I would like to know if it is possible to convert a USB signal to a RCA phone plug signal in order to close a switch contact. There are pushbutton switches that can remotely close a switch by means of a cable with a standard 3.5 mm plug on the end. These are attached to specialized switch boxes for adaptive devices for people with disabilities. Instead of having a physical pushbutton as the component sending the signal to the switch box, I would like to have the signal originate from a software program and be conveyed through USB or RS-232 and then be converted to the appropriate signal for closing a switch. This is a specialized need and I am not an engineer, so it's kind of hard to explain what I am needing. I do not really know how to look for such a solution. I appreciate any suggestions.
There are devices available such as this one (also available ready-built) that will do what you want. That might be bigger and more cumbersome than what you're looking for, however.
In reply to comment: Not much simpler, for USB, although you may find something smaller, with fewer channels - just search for 'USB Relay'. This looks the most promising route.
An alternative approach, if you do want something simpler,
then you may have to step outside of Windows - you don't say anything more specific than 'software' but I'm assuming you mean Windows or anyway a modern OS - but in the days of DOS, you had more direct access to the hardware, so could for instance use one of the RS232 control signals (DTR etc) plus a diode and a small relay. But AFAIK this isn't available under Windows. In fact - ref comments - this is still available under Windows too.
One way to handle this is to use an FTDI chip, such as the FT232R, which connects via USB to a virtual COM port, and on the other side of the IC presents a full set of RS-232 leads, at TTL levels, including TX/RX, DTR, RTS, CTS and DSR. It is available in a 28-pin SSOP package for $4.50 in single quantities.
You can then use the DTR lead to control a transistor or MOSFET to drive your relay. There are literally dozens of answers here describing how to do that so I won't repeat it here. The DTR lead can be easily manipulated using Windows software (we're doing it right now in one of our test boards to reset a processor using C#.)
There are a number of ways to approach the problem. The two I'd suggest you looking into first are phidgets, which should have a solution for you.
Another choice is an arduino with a relay shield.
There are certainly cheaper and more elegant ways to tackle this, but from the tone of your question I'm guessing you need something that is easy to use and has a large community to support questions you have as you progress. You'll find both in the above two options.