Final Design

    The development of fluorescence reader for a Hydration Sensor Module (HSM). The group developed a bench top test bed that will be designed to last the entirety of the HSM project. Therefore to account for any parts breaking, the design used easy to manufacture or off the shelf parts. Ease of use and maintenance were key ideas to maintain throughout the design process.

Figure 1a,b,c. Starting from the left: initial mock up design, prototype, final design

LED Simulator

Figure 2. Top and bottom view of LED simulator

    The requirement for the LED simulator was to replicate single wavelength two fluorescent proteins, have a single channel light, have variable adjustment for the brightness, be self-contained to be easily inserted and removed, and the ability to emit light at intensity close to the lowest intensity detectable by the photosensor. The LEDs were initially chosen in order to accommodate a certain height, which is why surface mounts were chosen. However since that is no longer a requirement the most important factor is choosing a LED that is close to the wavelength of the two fluorescent proteins. The design of the slides was to be similar to that of the filter so that they both can fit inside the housing of the test bed.

Figure 3a,b. Polarizing filters on the left. Bandpass filters on the right.

    Filter slides were used along side the LED slides in order in order to lessen the intensity of the LED. Bandpass and polarizing filters, as shown in figure 3a and b, were used to reduce the intensity of the LED and let certain wavelengths through. The filters used would help the LED simulator replicate the transient signal that would be produced from a test strip. The bandpass filter that were chosen correspond to the wavelengths that are desired with an accuracy of 10nm.

Test Bed

Figure 4. Annotated view of the interior of the test bed

    The requirement of the test-bed was that it must fit the LED and photosensor, be adjustable, minimize stray light, and contain electronic linkage of LED simulator, photosensor, and PC-based data capture system. The sliding rests on grooves which allow for it to restrict stray light from entering. The top of the door, and the perimeter of the box was applied with silicone gasket to further minimize stray light.

Figure 5. Potentiometer and gear system.

    To measure the distance between the sensor and the bottom of the housing unit a 10-turn rotation potentiometer was used, as seen in figure 4. It uses a 5:1 gear ratio which allows 5cm of linear motion, exceeded the requirement of 1mm to 10mm. The potentiometer fits the system well since it is able to be used with minimal programming and outputs to the DAQ system. The threaded rod chosen was a M6 x 1.0 which translate into 1mm of linear motion for each full rotation. A brass threaded flange has been put on top of the housing to prevent the rod from wearing out the plastic.

    The main plate holding the carriage was attached by a ball bearing which lets the threaded rod rotate without rotating the carriage. The initial use of similar bearings on the guide rails resulted in sticking at certain places. Therefore self-aligning linear ball bearings were used instead to prevent binding and allow up to 0.5 degrees of misalignment of the shaft. The longer linear bearings helped reduce reaction forces on the guide rails, which leads to binding. To measure the distance between the sensor and the bottom of the housing unit a 10-turn rotation potentiometer was used.