment geometry, fastening, wall thicknesses and internal clearances need further changes.
Anderson Todd
Engineering Portfolio