The apartment building I live in never has a working intercom. It has something that resembles an intercom box, and it is supposed to dial a preset phone number when you push a button for each tenant, but the landlord never bothered to keep the presets updated. My visitors can call my cellphone, but I often cannot have UPS or FedEx send packages to home.
Other tenants also have the same problem. They would leave notes repeatedly asking UPS to just leave their package at the entrance hallway, but UPS never do that due to policy reason. Some tenants attempted to leave a phone number, but once I ran into the UPS delivery guy---he told me the company does not issue them corporate cellphones, and he never uses his personal cellphone for work.
Although there are wireless doorbell systems that are very cheap (~$30), I don't want to just install a doorbell for my own unit. I want to have the doorbell send an instant message that can be broadcasted to anyone who is interested, including other tenants of this apartment. The question is how to build a system like that.
The first iteration of this idea involves using wireless sensor network, but they require interfacing with a gateway, which must be connected to a computer for Internet connectivity. I want to leave the computer out of the picture. Besides, wireless sensor networks aren't cheap. I forgot the exact pricing range, but a gateway could cost $300-$600 or more, and each node is also $100-$200. And since many of these are still academic prototypes, you don't find many places to buy them. Someone could steal the doorbell, and it could cost me $200 to replace it. This seems like an expensive doorbell.
Then I wondered if a "system on a chip" would be a good idea. I found the PIC Micro Web, which is really a small computer with a parallel port and an ethernet port. I could solder a push button to the parallel port, which would fire something off the TCP/IP on the ethernet side. The price is reasonable. The only problem is, it only works on Ethernet. In search of similar units, I found Digi Connect ME, which has a sibling wireless model Digi Connect Wi-ME. This theoretically allows me to connect the door bell push button to my wireless home network and send instant message over my DSL. However, it's still a bit pricy, at $130 per unit.
Then it occurred to me that I could use a Wireless to Ethernet bridge which may or may not support Linux. I found Linksys WGA600N, which based on a Ubicom chipset that has a Linux based SDK. With that device, maybe I could find out which one is the serial port, and repurpose that for door bell button connection. The cost is $80, but I think it is acceptable.