Maker Sebastian Harnisch has completed an impressively-professional programmable resistor, which he calls a “glorified decade resistance field” — in search of all of the world like a industrial piece of precision lab tools.
“In 2021/2022 I designed a DC digital load that might be extra succesful, but additionally rather more advanced than the same old DIY options,” Harnisch explains of the challenge’s origins. “Nonetheless, after constructing a working breadboard prototype of the analog circuitry with 12 ICs together with a number of precision and twin opamps, I believed that it could be higher to begin with a smaller challenge that might enable me to achieve loads of expertise and write a lot of the non-application particular code that I might use in a while for the digital half. And that is how I began engaged on a programmable decade resistor, a reasonably specialised instrument for area of interest purposes.”
This unimaginable home-brew piece of lab tools presents a high-precision programmable resistance. (📷: Sebastian Harnisch)
A decade field is, in impact, a extra exact model of a varistor: somewhat than turning a knob to regulate its resistance between two values, you possibly can program it to precisely what you want. Sometimes, that is very a lot a guide affair involving bodily contact switches or plug wires — however the Harnisch Programmable Precision Resistor sits on the desk and supplies not solely a precisely-controllable resistance however a spread of different options beside.
“The Programmable Decade Resistor consists of three principal practical blocks,” Harnisch explains. “Energy provide (Energy Provide Board). Programmable many years, management and driver circuits (Mainboard). Consumer Interface (Consumer Interface Board). The mainboard’s principal controller incorporates the enterprise logic to regulate the relays, learn the exterior inputs and run the USB and person interfaces. The principle controller communicates with the Consumer Interface Board over I2C which handles the multiplexing of the alphanumeric LED show, scans the swap matrix, decodes the alerts of the rotary encoder and drives the buzzer.”
The mainboard is pushed by an STMicro STM32G441KBT6 microcontroller, with an EEPROM linked over I2C for information storage. There are a complete of 39 non-latching relays, managed utilizing three low-voltage constant-current LED drivers appearing as a 48-bit shift register. “Admittedly,” Harnisch notes, “the selection of a continuing present LED driver as a relay driver is a bit odd. Nonetheless, the working situations of the half enable for it and the half was chosen to drive the LED show on the UI Board.”
The mainboard makes use of a sequence of relays to attach a community of resistors. (📷: Sebastian Harnisch)
Elsewhere within the machine are two temperature sensors, used for calibration, two exterior earth-referenced inputs which might act as digital inputs, set off inputs, or inhibit alerts, and, in fact, a USB connection — exposing the gadget to a number machine over a Commonplace Instructions for Programmable Devices (SCPI) interface. A entrance panel supplies extra native management, with a glowing alphanumeric show for rapid suggestions on settings.
This challenge is not Harnisch’s solely brush with the SCPI interface: again in April he confirmed off a desktop thermometer which offered SCPI management over USB, taking readings from a Texas Devices TMP117M sensor and printing them to a pair of Hewlett-Packard HPDL-1414 alphanumeric GaAsP-LED shows.
The challenge is detailed in full on Harnisch’s weblog, throughout a sequence of posts.
