What is the easiest way to make a swing gate closing sensor?

Question

Electronics

There are large swing gates, a gate that do not close tightly. What is the easiest way to
1) make a closing sensor
2) that could work from a battery / battery (so as not to drag wires)
3) make a signal transmission over a distance of about 15 m
?
I read about inductive sensors, but those that I looked at have a sensing distance of literally 5 mm. And you need to have a few cm, because in winter, due to snow, the gates converge differently than without snow, and they can also be swayed by the wind, and the upper edges will diverge

Answer 1

1) a reed switch and a magnet of appropriate power, the technology has been used in security alarms for many decades
2) the reed switch does not need power, on the contrary, for example, with the gate closed, you can completely turn off the transmitter by supplying power to it through this very reed switch
3) any transmitter that can transmit message for breaking/closing the circuit. 15m in the absence of obstacles, almost any budget transmitter will provide.
Correctly noted in the comments, the reliability of batteries in the winter on the street is a big question.

Answer 2

Alright bullshit.
So take the tape from the tape measure, select the length, attach a mirror to this tape or, better, a corner reflector.
At the control point, place an IR laser pointer + photodiode + strapping.
As a result, the battery is not needed, stupid mechanics.

Answer 3

Hinge angle sensors ? Transmission over WiFi (esp8266 for $1), for example.
Or a surveillance camera, and periodic analysis of the picture. It's a little more expensive, but the camera can hang on the house and monitor the gate from afar, which solves the problem of wires.

Answer 4

IR barrier?
Data transfer - to taste. The minimum power consumption, I think, will be for a wired communication line.

Answer 5

Ordinary AA batteries Energizer Ultimate Lithiu may well work in cold weather.
Therefore, you can find a battery-powered wireless switch module in China, connect the power of the transmitter to the limit switch on the gate. so that he does not eat batteries while the gate is closed.
Assemble the circuit so that while the gate is closed, the limit switch tore the power supply of the module.
The gate opens, the transmitter turns on, there is a signal. The car passed, the gate closed, the transmitter turned off. Well, or what kind of logic is required.