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TeraHz developer's guide
This document explains how TeraHz works. It's a good starting point for developers and an interesting read for the curious.
Hardware
TeraHz was developed on and for the Raspberry Pi 3 Model B+. Compatibility with
other Raspberries can probably be achieved by tweaking the device paths in the
app.py file, but isn't confirmed at this point. Theoretically, 3 Model B and
Zero W should work out of the box, but models without Wi-Fi will need an
external Wi-Fi adapter if Wi-Fi functionality is desired.
TeraHz depends on three separate sensor boards:
- AS7265x spectral chipset
- VEML6075 UV sensor
- APDS-9301 lux-meter
The sensors leech power from the GPIO connector, thus eliminating the need for a separate power supply. The necessary power for the whole system is delivered through the Raspberry's USB port. This also allows for considerable versatility, as it enables the resulting device to be either wall-powered or battery-powered. In a portable configuration, I used a one-cell power bank, which allowed for about 45 minutes of continuous operation.
AS7265x chipset
This chipset supports either I2C or UART. Because transferring large amounts of data over I2C is rather cumbersome, TeraHz uses AS7265x in UART mode.
This chipset consists of three rather small surface-mounted chips and requires an EEPROM. To lower the complexity of assembly for the end-user, I recommend using a breakout board.
The serial UART connection operates at 115200 baud, which seems to be the standard for most recent embedded peripherals. As with most serial hardware, the TxD and RxD lines must be crossed over when connecting to the processor.
Communication with the sensor is simple and clear through AT commands. There's
a lot of them, all documented inside the datasheet, but the most important one
is ATGETCDATA, which returns the calibrated spectral data from the sensors.
The data is returned in the form of a comma-separated list of floating point values, ending with a newline. The order is alphabetical, which is different from wavelength order. See the datasheet for more information.
VEML6075
This chip communicates through I2C and provides TeraHz with UVA and UVB irradiance readings. It's not an ideal chip for this task, as it's been marked End-of-Life by Vishay and it'll have to be replaced with a better one in future hardware versions of TeraHz.
The chip resides at the I2C address 0x10. There's not a lot of communication
required: at initialization, the integration time has to be set and after that,
the sensor is ready to go.
16-bit UV values lie in two two-byte registers, 0x07 for UVA and 0x09 for
UVB. For correct result conversion, there are also two correction registers,
UVCOMP_1 and 2, located at 0x0A and 0x0B, respectively.
To convert these four values into irradiances, they must be multiplied by certain constants, somewhat loosely defined in the sensor datasheet. Keep in mind that the way of computing the "irradiance" is very much experimentally derived, and even Vishay's tech support doesn't know how exactly to calculate the irradiance.
APDS-9301
Datasheet Buy breakout board This chip measures illuminance in luxes and like the VEML6075, connects through I2C. Unlike the VEML6075, this chip is very good at its job, providing accurate and fast results without undefined mathematics or required calibration.
At power-on, it needs to be enabled and the sensor gain set to the high setting, as the formula for Lux calculation is only defined for that setting. This initialization is handled by the sensors module.
The lux reading is derived from two channels, descriptively called CH0 and CH1,
residing in respective 16-bit registers at addresses 0xAC and 0xAE. After a
successful read of both data registers, the lux value can be derived using the
formula in the sensor's datasheet.