Bob Harbour Brings Modern Usability to a 1970s Electronic Load Box with an SCPI-Inspired Interface

Electrical engineer Bob Harbour has put a bit of modern usability spin on a vintage piece of test equipment, adding computer control and remote monitoring capabilities to a 1970s electronic load box.

"An electronic load box is a tool for working on power supplies. It provides a stable, adjustable load, turning power into heat," Harbour explains. "I picked up an old Transistor Devices DLR50-15-150 at a hamfest back in October for a very reasonable price. This is a completely analog instrument that can provide a constant current sink, a constant resistance and sort of a battery simulator. All of these configurations are adjusted with a knob on the front panel."

For the 1970s, front-panel knobs and analog dials were perfectly acceptable as a user interface. Modern equivalents, though, offer remote configuration and monitoring capabilities missing from vintage gear — but rather than throw it out and buy a modern replacement, Harbour decided to give the gadget a makeover while retaining as much of the original device's innards as possible.

"A [Microchip] SAMD21E18 MCU [Microcontroller Unit] on a board of my own design was used to control this project," Harbour explains. "A 16 character [by] two line LCD display is interfaced through the GPIO [General-Purpose Input/Output] pins. The user interface board is also interfaced to the CPU via an I/O expander chip to the I2C bus.

"Isolating the user interface board is done with an Analog Devices ADM3260 I2C isolator chip. Control voltage for the load box is controlled by a 16-bit DAC [Digital to Analog Converter] whose output voltage is boosted with an op amp to the required 0 to 6V range."

The new hardware includes temperature probes and a USB to UART bridge for controlling the hardware from a remote system, using an interface inspired by the Standard Commands for Programmable Instruments (SCPI) protocol. The new boards themselves are housed in a secondary box which sits atop the original chassis, in order to preserve the load box's original guts.

The full project write-up is available on Harbour's Hackaday.io page, along with schematics for the custom hardware.