Open gate for lawnmower only

Hi. A friend of me have an automatic lawnmower and a property that is divided in two with a gate in the middle. He askes me for an automatic gateopener that opens for the lawnmower but for his small child. I can't find a way to achieve this when searching the internet, i.e. to identify when a specific object is within a certain distance (so the gate have time to open) but don't open for anyone. Any suggestions?

Ideally: Low power on object side for battery to last. Not to expensive Robust design

RFID seems to have to short distance. Can 433mhz, ble or wifi or any similar be used for as distance measurement to unique object maybe?

Anyone with a feasible idea?

There are various RF modules that you could use that are low power. WiFi is not low power. Some RF modules allow the sketch to read the RSSI (Received Signal Strength Indicator). Perhaps your sketch could detect when the mower is close to the gate using the RSSI reading.

IR may also be worth investigating. An IR emitter with a narrow beam pointing in the direction that the mower is moving could be picked up by a detector at the gate. Sunlight, especially on hot days, could be a problem for this idea.

Rather than trying to detect when the lawnmower is close to the gate why not have it send an explicit "please open the gate now" message when it needs the gate opened. That could easily be organized with a pair of Arduinos and a pair of cheap nRF24L01+ transceivers. Actually just send 'O' for open and 'C' for close.

Simple nRF24L01+ Tutorial

...R

I would not let the lawnmower and child on the lawn at the same time! Open the gate when the lawnmower is operating and close the gate when the lawnmower is done.

How programmable is the lawnmower? Does it follow a programmed path or is it moving semi-randomly and bounded by a fence or underground wire?

@PaulRB Thanks. Will look into If my 433 stx882 nicerf modules can read the rssi. They are quite Low power i think (34mA at 3.3V) and since the message will be so short it might work. Will calculate if it's feasible or not.

@Robin2 Thanks for the suggestion, but i will still need to know when to send the open/close signal and for that i need the proximity to the gate. I do not know how the lawnmower thinks and I do not want to hack the lawnmower. ;)

@johnwasser Thanks for the reply. That's a valid concern and will ask him if the mower is "childproof" or if they have considered the safety issue. Thanks.

The "open gate when away" approach is the easiest and probably absolutely best approach and was exactly what his wife told us sarcastically when discussed the problem over a beer. But what's the fun in easy solutions when you can make a complex and challenging solution?

Makeorbrake: The "open gate when away" approach is the easiest and probably absolutely best approach and was exactly what his wife told us sarcastically when discussed the problem over a beer. But what's the fun in easy solutions when you can make a complex and challenging solution?

I don't think that was sarcasm. There's a place for fun and whimsy, but not when there is even a hint of a risk of injury, in my view. I had the misfortune of receiving a fund raising email at work from an employee who accidentally ran over their kid with a riding mower. Kid lost both legs.

Just keep Asimov's first law foremost:

  1. A robot may not injure a human being or, through inaction, allow a human being to come to harm.

Makeorbrake: @Robin2 Thanks for the suggestion, but i will still need to know when to send the open/close signal and for that i need the proximity to the gate. I do not know how the lawnmower thinks and I do not want to hack the lawnmower. ;)

I was certainly not proposing any hacking of the lawnmower.

Maybe the lawnmower could break a beam which would cause the Arduino on the gate to say "You are near me, do you want the gate opened?" If it was anything other than the lawnmower that breaks the beam there would be no response and no gate opened.

...R

Makeorbrake: Thanks for the suggestion, but i will still need to know when to send the open/close signal and for that i need the proximity to the gate. I do not know how the lawnmower thinks and I do not want to hack the lawnmower. ;)

The lawn mower may have a predetermined path, or it may moving randomly. In both cases, the mower is likely to get near to the gate without wanting to go through it (e.g. when mowing the grass in the vicinity of the gate). So besides detecting the mower's presence, you still need to have a way to find out when the mower wants to go through that gate, or when it is just passing by.

My first thought was indeed also in the lines of Bluetooth or similar short-range wireless connection, and the mower actively asking the gate to open for it, when the mower is done with one half of the property and then wants to move on to the next.

If the gate is inside a small alleyway or so, and the mower only gets in there to go through the gate, you may consider installing an IR beam in front of the gate. If that beam is broken, the gate can send a short range wireless request to the mower to confirm the mower's presence, and open the gate upon a positive response. If the mower is out of range (this of course should be limited to maybe a few meters, or whatever you think appropriate) the gate doesn't open as it's probably the kid, the dog, or a bird.

@aarg That was truly tragic. Safety first of course. To be honest do I not even know how they work regarding on/off when you are home. Think they are on all the time and you are supposed to have full supervision when your child is in the garden which you have anyway. I will ask him and discuss the perils you all have brought to my attention. These risks are not connected to the automatic gate though as the risk is there already as the mower charging station is in the main garden. The main peril we saw before was that the child could run out in the street accidentally if the gate opened incorrectly. Thanks for the concern.

@Ronin2 I do not understand. How will the beam know if it's the lawnmower or not? Can you explain please. Se below section though.

@Robin2 and @wvmarle Unfortunately the gate is in the open and not in a way a beam is possible.

@wvmarle Thanks for the extensive answer. I think I need to think of the mower as a erratic thing. I was thinking of a simpler solution. Open the gate always when it is within a radious so that if it wants to go through the gate is already open. Opens to often but simpler. Opens up new safety concerns though as.. but this is just brainstorming. This whole idea will probably go in the "not safe" category but the technical question of how to sensor if "uniquely x is within radious of y" is still interesting.

Makeorbrake: @Ronin2 I do not understand. How will the beam know if it's the lawnmower or not? Can you explain please.

It does not need to. No matter what breaks the beam the gate sends a signal to the lawnmower. But it is only the lawnmower that you will have programmed to reply.

Unfortunately the gate is in the open and not in a way a beam is possible.

A problem for every solution :) I am reminded of the humourous song "There's a hole in the bucket"

Try some ingenuity. What about a reflecting beam? Or adding posts to attach the beam sensors to?

...R