Lights

This is probably the easiest useful application of home automation. The quick way to do this is to use a single TTL bit and use this to directly drive a Solid state relay. These devices cost about $15 and make for very simple circuits.

• Faster - Solid state switching is to the nearest cycle, relay bounce and speed of actuator make relays slower.
• Longer life - Carlo Kopp thinks that SSRs only last 100000hours or less, but David Morgan counters that relays tended to wear faster than this(only time will tell :-)
• Better noise immunity - SSRs switch only at 0V points, which means that a) they switch inductive loads with only small back-EMF; b) they don't bounce like relays tend to do - increasing the life of power supplies of compact fluoros and electronic devices.
• No need for external supply - mechanical relays need a chunky power supply and some decent transistors to operate. SSRs do this automagically. Of course, I just steal 12V from the computer's power supply.

A cheaper circuit uses mechanical relays driven by ULN 2803 drivers to get the same switching circuit. This has a few advantages:

• Cheaper - mechanical 250V@1A relays cost $3
• Better supply - Solid state relays actually put a small voltage drop in the circuit(0.8V I found), where as a relay has a resistance <0.0001 Ohms.
• Better isolation - according to Carlo, solid-state relays have a weaker voltage isolation than mechanical varieties of a similar cost.

I've tended to use relays in small inductive, or larger resistive loads suchs as solenoids, gas valves and lights. I use SS-Relays for controling motors like fans and pumps, because of their better handling of inductive loads.

An even more advanced circuit, which is still at the testing and design stage consists of a Triac switch with variable on time for dimming purposes. This is quite difficult, as the std. variable on time dimmer circuits use a variable resistance, rather than a variable voltage or digital input. This circuit may cost more than a SSR(but it will dim... :-).

The basic design will use a triac switched by something like the MOC3021. the tricky bit is converting a digital signal into a phase angle. Two posibilities I can think of: use the standard light dimmer circuit with a digital (tricky - isolate the pot from the 1-wire network) or do the phase calculation on the isolated side and use some kind of programmable delay to trigger the triac (probably easier).

For halogen bulbs I have a rather simple, yet cute circuit using a 2N3055 and a rectified and filtered 12.8V supply. This already does dim, but it is not as efficient, or high powered as the triac based system.