Sequent Microsystems like to make stackable Raspberry Pi HATs. After their stackable 4-relay board allowing for up to 32 relays controlled by a Raspberry Pi board, the company has now launched MEGA-RTD 8-channel RTD Raspberry Pi HAT enabling up to 64 resistance temperature detectors via 8x MEGA RTD board stacked on top of a Raspberry Pi board.
If you haven’t heard about this type of temperature sensors, we wrote a post explaining the advantages and drawbacks of resistance temperature detectors last year. Basically, those temperature sensors contain a resistor that changes resistance value as their temperature changes, and support much wider temperature ranges than traditional temperature sensors such as DHT22 while keeping a <0.1°C accuracy. The price of the sensors themselves is similar to traditional sensors, but the total price is more expensive since they require extra hardware to measure the resistance/voltage value.
MEGA-RTD key features and specifications:
- MCU – STMicro STM32F030 Arm Cortex-M0 microcontroller handling I2C commands from Raspberry Pi SBC
- Sensors Inputs – 8x terminal blocks for 3-wire RTD-100 thermistors
- A/D converters – 2x Texas Instruments ADS1248 24-bit Delta-Sigma A/D converters good for up to 0.01 °C accuracy
- Power Supply – Via DC jack; separate 3.3V power supplies for analog and digital components to increase accuracy
- Dimensions – 65 x 56mm (Raspberry Pi HAT)
Sequent explains how it works in a bit more details:
The A/D converter measures the temperature by sending a 1mA current through the RTD PT100 thermistors and measuring the voltage. The current generator is using a 820 Ohms, 0.1% resistor. Thus, the accuracy of an un-calibrated device is limited to 0.1%.
To achieve better precision, we provide a software “calibrate” command. Field calibration up to 0.01% is possible using a 0.01% 100 ohms resistor.
Command-line tools and Python drivers will be available on Sequent Microsystems’ Github account. They also integrated the board with NodeRed using eight RTD thermistors to show the data in a neat web-based graphical dashboard.
There’s also a user guide but shared in an “interesting” way as you have to go through a checkout process to order the free e-book.
MEGA-RTD has recently launched on Kickstarter with a $1,000 funding target that has already been surpassed. Rewards start at $30 for a four-channel board, $40 for an 8-channel board, and go up to $320 for a kit with eight MEGA-RTD boards supporting 64 RTD thermistors. Shipping adds $5 to $15 depending on the selected rewards. Note you’ll need to add your own Raspberry Pi board, as well as purchase 3-wire RTD thermistors such as PT100 resistance temperature detectors available for $3 and up on Aliexpress. Rewards from the crowdfunding campaign are expected to ship in October 2020.