Enviro pHAT and High Altitude Balloon Missions

Enviro pHAT

The  Pimoroni  Enviro pHAT is an alternative sensor package to the Sense HAT for use on the Raspberry Pi.  It is smaller, lighter and less expensive than the Sense HAT.  It does not have the joystick inputs or LED display.  Its motion sensor is somewhat less capable.  It adds a color sensor that may not be useful unless mounted outside the payload or at a window.  It does add an A/D converter that would be extremely useful for adding sensors, including an external temperature sensor.  However some sensor capability is lacking  It includes:

  • BMP280 temperature/pressure sensor
  • TCS3472 light and RBG color sensor
  • Two LEDs for illumination
  • LSM303D accelerometer/magnetometer sensor
  • ADS1015 4-channel 3.3v, analog to digital sensor (ADC)

Its weight is 6.8g instead of the Sense HATS 20.4g, though neither weighs much.  It is also sized to fit on the Pi Zero.

The BMP280 is slightly less capable that on the Sense HAT.  It has a pressure range or 300 to 1100 hPa.  It should work to 30kft, instead of 34kft for the Sense HAT.  The temperature measurement has a slightly wider range with comparable accuracy as the pressure sensor on the Sense HAT.  But the Temperature from the humidity sensor on the Sense HAT is much more accurate.  The Enviro pHAT is missing a humidity sensor that might give information about cloud altitudes.

TheTCS3472 light sensor is probably best used qualitatively.  The uncertainty in its sensitivity is about ± 20%.  It has four color channels, RGB filtered, and Clear (nearly full spectrum).  It is IR filtered.  The sensor has a 16-Bit count range with full scale being near 1W/cm^2,   Pointing it at the sun may take it near saturation.  The lower airmass of a balloon flight may take it into saturation, at least for the Clear sensor, if pointed at the Sun.

The LSM303D is a 3D Accelerometer and 3D magnetometer.  It does not have the Gyroscopic rotation sensor that the Sensor HAT does.  Its acceleration scales range from ±2 to ±16g.  Earth normal would be 1g.  the Magnetometer scales from ±2 to ±16 gauss.  The Earth’s field varies around 0.6 E-5 gauss.  A payload’s rotation rate might be obtained from rapid, continuous measurements of the magnetic field.

The ADS1015 ADC open the possibility to add user measurements to the pHAT.  It is a 4 Channel 12-Bit sensor.  12 -Bits suggests about 1:4000 measurement, about 1 mV resolution.  For external temperature measurements, I have used an RTD in series with a high precision resistor.   Omega.com F3145 is an example.  A smaller sensor should respond more quickly to changes, and a higher resistance should dissipate less power.  These sensors should be good to -70C.  I usually choose a 1000 Ω RTD in series with a 1000 Ω resistor, giving a signal near 2.5V.  But a higher series resistor would dissipate less power at the expense of lower resolution.   The power used by the sensor can become an issue at lower pressures, where thermal convection is less able to dissipate the power.  This voltage divider circuit can be driven by the 5V supply.  a 1kΩ RTD with a 1kΩ series resistance should dissipate 2.5mW in the RTD.    Be aware that the series resistor also has a temperature coefficient.

I intend to use both the Sense HAT and the Enviro pHAT.  They have similar specifications.  But I want both the humidity sensor of the sense HAT to determine the cloud altitudes, and the ADC of the pHAT, to determine external temperatures, and maybe read a few photodiodes.

Update 4/5/2017

I came across the Explorer pHAT  which is more suited to complimenting the Sense HAT, by adding an ADC for about $12 from adafruit.com.  It has the ADS1015 12-Bit ADC as the Enviro pHAT above does.  I expect to order one of these to fly with the Sense HAT.  The Pimoroni.com description is here.

Now to add an external humidiy sensor …



© 2017 David B Snyder