The following is a list of materials and tools needed to create the sensor:
Remove 3 inches of wire casing from each end of the phone wire to expose the wires inside. For the heart rate monitor, you will be using four wires to attach the LED and photoresistor. Trim two of the wires by 1.5 inches on the sensor end, as shown in Figure 1 below. Use the wire stripper to remove the casing from each of the four wires on both the sensor end and header end.
Figure 1: Wire prepared for attaching sensors
Slip a small piece of heat shrink over each on the header end of the phone wire. Next, solder each wire to a pin on the header and carefully shrink the heat shrink over each solder joint using a lighter or the heat of the soldering iron.
Figure 2: Header pins soldered to header end of sensor cable
As in Step Two, slip a piece of heat shrink over each of the wires on the sensor end of the phone wire. Solder the LED to the set of shorter wires, being careful to minimize the amount of heat placed on the LED wires to avoid damaging the light-emitting diode. Next, follow the same process to attach the photoresistor to the set of longer wires on the sensor end of the cable, again ensuring that you minimize the heat on the wires to prevent damage. Finally, shrink the heat shrink on all four wires using a heat source.
Figure 3: Photoresistor and LED
The next steps are designed to create the wrapping to keep the sensors in place around the test subject’s finger. First, we will lay the photoresistor and LED in line on a piece of medical tape about 5 inches long. Make sure to keep the tape sticky-side up; the wires should be stuck to the adhesive.
Figure 4: Sensor backing
Next, cover the exposed wires by trimming tape strips to fit around both the photoresistor and LED. The sticky side of the new tape strips should be face down. The end result will be the sensor base, with the two components enveloped between the tape with a smooth outer layer.
Figure 5: Cover exposed wires
Figure 6: Preparing the covering
Figure 7: Cut notches for easy wrapping
Figure 8: Build padding to support the LED and Photoresistor
The sensor is now complete. To use the sensor, carefully align the LED and the photoresistor on either side of the patient's thumb, and wrap the tape around the thumb to block out light. If you are not receiving clear readings, make sure that the LED and the photoresistor are parallel to each other, and that they are aligned on opposite sides of the thumb.
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