A 120V AC Plug Controlled by TTL
So getting a good TTL control signal out of ones computer is not all that difficult. Use the UBW or use a parallel port. It is trickier to make that 5v TTL signal control high power/high voltage systems such as a household plug. The voltage itself is not to much of a concern, but the current sure is. At 15+ amps for devices such as heaters, AC, and microwaves, control of such a plug will need to be carefully thought out.
This device is essentially a household 120VAC outlet with the hot wire switched through a TTL controlled solid state relay rated at 18 amps.
The Construction
The household outlet is wired as one expects: white (neutral) to silver screw, black (hot) to colored (gold in picture) screw, and green (ground) to the green screw. The neutral and ground wires go directly from the power cord to the device's outlet. The hot wire, however, is passed through a solid state relay. We used the Crydom EZ240D18 as it will switch on TTL levels and is good up 18 amps. The devices outlet wiring can be seen below. Note the wiring is all quite butch to allow for 15+ amps.
The relay and its wiring can be seen here. The relay will burn somewhere in the neighborhood of 15W at 18 amps. This is a real concern, if this heat is not dealt with it will burn up the relay. A heat sink is what is called for, but standard US electrical boxes do lend themselves to mounting large heat sinks. We tried mounting the relay to the standard galvanized cover plate that came with the electrical box. What we got was a metal plate that got very hot in the immrdiate area of contact with the relay, but cool elsewhere. This was not good, steel sucks as a heat sink. We machined a new cover plate out of 1/4 inch aluminum plate, which kept the temperatures under control (and more even).
The final box has a power cord for plugging into 120 AC and two leads for the TTL control signaling. The larger relay box has been made intentionally large to house other electronics. For instance, USB powered micro controllers can be housed here to provide sophisticated control functions.