I seem to be on a bit of a roll at the moment, just got a prototype working of a PSoC based USB 5 to 1 switching unit, this is using relays salvaged from old ebay HP/Agilent 44471A relay cards.
It's very simple, really just the PSoC and a couple of ULN2003's ... initial testing looks good, so I just need to repurpose the USBTMC code from the Yaugi to provide a USBTMC or USBUART interface.
I've used the same approach as the HP modules ... so the quad pole relay is paralleled to act as a dual pole, presumably this was to reduce contact resistance, but also to cope with isolation limits with pin proximity.
![DIY USB Switching Unit](https://www.eevblog.com/forum/index.php?action=dlattach;topic=218618.0;attach=873528;image)
The picture mostly hides the massive schoolboy error of not checking the footprint yet again and having too small holes for the relays, but luckily with a "just the right amount of flux" I managed to get them all soldered on ;-) ... clearly I'm not leaving school anytime soon!
I've just ordered an enclosure and front and rear (pcb) panels ... the intention is to have some led's and a couple of tactile switches on the front panel (for manual selection.) I was going to use Pomona binding posts, but that's £120 in posts which seems mad, so I'll just go for Cliff gold plated ones for now.
Slowly I'm getting to the point where I hope I can do something remotely metrologisty ... I now have three LTZ1000 references (in various states of completeness), a USB temp/pressure/humidity sensor (v2 coming soon), a USB switching unit, and a selection of USB GPIB interfaces .. all home made ... which goes a small way in offsetting how much I've spent on actual kit in the last year or so!