USB Relay Mux box

This repository contains hardware and firmware for a USB Relay Mux that has two independent halves, each of which multiplexes a pair of signals to four channels. It is designed for use in automatic testing, for example to switch input signals to multimeter or to change polarity of a power supply.

Schematic - BOM - Gerbers

Design

Many of the cheap USB relay boards are designed for switching large currents, and not for lab usage. They have several shortcomings:

• Poor quality relays, which have varying amount of resistance when switching small currents.
• Weird custom protocol with no feedback from device.
• Excessive current draw from USB bus when all relays are activated.

This project implements the standard SCPI command set and uses higher quality Panasonic TQ2SA-5V relays.

Specifications

• Switching time: 10 ms
• Maximum current and voltage:
  • up to 30 Vrms: AC 1 A or DC 2 A
  • up to 125 Vrms: AC or DC 0.5 A
• Parasitic capacitance: 40 pF each signal to GND
• Wetting current: 10 µA
• Contact resistance: 50 mohm typical, max 150 mohm
• Power supply: USB 5V 300mA
• Control: SCPI over USB CDC-ACM serial port

SCPI command set

• *IDN?: Query model name and serial number
• OPEN @(1,2,5:8): Open (disconnect) one or multiple relay channels
• CLOSE @(1,5): Close (connect) one or multiple relay channels
• CLOSE? (@1): Query whether a specific channel is closed
• CLOSE:STATE?: Return a list of all closed channels
• OPEN:ALL: Open all channels
• SET 255: Set channel states as 8-bit integer, break-before-make
• SET:MBB 255: Set channel states as 8-bit integer, make-before-break
• GET?: Get channel states as 8-bit integer

The commands follow "Signal Switchers" instrument category of SCPI 1999 standard.

All commands wait for relay switching, so consecutive instructions will follow correct make/break sequencing.

Parts

Part	Info	Approx. cost (without taxes)
PCB	120x100 mm, gerbers	8 EUR
Components	Listed in BOM	18 EUR
Binding posts	20 pcs, 5 mm mounting hole	10 EUR
Enclosure	Hammond 1550MBK	15 EUR
Total		51 EUR

The PCB is designed for mounting the binding posts directly with a nut to the 5 mm hole. How well this works depends on the specific connector model. If needed, the hole can be drilled larger without damage to the traces, and a soldering area is provided in case there is no electrical contact through the screw. Some solder is a good idea in any case to prevent it from the connector spinning in the hole. Cheapest binding posts can be found on eBay, but some of them lack a hole for a wire.

The PCB replaces the original lid of the enclosure. A small notch needs to be filed for the USB-C connector to fit through.

Firmware

STM32F042 firmware is based on STM32Cube USB-CDC library and scpi-parser library.

It can be built using PlatformIO:

pio run
...
RAM:   [=====     ]  52.5% (used 3224 bytes from 6144 bytes)
Flash: [========  ]  80.3% (used 26300 bytes from 32768 bytes)

The board can be programmed through USB DFU protocol using STM32 built-in bootloader. The bootloader is activated by holding down Clear button while plugging in the cable.

License

The electronics design is licensed under CC-BY-4.0. Suggested attribution format for derivative designs:

Based on USB Relay Mux by Petteri Aimonen, https://github.com/PetteriAimonen/relaymux/

The firmware is licensed under BSD 2-Clause license.