From Catholicpenguin

Overview

Recently I've been experimenting with measuring the electricity use of my fridge via audio. By placing a laptop with a sound recording program near the fridge, and leaving it running all night, clearly observable duty cycle patterns can be observed and easily calculated.

Tonight I'm leaving it running to profile AC usage, and this got me thinking. With a cheap microcontroller hooked to a sensitive microphone, these two huge energy users of a typical home could easily be monitored.

Obviously, especially during the day, significant noise would need to be filtered out from noise generated during normal home use. Still, since their duty cycle is rather long, it would be fairly easy to isolate outliners, and a baseline profile for what on/off cycles should look like could easily be established by monitoring at night.

As a first generation product, I'm envisioning something that's battery powered, clips or is placed near either a window air conditioner or by the thermostat of a normal unit (this may not prove to capture satisfactory audio), and would read out performance characteristics of the unit, of particular note the duty cycle and the electric cost per day.

If made cheaply, and I think it could (<$10 in bulk), this type product could provide a significant niche product to the concerned green consumers out there looking to reduce their electric bill and environmental footprint.

The electricity consumed by the unit would need to be calibrated, but this could be done easily for users with old-style electric meters by having them time the number of seconds the meter takes to spin without the AC on, and with it. With these two simple pieces of data, would could be programmed over a USB port when connected to the computer, and the electric rate of their utility, the power per day could be very easily calculated.

Future additions for a ver 2.0 product would include ZigBee wireless additions and a temperature sensor, allowing the unit to communicate with others and a central computer for data logging, notification of low battery, etc.

Value added services that could be included would be a report of the average temperature of the room if placed well, and advisement on energy policy. Also, the duty cycle could be monitored, and when increased beyond a certain point, the unit could notify the user that the air filter should be changed, for example.

Technical

Components wise, a small microcontroller like the ATTiny25, with a A/D converter and built in differential amplifier would work well for the first version. This could be coupled to a 16x2 LCD display for output, although this would likely consume too much power, even without the backlight, or a low power 7 segment LCD, tho this would require expensive custom LCDs or some external labeling. The sound measurement could be provided with any cheap electric condenser mic, of the kind used in the cheap digital voice recorders or tape players. The built in 20x differential ADC should be just fine for this sort of application, and it's fast enough that small samples of actual audio can be recorded and analyzed.

The chip has on board temperature sensor, however, it requires chip specific calibration to be accurate, and even at its very best, is only accurate to within +- 1C. The datasheet only rates +- 10C. This would probably be enough for average temperature advisement, but certainly not enough for anything more accurate. On the other hand, the Dallas 1-Wire DS1820 sensor is super accurate, and fairly cheap.