Now I realize that using this particular sensor is a bit impractical at $40 a piece; I certainly wouldn't have one if I hadn't procured it in the midst of a dumpster-dive, but I do believe that the idea does have merit, as clock radios do actually suck up a fair bit of power needlessly.
Panasonic NaPiOn PIR Sensor with a PIC12F629 Microcontroller
The NaPiOn sensor runs off +3.3V and outputs a logical "1" whenever it senses motion. Since the output only stays high for a second or two, we need something to keep the display on for a useful amount of time, and to provide a little bit of hysteresis to prevent rapid cycling, which would be annoying. The cheapest, easiest, smallest way to do this is with a microcontroller.
Could you do this with discrete logic? Sure. Could you do this with discrete logic for $1.30 and fit it into one square centimeter of PCB space? Not a chance. The use of a microcontroller also gives us the ability to change any parameter we want (time on, time off, etc) with a few lines of code.
When you consider that the sensor draws about 60uA in standby, and the PIC could probably be made to draw as little as 20uA if the firmware were optimized, it is plausible that the whole sensing circuit could end up drawing less than 200uA including the power supply.
Some rough, ball-park calculations:
13mA Per Segment * 7 Segments * 4 Digits = 364mA (13mA per diode is a rough guess)
200uA = 0.2mA (this one is right)
This would mean that you could theoretically save a fair bit of power (relatively speaking).
Take my clock radio as an example. It's a Sony Dream Machine, and it's power rating is 12W. When compared to your 4500W clothes dryer, 12W is not very much, however we have to remember that clock radios generally run twenty-four hours a day, 365.25 days a year. So in effect:
12W x 24h x 365.25 = 105.192kWh/Year
(12W = 0.012kW)
As you can see, 12W happening all the time really does add up to a substantial amount at the end of the year. When you consider that most homes have more than one clock radio, and that they are only being "looked at" maybe 2% of the time they're powered on for, the potential for savings over the lifetime of the appliance does exist.
Tomorrow morning I'm planning on hacking open a clock radio, patching this circuit and taking down some real numbers. Stay tuned...