By: Stephen Cortez (Electronics Engineer, Central Seizure Suite)

Rose Leidenfrost (Electronics Engineer, Seizure Watch)

This document will discuss the individual thought process behind the selection of each sensor to be used for the A-TeChToP Project. This will include trade-off studies, sensor quality comparisons, and sensor price comparisons. Ultimately, each sensor picked will be done so with the intention of utilizing it within the project’s final design.

Central Sensor Suite

Stephen Cortez (Electronics Engineer)

Accelerometer

Trade-Off Studies:

This sensor is required to be accurate enough to measure the difference between idle, anaerobic, and erratic or random movement while attached to a child. The following requirements governed the selection process of the final product:

1. Sensor is to be placed upon the chest of the child along with the remainder of the primary unit. This will allow for simplicity in determining the orientation of the child wearing the device since measurements will be taken from his or her center of mass.
2. Sensor should be accurate up to the range of at least ±16g, however the range ±8g and below are preferred. This will ensure that the device is sensitive to the desired point.
3. Size of the breakout board that contains the sensor should be no larger than 22mm x 22mm x 5mm. These dimensions were declared considering the potential sizes of varying accelerometer devices.
4. Additional features such as free-fall detection are preferred but not required.
5. The sensor should have existing, functional libraries made specifically for Arduino IDE code since this will be the medium used to communicate with it. If a library does not exist for the specific sensor, then a library should at least be available for its series type of accelerometer.
6. The price of the sensor should remain below $20.00 in order to remain within a reasonable range for the group’s overall funding.
7. Whether the Sensor is analog or digital is not exceedingly important, assuming that the previous requirements are met. Digital devices, however, must have an output resolution of at least 10 bits.
8. Sensor should preferably have either an on board voltage regulator or at least be capable of tolerating between 1.8V to 5V.
9. Sensor must be Pb free compliant as required by RoHs standards.

Potential Sensors (Properties):

ADXL335:

• Analog
• Pb free (RoHs compliant)
• Arduino IDE compatible
• ±3g accuracy range (only)
• 3 DoF (3-Axis X, Y, Z)
• 16mm x 18mm x 3mm (roughly, slightly smaller actual)
• 1.8V to 3.6V range only, no power regulator
• No extra features
• $14.95 from SparkFun
• GitHub contains available libraries
• Datasheet: https://www.sparkfun.com/datasheets/Components/SMD/adxl335.pdf

MMA8452Q:

• Digital
• Pb free (RoHs compliant)
• Arduino IDE compatible
• ±2g, ±4g, ±8g accuracy ranges (Programmable)
• 3 DoF (3-Axis X, Y, Z)
• 12-bit resolution (programmable to 8-bit)
• 16mm x 17mm x 3mm (roughly, slightly smaller actual)
• 1.95V to 3.6V range only, no power regulator
• Extra features include free fall detection and shock/vibration monitoring
• $9.95 from SparkFun
• GitHub contains available libraries
• Datasheet: https://cdn.sparkfun.com/datasheets/Sensors/Accelerometers/MMA8452Q-rev8.1.pdf

ADXL345:

• Digital
• Pb free (RoHs compliant)
• Arduino IDE compatible
• ±2g, ±4g, ±8g, ±16g accuracy ranges (Programmable)
• 3 DoF (3-Axis X, Y, Z)
• 13-bit resolution (programmable to 10-bit as well)
• 16mm x 20mm x 3mm (roughly, slightly smaller actual)
• 2V to 3.6V range only, no power regulator
• Extra features include tap/double-tap sensing, free fall detection, and low power detection
• $17.95 from SparkFun
• GitHub contains available libraries
• Datasheet: https://www.sparkfun.com/datasheets/Sensors/Accelerometer/ADXL345.pdf

GY-521 (MPU6050 Series):

• Digital
• Pb free (RoHs compliant as well as Green compliant)
• Accelerometer and Gyroscope combination board (improved orientation accuracy)
• Arduino IDE compatible
• ±2g, ±4g, ±8g, ±16g accuracy ranges (Programmable)
• 6 DoF (3-Axis X, Y, Z)
• 16-bit resolution (programable)
• 16mm x 17mm x 3mm (roughly, slightly smaller actual)
• 2.3v to 6V range (possible damage to device around 6V however)
• Extra features include free fall detection, low power detection, and ESD protection
• $39.95 ADXL345/ITG3200 version from SparkFun ($5.79 for GY-521 version on Amazon)
• GitHub contains libraries for GY-521 and large amounts of online support
• Datasheet (MPU60X0 Series): https://courses.cs.washington.edu/courses/cse466/14au/labs/l4/PS-MPU-6000A-00v3.4.pdf

Final Decision:

