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Asked: 2026-05-17T14:55:29+00:00

I am working on a personal project involving sending simple signals from my computer

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I am working on a personal project involving sending simple signals from my computer to a circuit via USB. Basically I am using the USB signal as the gate signal for a MOSFET which will in turn activate a relay to turn on/off various AC peripherals. For example if I want to turn on a light bulb for 5 seconds every minute I would be sending a 1 down the first wire for the first 5 seconds of every minute.

This is my problem: I have no idea how to manually send a 0/1 down a specific wire on a USB cable, or even interact with a USB port at all 🙁

So I guess there are multiple parts to this question, is it possible to interact directly with the bits being sent via a USB port? If so how would I do this? I am familiar with C++ and C#, so I really hope that you can do it in one of those…

Thanks!

edit Hmm so it looks like the USB port actually only has one 5V pin so direct USB interaction wont work. Going to take a look at a parallel adapter and get back on it.

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1 Answer

  1. Editorial Team

    USB is a bad fit for anything that doesn’t have a USB interface at the other end of the wire. If you don’t want to get into building your own USB device, I’d suggest buying a USB to serial adapter, which gives you two directly-controllable output lines (the flow control lines), or a USB to parallel adapter, which gives you more than 8 lines.

    Chris Johnson’s answer has a link to instructions for Windows serial port programming. You’ll want to look at section 7 — the SETDTR, CLRDTR, SETRTS, and CLRRTS are your flow control line toggles (for the DTR and RTS lines, respectively).

    As far as hardware goes, a “1” (SET) value on a flow control line is +3 to +15 volts on the line, and a “0” is -3 to -15. Actual voltages can vary between devices; measure it to be sure. (EDITED; I got the 1 and 0 mixed up. The control lines use the opposite convention from the data lines.)

    Here are Wikipedia pages for voltage characteristics and pinouts.

    EDIT: Having done some more research on USB-to-parallel adapters, I don’t think they will give the needed level of control. For best results, you’ll need a PCI or PCMCIA parallel card, or a parallel port built into the motherboard.

    I’m not a Windows programmer, but this library might be useful for controlling the parallel port’s lines from Windows.

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