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Stackable Raspberry Pi HAT provides up to 64 Resistance Temperature Detectors

Jul 28, 2020 — by Eric Brown — 2493 views

Sequent Microsystems is Kickstartering a stackable, $30 and up “Mega-RTD” HAT that supports up to 64 Resistance Temperature Detector (RTD) channels for precise temperature measurements.

Resistance Temperature Detector (RTD) sensors, which are used for highly precise temperature measurements in industrial and laboratory systems, can be found on embedded systems such as Advantech’s WISE-710 IoT gateway and NI’s CompactDAQ controller. Now Sequent Microsystems has won Kickstarter funding for a Mega-RTD HAT that can be stacked to provide the Raspberry Pi with up to 64 RTD channels. Shipments are due in October.

Mega-RTD (8-channel) with (left) and without Raspberry Pi
(click images to enlarge)

RTD sensors include resistors with resistance values that vary as the temperature changes, enabling accuracy within within 0.1°C to 0.2°C. The Mega-RTD offers “up to” 0.1°C sensitivity at prices far below those of dedicated RTD systems, says Sequent, which has found Kickstarter success with industrial-minded Raspberry Pi HATs such as the recent 4-Relay.

The 65 x 56mm Mega-RTD is available in 4-channel ($30) or 8-channel $40 versions, with one or two 24-bit Delta-Sigma ADS1248 ADCs, respectively, from Texas Instruments. Each ADC can measure temperatures from four 3-wire RTD-100 thermistors by sending a 1mA current. All RTD channels can be read individually or simultaneously.

The HAT is controlled with a 32-bit STM32F030 MCU that receives commands from the Raspberry Pi via I2C. The HAT also provides a watchdog that will perform a 10-second power cycle and reboot the Raspberry Pi if it locks up.

Dual stacked 8-channel Mega-RTD HATs (left) and block diagram
(click images to enlarge)

If you don’t want to add your own RTD sensors, packages are available for $70 (4-ch.) or $120 (8-ch.) with integrated sensors for each channel with a 0 to 400℃ range. The sensors connect via pluggable connectors and bundled 2-meter wires. The HATs can be stacked eight high to enable up to 64 channels. Kickstarter packages without RTD sensors are provided with 16 channels ($80), 32 channels ($160), and 64 channels ($320).

The HAT uses an 820 Ohms current generator, which limits the measurements to 0.1%. However, field calibration is possible by integrating a precision 100 ohms resistor, thereby enabling measurements as granular as 0.01%.

3D printed enclosure (left) and NodeRed examples screen
(click images to enlarge)

A GitHub page for the Mega-RTD provides command line code, Python drivers, and a NodeRed data acquisition example. There are also 3D printing files for building an enclosure.

Further information

The Mega-RTD is available on Kickstarter through Aug. 27 in packages starting at $30 with worldwide shipments due in October. More information may be found on Sequent Microsystems’ Mega-RTD Kickstarter page.

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