The Hotel Air Conditioning Mystery Box

While staying at an inexpensive but clean hotel, I noticed this little box attached to the wall, which the air conditioner power cord was pluged into. It had a little flashing LED. I first thought it might be a power line filter or one of those 'watt-misers' (watt misers are a good thing, I use an "Energy Smart" brand one on my refridgerator), but upon closer inspection, I found a photocell and what looked like a wire antenna.

The unit monitors the air conditioner's power consumption, room temperature, and room light level in the room, and communicates with a host computer in the office. The host computer then sends signals to the unit to turn the A/C's power on or off. This can override the air conditioner's thermostat in a case where you have set the A/C to a cooler temperature than that provided by the programming in the host computer. The units are addressable individually.

If it is too hot in the room, you can call the front desk and ask them to program the box to allow the air conditioner to reach a colder temperature. You can literally ask for a lower limit, in degrees, to be programmed in. I assume it works with the heat as well.

This was all very interesting, and later I got sent to me some photos sent to me of the inside of a similar box, which revealed that there was indeed a relay to shut off power to the air conditioner, as well as a circuit board which had a photocell, a small flexible pc board which came out the bottom with an LM34D IC (temperature sensor) on it, and indeed, an antenna. One wire from the air conditioner power circuit passed through what looks like a brown colored current transformer.

Two crystals on the board are 39.150 MHz and 10.15 MHz. Other IC's are LM339N, NE615N, LM324, and an ASIC or similar device. There is also an RJ11 jack, which would seem to be a programming interface, probably a serial port. The antenna wire goes to the area of the board with the NE615N, and there is also an RF coil there. I suggest that the 10MHz crystal runs the logic, and the 39MHz crystal runs the radio.

The property manager's computer turns on or off air conditioners, by radio, based upon room occupancy. It was linked into the same computer that sets up the room key cards. The photocell may be used to sense when the lights are out. such as when the occupant is sleeping or has left the room, and cut off the air conditioner when the temperature is below a set point, providing a higher but more energy-saving temperature setting than a careless user may have set on the air conditioner's control panel.

I had an unfortunate experience with this room. It was 74 degrees minimum during the day, limited by the control box, and at 2 AM, the A/C unit was being shut off. I called the desk, and they did send the amintenance man out, twice, but he said nothing was wrong with the A/C. The mamager at this hotel had no idea what to do about it. I can only imagine they did not know how to get into the program that controls the units, or even that it existed. Might have been part of some package they bought when they computerized the hotel. Who knows.

As a last resort, I ended up unplugging the air conditioner from the box, and plugging it right into the wall. Then I was able to get the 70 degrees I need for comfort during the hot summer. When I checked out, I plugged the A/C back into the box.