Controlling simple relay switch via USB

Question

I'm looking to control a mains powered light from a simple relay switch connected via USB to the computer.

The relay switch isn't even a USB device, it's just a simple switch that requires the USB voltage to turn it on. When the voltage drops below a threshold, the switch will turn the light off.

My problem is that I can't control the power output of a USB port. I'm happy to do it using any language on Windows or Linux (but preferably Java because I'm used to it).

Answer 1

Unfortunately, in most cases you cannot control the power supply to the USB port. The power supply is usually hardwired through, and not switchable in software. You can send a reset to a USB device, but that won't work in your case.

There are a number of projects on instructables that do similar to what you describe, but unfortunately they seem to either be quite complicated or require expensive parts.

EDIT: There is actually a product currently in the news which would do want you want, but it doesn't appear to be shipping yet: http://www.pwrusb.com/

EDIT (again): Apparently you can do this with some usb hubs. This post sugggests the Linksys USB2HUB4 is one that works.

EDIT (and again): Apparently there are a number of similar questions, but there don't seem to be any more useful answers:

• https://stackoverflow.com/questions/405269/custom-usb-device-that-disables-power-to-usb-devices-plugged-into-it
• Power off an USB device in software on Windows
• Is there software or code to alter USB power output
• Can I write a program that swiches USB on/off

Answer 2

Most of the USB to Serial or USB to RS232 $10 converters support hardware handshaking. Use one of those as a single channel digital io.

Connect your transistor that will drive the relay to DTR on the converter board and command DTR ON/OFF with the converters driver.

A 2N7002 is a good transistor to use (FET actually) since it will work from 3V and doesnt need any resistors anywhere.