Sebastian Harnisch's Desktop Thermometer Offers a Smart SCPI Interface, Cheery Glowing Display

Maker Sebastian Harnisch has built a desktop thermometer with a retro style, using a high-precision sensor and providing a Standard Commands for Programmable Instruments (SCPI) interface over USB as well as a glowing digital readout on its face.

"For a long time I wanted to build an SCPI capable thermometer to keep track of the ambient temperature in my small lab," Harnisch explains. "The list of requirements was fairly short: reasonably high precision ([a] sensor with <= 0.3°C max. uncertainty); power and SCPI communication over USB; [a] compact enclosure with a display."

Tackling the sensor first, Harnisch settled on the Texas Instruments TMP117M sensor — a device which boasts a ±0.1°C maximum error between 25°C and 50°C (77-122°F) and a typical accuracy of ±0.05°C across comfortable room temperatures. The only problem: it's provided in a compact WSON6 package measuring just 2×2mm (around 0.08×0.08"). "As it later turned out," Harnisch notes, "soldering the IC [Integrated Circuit] using a hot air soldering station was a bit more time consuming than e.g. a typical SOIC package (preparation), but not that difficult."

Elsewhere on the build's custom circuit board is an STMicroelectronics STM32G441 microcontroller running the FreeRTOS real-time operating system, providing USB peripheral support and the SCPI interface — allowing the sensor to be queried and controlled using a standard protocol over its USB connection. "For the SCPI interface I studied the SCPI specification (1999)," Harnisch explains, "and took some inspiration from SCPI implementations found in DMMs [Digital Multimeters] (Agilent 34401A, Keithley DMM7500)."

"I couldn't resist using two 'vintage' [Hewlett-Packard] HPDL-1414 alphanumeric displays, mounted on their own vertically mounted PCB," Harnisch adds. "The deep red glow of the GaAsP-LEDs always puts a smile on my face — pictures often don’t do it justice. The issue with these though is power dissipation. If it wasn't already necessary to position the temperature sensor far away from the electronics because of the fairly powerful microcontroller, it is now completely unavoidable. Therefore I decided to design a separate tiny PCB for the sensor in order to connect it to the main PCB via a cable."

The main PCB and its display daughterboard are housed in a smart metal chassis with a single push-button for switching between immediate, minimum, maximum, and average display modes, as well as showing current trigger settings. A full-size USB connector sits at the rear for SCPI and power, with the sensor connecting on a flying lead to make sure it isn't affected by the surprisingly high heat output of the main unit's display.

"In the meantime I built a second unit," Harnisch concludes. "When bringing the both sensors in close proximity (not thermally coupled), the sensors agree reasonably well – the values differ by less than 0.07°C. Nice!"

The full project write-up is available on Harnisch's website, though the source code and design files have not been publicly released.