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.

PetteriAimonen / relaymux