To monitor the garage at my current residence, I first thought running a wire attached a sensor would be required. Because of the difficulty involved in running a wire I began to search for alternatives. I stumbled upon some low cost and quite minute transmitters and receivers on Ebay that operate on the (unbeknownst to me at the time) 70cm amateur radio band, and are governed under provisions outlined here.
After some reading, I decided that it would be possible to add some some door sensors to the garage doors, and relay their state (open or closed) wirelessly at certain time intervals. The venerable Atmega328p would once again be selected for the task of encoding and decoding signals, using amplitude shift keying (ASK), a task made much easier with the help of the RadioHead ASK libraries.
After some breadboarding, a prototype was made which worked for several years, until I decided to take some time and create a more capable design. Because of the limitations on transmit time per hour, a pulse would be sent out informing the receiver of the door state at a rate that would surpass the time needed for an individual to open and close a door, so a state would not be missed. This leads to quite frequent, but short pulses.
In the final design, room for four sensors was allocated, and a boost converter was selected in case more power was needed to relay the signals, as 5 volts through USB would be supplied for the transmitter, but 12volts could be used. On the receive side, a tricolor LED was used to differentiate between three states. A received packet indicating all doors closed lit a green pulse, a packet that indicated an open door shown red, and lack of a packet for 10 seconds shown blue. If a packet that indicated a door was open arrived, besides the red lit LED, a Piezo buzzer would sound, until an all doors closed packet arrived, or a button would be pressed to silence it.
The transmitter receiver pairs operate using a sawtooth crystal that oscillates at around 433MHz, because of the variability of the frequency due to things like voltage, temperature, and different manufacturing tolerances, these devices can be finicky, and finding a well matched pair or doing some fine tuning can be useful. All in all, this radio setup served me well for many years, but is no longer in operation, partially due to sharing the garage.