The Hotel Air Conditioning Mystery Box

Not being excited about the possibility of having my air conditioning being controlled 'for my own good', or the possibility of being spied upon, I was compelled to investigate.

Inspection of the box 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. I was unable to investigate this. The antenna wire goes to the area of the board with the NE615N, and there is also a coil there, suggesting a radio. I suggest that the 10MHz crystal runs the logic, and the 39MHz crystal runs the radio.

The question, of whether it transmits or not, is a good one. The presence of a current transformer, a device which generates a signal when power is being drawn, indicates that the unit knows whether the A/C is on or off.

This could be used for data reporting in a scenario where the unit is capable of transmitting, or merely for providing input to the economizing logic where the unit is just a fancy energy saver. Personally, I don't think it transmits.

The prevailing suspicion is that the box has an ID which is set up by the RJ11 connection at install time. The property manager then turns on or off air conditioners, by radio, based upon room occupancy. 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 the user has set on the air conditioner's control panel.

I can see why the hotel management would want to make sure that the A/C was off or minimal when the room was not being rented, but I would strongly disagree with any practice that would interfere with my coming back to a nice cool room after a hard day's work, as well as anything that would cut off the A/C at night. I like it cold when I sleep, and I am paying for the room in any case. I am entitled to all comfort services unless notified in advance. Aren't you?

In any case, the device was carefully reassembled and put back. It was not my property, so I took great care to leave it as found. My policy- never damage anything! To avoid subjection to this sort of device, just unplug it from the wall, plug it into itself (so the housekeeping won't notice the empty plug ) and plug the A/C right into the wall. When your stay is over, plug things back in the way they were.

Note to sweaty hackers: You know if you break this thing you are gonna have to pay for it. So don't go fiddling with it. The best thing to do is to call the front desk and ask them to set the temperature lower. If they have no clue what the hell you are talking about (as in this case) Just leave it alone and plug the a/c into the wall till your stay is over.

Note to hotels: I don't advocate anyone damaging your equipment. You need to understand that people who are paying $120 per night for a room with air conditioning expect to have control over the temperature, and not have it sneakily cut off when they go out for business or have it set to 78 degrees in the middle of the night. This stuff is controlled by a computer, probably in a closet somewhere. If you don't have priveleges on that computer, get them, so that if someone complains it's 76 in their room and they prefer 70, you will be in a position to provide service to your customers. And no more hate mail you pompous poppinjays! It's not my educational article, and Mr. Zool is deceased.