Home Automation with Standard Modules
The key requirement for wiring X-10 modules into your home is that they always have a connection between the hot and neutral power lines. Figure 17, for example, shows how you connect a basic dimming receiver to a light. The receiver connects in series with the light, not across the power line in parallel, and should be in the hot line, not the neutral. If you connect it in parallel, you’ll blow the receiver the first time you turn it on.
FIGURE 17: Basic receiver/lamp circuit
The receiver draws operating power from the circuit through the lamp. The light has a much lower resistance when it’s off than when it’s on, typically by a factor of 10 or more, so as the equivalent circuit of Figure 18 shows, there’s less voltage drop across the receiver when on. The X-10 signal drops proportionally, so there’s a smaller X-10 signal available to the receiver when the light is on. The effect is worse for small-wattage lamps — night-light bulbs have such a high on resistance that reliable X-10 control with a two-wire dimmer is unlikely. This effect causes Off commands to be less reliable when the X-10 signal strength is marginal, a condition you may be able to correct with an amplified repeater.
When you must control a small-wattage lamp, or if the system delivers marginal signal strength, the reduction of the X-10 signal to receivers in the on state can account for units you can turn on but not turn off reliably. You have to boost the received signal to solve that problem, or convert the installation to use a three-wire dimmer (Figure 19; don’t confuse a three-wire dimmer with a three-way dimmer). Connecting the three-wire receiver directly across the hot and neutral wires lets it receive the X-10 signal unaffected by the on or off state of the lamp.
FIGURE 18: Equivalent circuit — light on
FIGURE 19: Three-wire dimmer/lamp circuit
Perhaps the most confusing X-10 installation topic is the requirements for wiring X-10 dimmers into a three-way circuit — that is, a circuit with two switches controlling a single light. Figure 20 shows the one way to wire conventional switches into a three-way circuit. The light will be on if both switches are up or are down, and off otherwise.
FIGURE 20: Simple three-way circuit
Unfortunately, the circuit in Figure 20 will not work if you simply replace one of the threeway switches with an X-10 dimmer, because for the wrong setting of the second three-way switch, there’s no complete circuit through the light, and the dimmer gets no power. Because it gets no power, it never sees the signal to turn on. For that reason, X-10 three-way circuits are entirely different than the mechanical equivalent of Figure 20. An X-10 dimmer must connect directly to the light at all times, so the fundamental circuit has to be the same as the standard dimmer circuit of Figure 1-17. The second switch no longer controls the primary circuit; instead, it signals the dimmer. Figure 21 shows the idea — the second switch drives a control input to the dimmer, telling it to turn on or off by a pulse on the input. The light is always under control of the dimmer, and never directly under control of the slave.
FIGURE 21: Basic three-way X-10 circuit
If you imagine the two wires between the switches in Figure 20 as corresponding to the two wires in the gap between the slave and the dimmer in Figure 21, you can see how you might wire the dimmer/slave pair in practice. There are a lot of different ways to wire three-way circuits, however; for more detail and explanation, and for how to extend the idea to one-way circuits.
Of course, all of that is the hard way. If you simply install X-10 switches that output signals to the power line, you can then use your PC to signal any number of devices to turn on, off, or dim, regardless of the actual connections (or lack thereof ) between the switches and anything else. No X-10 design problem has only one answer.