Recently I wanted to build a USB controlled power strip that powers some peripherals as soon as my computer starts up. Luckily, I managed to find a commercial USB-Power On switch in my attic (see the lead image) maybe I got it as a gift from someone a few years ago, cannot remember right.
That is over but later I realized that it would be better to build a custom usb power box myself for certain reasons that I do not want to share here. So here is another automatic usb main switch design that demands only a few parts from your treasure box!
It is noticed that almost all available usb power strips are quite simple as they simply use the +5VDC from the usb port to drive a 5V electromagnetic relay, and that setup is more than adequate for the task. In fact, there is no need to develop a relay circuit because there already exists a number of dirt-cheap Chinese 5V relay modules.
But a slightly better design is presented here. See its simple and self-explanatory schematic below.
This circuit is pretty elementary as it only contains a few jellybean parts. As you can see, it uses an RC filter to turn the relay on with a little delay. The transistor delay circuit may be helpful to learn some electronics fundamentals.
Note, any 5VDC relay can be used here as long as its coil needs less than 100mA current (recommended). But be aware that using a relay to drive inductive loads may lead to accelerated aging of the relay. So, do some searches for hints on reducing this type of problem.
After I was done making a breadboard version for a quick test (see below images), I made a second version on a small perfboard. Then the whole contraption was put into a ready-made power strip and fixed it under my workbench.
That is all for now. Well, now simply plug the USB cable of your own automatic USB main switch to your PC and let it control the outlets from there, cut and supply power without your routine intervention. Have fun!