The final decision for the Accelerometer device is the GY-521 MPU6050 combo board. This sensor was chosen considering that it met all of the initial requirements with ease, as well as surpassing all of the other devices in terms of resolution, g-range accuracy, and voltage range tolerance. Also, this device contains a gyroscope as well, which will be crucial in detecting the orientation of a child in motion, which is a feature that the other sensors were incapable of (only static orientation was detected). The programmable ±2g, ±4g, ±8g, ±16g accuracy ranges are an excellent and useful feature as well. The price for the GY-521 unit is very reasonable and the size of the device is within the desired range. The final decision behind this device is the large amount of libraries that exists for this device, allowing for a guaranteed method to properly operate it. The libraries can be found on GitHub through this link, where the library that is intended to be used based on inspection is the KuoE0/GY-521 RAWDATA library:

https://github.com/search?utf8=%E2%9C%93&q=gy-521

Temperature Sensor

Trade-Off Studies:

This sensor is required to simply measure the temperature of the wearer of the device from their armpit. The armpit placement was chosen since oral, rectal, and temporal temperature measurements would be counter-productive for the subtlety comfort, and overall design of the device. The following requirements governed the selection process of the final product:

1. The sensor must be capable of measuring a minimum temperature range between 35˚C to 45˚C, although any wider range is acceptable. This range requirement was chosen since these values represent the temperatures in which a human may experience serious or even fatal conditions to their health. Converted, this range is approximately 95˚F to 111.5˚F.
2. The sensor must be of a reasonably small size, around 5mm x 4mm or the size of a transistor, considering that it will be placed within the armpit of the child. It’s size and orientation will be key factors in the comfort of the device.
3. Sensor must be Arduino IDE compatible and have existing libraries available.
4. Sensor must be capable of supporting a minimum range of 1.8V to 5V in order to remain consistent with the other sensors of the A-TeChToP suite.
5. Sensor must preferably be somewhat resistant to moisture since the armpit placement may require the device to experience moisture from sweat.

Potential Sensors (Properties):

LM34:

• ±1˚F accuracy
• 5V to 30V operation range
• -45˚C to 150˚C
• Not moisture resistant
• About 5mm x 4mm (transistor size)
• $7.49 from Amazon (can be requested as a sample from Texas Instruments)
• Datasheet: http://www.ti.com/lit/ds/symlink/lm34.pdf

DS18B20:

• ±1˚F
• 3V to 5.5V operation range
• -55˚C to 125˚C
• Moisture resistant
• About 5mm x 4mm (transistor size)
• Utilizes only one wire to communicate
• 9 to 12-bit programmable resolution
• $4.25 from SparkFun
• Datasheet: http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf

TMP36:

• ±2˚F
• 2.7V to 5V operation range
• -40˚C to 125˚C
• Not moisture resistant
• About 5mm x 4mm (transistor size)
• $1.00 from SparkFun
• Datasheet: http://cdn.sparkfun.com/datasheets/Sensors/Temp/TMP35_36_37.pdf

Final Decision:

The final decision for the temperature device is the DS18B20. Since temperature range and size could not help in the selection process from the previous potential devices, these parameters were ignored. The placement of the temperature sensor within the armpit of the wearer raised the awareness of moisture being a potential environmental hazard. Thus, the moisture protection benefits from the DS18B20 were preferred. Also, this device is easily programmed and has existing Arduino libraries available for use as well as programmable resolution of up to 12-bits. It is also believed that the single wire needed for communication will help in the optimization of the overall device size.

Pulse Sensor

Trade-Off Studies:

This sensor will be chosen based upon some basic requirements in order to meet the needs of the A-TeChToP sensor suite. The positioning of the sensor was taken into consideration when choosing a sensor for the main device, since the placement is important for the subtlety and comfort of the device.

1. The sensor should successfully provide heart rate physiological feedback in the form of beats per minute (bpm).
2. Sensor should be capable of operating from a discrete location so the wearer is not hindered in any way.
3. The price of the sensor should not exceed $45.00 in order to meet remain within the funding of the group.
4. Sensor must be Arduino IDE compatible as well as have existing libraries available for use.
5. The sensor must be able to operate within the minimum range of 1.8V to 5V voltage from a power supply.

Potential Sensors (Properties):

Pulse Sensor Amped (SEN-11574):

• Small, potentially concealable
• Can be applied to multiple surfaces (ear lobe and finger) with 24” cable
• 3V or 5V (from Arduino) operation voltages
• Comes with a clipper for attachments
• $24.95 from SparkFun
• Arduino IDE compatible and libraries available for use
• Getting started guide: file:///C:/Users/styph/Downloads/PulseSensorAmpedGettingStartedGuide.pdf

AD8232 Single Lead Heart Rate Monitor:

• About 90mm x 75mm, which is not an ideal size for subtlety
• Placement would be located in center, main unit with wires expanding out
• 3.3V operating voltage (only)
• Capable of monitoring both ECG and heart pulse
• Arduino IDE compatible with existing libraries
• $19.95 from SparkFun
• Datasheet: https://cdn.sparkfun.com/datasheets/Sensors/Biometric/AD8232.pdf

Final Decision:

Considering that there was a limited pool of pulse sensors available to purchase on their own, the Pulse Sensor Amped was chosen for this sensor. This decision was made considering the versatility of this sensor due to its small electrode surface area. Also, since the ear lobe and finger are not ideal locations for this sensor, it has been considered to measure the pulse of the wearer by attaching the electrode directly to their chest, just outside of the main unit of the device. This positioning is presumed to emit a strong enough electrical pulse for the sensor to simply detect a signal and relay it to the Bluetooth device. This method will optimize device size and concealment while still performing the necessary functions.

