PROMPT - Project for Recording, Observing, and Measuring Parachute Tube

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Since this board needs to monitor 2 remote temperature sensors, choosing a relatively low-cost and compact temperature sensing solution is important. Thermistors are immediately out of the running since the motor's temperature can reach up to 212C, far beyond the range of most NTCs. So, a choice is left between RTDs and Thermocouples. While RTDs would be more accurate/precise, they are more expensive and available in fewer packages than a thermocouple would be. Based on this, thermocouples seem to be the direction the team will go in. To compensate for the lower precision, a J-type or similar thermocouple can be utilized instead of the much more common K-type thermocouple.

The temperature sensing inside the parachute bay will be accomplished using a temperature sensor screwed directly into the bulkhead between the avionics and payload bays. For the booster, the temperature sensor will be on a set of breakaway wires through both bulkheads. The most likely approach that will be taken to implement this is to put a barrel jack on the bulkhead of the booster, which is then connected to with a barrel plug. This will provide the greatest chance of a proper pull-out, but may not provide a reliable connection and may introduce errant resistance, skewing measurements. 

Pressure Sensing:

The pressure inside a standard parachute bay should be between 8 and 15 PSI when the parachutes deploy. In case that value is incorrect, a significantly overrated sensor was chosen. The selected sensor also has considerations for the attachment of a measuring tube, to allow the sensor to be situated away from the bulkhead, therefore reducing wiring complexity. This measurement tube will be run through the bulkhead and to PROMPT wherever it is mounted.

Remote Connection:

PROMPT will be connected to the backplane through the standard RS485 interface provided on the sensor board. This interface also provides a 5-volt supply for PROMPT. In the event that an RS485 connection cannot be established, PROMPT will log its data locally.

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Component Selection:

0-60PSI pressure: ABP2LANT060PG2A3XX

• Used by Sparkfun
• i2C interface
• Not on JLCPCB

Thermocouple Amplifier: MAX31855 

• Used by Adafruit
• 3-Wire SPI interface
• Cannot be used with grounded thermocouples
• JLCPCB: C52028

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• Unknown, probably STM32

Memory: W25Q128JVSIQ

• Same as most other devices on the backplane
• JLCPCB: C97521

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