The other day I bought a 5v USB relay from eBay for a project I’m working on. The ultimate goal is to use it on a Raspberry Pi in a build where the GPIO pins are already being used by a touch screen. For just shy of $5 AUD with free shipping, it was a good buy I reckon.
The eBay listing says it comes with a Windows DLL for controlling it, but it never did (just the device in one of those silvery electrostatic bags and no download links in the listing). Plus the listing seemed to suggest the DLL was closed source and who needs that kind of negativity in their life? So I looked around to see if anyone else had worked with this.
Turns out they’re a fairly common device manufactured by “www.dcttech.com” and several people have written libraries that control these. The relay is basically a HID device. You send a feature report along with the relay you wish to activate and the state, plus some padding bytes, and you can control it without some unknown DLL file.
And turns out there’s an excellent cross-platform node.js library called node-hid. Give it the path or VID / PID of your device and you can read, write, set feature reports, get feature reports, and so on. Brilliant!
But no matter what I tried, I couldn’t get the damn thing to operate. In the end I “brute forced” it by writing a for loop that incremented some of the bytes until the relay clicked on. Slowing it down with a setInterval told me what bytes to send to turn them on and off. To turn it on, send 0xFE (254) to Report 0. To turn it off, send 0xFC (252) to Report 0. The 8th byte of report 0 when you request the feature report is 0 for off, and 3 for on. In other words, zero = off, nonzero = on
So here’s a short demo node.js app that will find the first
USBRelay device and toggle it every second.
Note that this has only been tested on the one-relay board. However byte 3 when sending should be the relay number you wish to control