Oximeter

Trade-Off Studies:

For this sensor, there were few options during the selection process, with sensors that are already assembled being too high in price or not quite tailored to the requirements of the A-teChToP Suite project. Therefore, the following criteria were used to select a Light-to-Frequency sensor that could be implemented by the group to detect the amount of oxygen saturation within the blood of the wearer by measuring the light shone through a section of tissue and outputting data based upon the amount of light:

1. Sensor should measure the proper light for left over against the tissue of the wearer. This means that the preferred range of measurable light would include either Ultra-Violet light, Near-Infrared light or both. This range is 10nm to 400nm for UV rays and 100mm to 700mm for infrared light.
2. The sensor should operate within the range of 1.8V to 5V at least in order to remain consistent with other sensors.
3. Sensor should not require an excessive amount of external equipment in order to accurately convert light to a linearly equivalent frequency.
4. Sensor should be small and easy to implement into the PCB board for the central unit, no larger than 5mm x 4mm.
5. Sensor should be compatible with Arduino IDE and have existing libraries available for use.
6. A tutorial on how to implement this sensor into a pulse oximeter is necessary for simplicity.

Potential Sensors (Properties):

TSL235R Light-to-Frequency Sensor:

• 2.7V and above voltage operation range (includes desired 5V)
• 4mm x 4mm large, within expected size range
• $2.95 from SparkFun
• Tutorial: https://www.youtube.com/watch?v=GdN5IRVJOXI
• Datasheet: https://www.sparkfun.com/datasheets/Sensors/Imaging/TSL235R-LF.pdf

Final Decision:

The decision for this sensor is straight forward considering that there are no reasonable commercially available pulse oximeters for the group to purchase. Therefore, a custom made pulse oximeter is required to be made using the TSL235R Light-to-Frequency sensor. This particular sensor is available at a very reasonable price and has existing tutorials for how to construct the actual oximeter sensor. This will save the project group valuable time and funding as well as create room for customization.

Electrocardiogram (ECG)

Trade-Off Studies:

The selection process for this sensor did not require any comparisons between devices since the ECG of the child wearing the sensor suite will be obtained by a custom made filtering circuit receiving signals from specially positioned electrodes. These electrodes, in order to reduce visibility, will be placed in the inner, uppermost part of the child’s left arm. Two electrodes will be used for the arm placement after being tested for the most optimal positions. Position testing is necessary in order to reduce the external noise from the surrounding muscles and only measure the desired ECG signal. However, the customized filtering circuit will be designed with both a high pass filter and a low pass filter in order to remove large electromyography signals and small internal noise respectively. These circuit design specifications defined the criteria for the ECG Trade-Off Studies:

1. ECG circuit should be capable of providing a clear ECG signal, with discernable characteristics for the P-wave, T-wave, and QRS complex.
2. The circuit should be compact on the custom PCB board, taking up no more than 20mm x 20mm of surface area.
3. The leads for the electrodes of the sensor should be thin and easily concealed, along with the electrodes themselves.

Final Decision:

The ECG circuit design that will be used for the A-TeChToP Sensor Suite can be found at the following link:

https://www.youtube.com/watch?v=Uj9OXNg_p78

This design will be used as a reference for the group’s final implementation of the ECG sensor. Once ideal electrode positions that can be used for the standard of the device are found, the capacitor and resistor values necessary for the particular filters will be chosen. The overall cost of this sensor along with its circuit does not seem like it should exceed $25.00.

Seizure Watch

Rose Leidenfrost (Electronics Engineer)

Accelerometer

The ADXL345 from Analog devices is the accelerometer of choice for our watch system due primarily to its versatility. The sensor may be interfaced as either SPI or I2C with the MCU which allows for flexibility with implementation and testing. This accelerometer also satisfies applicable level 2 requirements regarding measurement range, resolution, and sampling frequency. The size of the ADXL345 is also small enough to be included in the wrist worn device with ease. Although it is the most costly of the accelerometers considered it is still within budget and worth the extra cost.

Features –

        SPI/I2C Interface options

        ±2g, ±4g, ±8g, ±16g Measurement range options

        10 bit Resolution

Electrodermal Activity (EDA)

Battery

Clock