Yes…
When the RF signal detected is “something” trigger automation.
It’s a fairly basic thing to do
Will the Sonoff RF Bridge work for this? With a regular 433Mhz remote?
Yes, I assume it does. Not sure what it exposes in HA though.
But flashing it will probably give you better control (if that is possible)
I have never used the bridge.
Here is a topic about RF.
Keep in mind that you need to place the receiver close enough that you can operate it from the gate, if that is an issue.
Or you will need to place the receiver outside.
Simply use a ShellyButton. 
It’s Wifi based, and it is, what makes it ideal for this use case, always in a deep sleep and only activates on button pushes.
You link the button to your wifi. After that you can set up four different types of button pushes (one click, two clicks, three clicks, long click = push and hold), that can trigger anything in HA, eg. an automation. You can select the time, that the button stays connected, after that it goes into deep sleep (good, because the battery doesn’t need to be drained while searching for a Wifi that’s not in range) and activates after a push.
after pressing it, how many seconds until it’s usable on wifi?
According to Shelly less than two seconds (as stated on the link I provided). My personal experience shows that to be mostly true. My longest response time was not more than three or four seconds.
Another advantage is, you get a visual feedback, if the connection and everything else worked (the small circle on the button lights up in different colors).
I am using it for the opposite. I have an alarm on my garage door, if it is open too long, it sends an alert. To disable this alert, eg. while cleaning up the garage, I use the Shelly Button1. It works like a charme and the battery is still over 95% after five or six months.
I’ll order a couple - thanks for your help!
Oh, I forgot. If you want an instant reaction from the button, you just need to connect it to USB. If the button is powered via USB (not battery), it always tries to connect to the Wifi set up. There you will have an instant reaction.
That would make sense, if you have a power outlet in your car, that switches off, while the car is not running. As long as you drive the car, the button is powered (so searches for your Wifi), but if you switch off the car, it won’t drain your car battery but still works.
The button is only programmable while connected to a USB power. 
Honestly, this thing is made for your use case.
EDIT:
sensor:
- platform: mqtt
name: "Shelly Button1 Battery"
state_topic: "shellies/shellybutton1/sensor/battery"
unit_of_measurement: "%"
icon: mdi:battery
- platform: mqtt
name: "Shelly Button1 Error"
state_topic: "shellies/shellybutton1/sensor/error"
icon: mdi:battery
binary_sensor:
- platform: mqtt
name: "Shelly Button1 Kitchen Charger"
state_topic: "shellies/shellybutton1/sensor/charger"
value_template: "{{ value }}"
automation:
- id: Shelly Button1
alias: Shelly Button1
trigger:
platform: mqtt
topic: "shellies/shellybutton1/input_event/0"
action:
- choose:
- conditions:
- condition: template
value_template: "{{ trigger.payload_json.event == 'S' }}"
sequence:
- service: input_boolean.turn_on
entity_id: input_boolean.all_lights_or_anything
- conditions:
- condition: template
value_template: "{{ trigger.payload_json.event == 'SS' }}"
sequence:
- delay: '00:00:01'
- conditions:
- condition: template
value_template: "{{ trigger.payload_json.event == 'SSS' }}"
sequence:
- delay: '00:00:01'
- conditions:
- condition: template
value_template: "{{ trigger.payload_json.event == 'L' }}"
sequence:
- delay: '00:00:01'
1 Like
Maybe I’m a bit late to the party, but I tend to have this passion for nfc tags. I always have my phone with me. Got an NFC tag hidden away nicely in my car. And when I reach my phone there, it triggers anything I program it to.
In this case I could make it to so do a REST API command to Home Assistant to open or close the garage door (respectively of current status, so toggle maybe).
Your phone is always connected to HA (either wifi or cellular network) if your instance is also accessable outside LAN. Also less likely to be copied. RF can be easily copied. Not to mention, this is also the cheapest and simplest solution. You can buy 10 nfc tags for less than 10 bucks
That’s a great idea too. Any tips on which NFC to buy? Are there good and bad NFC tags?
Not particularly any good or bad. These are the ones I got. Nfc tags as a sticker. Amazing
I’m not sure why a button is better?
What other kind of actions do you want to accomplish when you push the button?
I use a Lutron Pico remote (Home/Away marked) for an in-car activation device, though I am using mine to close the garage doors and set the alarm, you can tie it to any automation you want. If you already have Caseta devices, but not the bridge, that is the next step, and you can make the remotes do whatever you want.
Response time is measured in milliseconds and the range is dependent on the bridge placement, but I can arm the alarm from well over 50 feet down the road
Because it is illegal to use your phone while driving?
1 Like
Who said anything about using the phone while driving? 
Even if I had suggested to use the phone as a control button (which I didn’t) the OP could still stop and push a button on the phone. Still not using the phone while driving either.
What I was asking was to see why they didn’t think the phone was as good as a button. I was asking to find out what their thought process was in that regard.
If they thought the phone could only be used as a button I would have next pointed out that if the phone is already connected to HA the OP could use the phone’s location to do things without the need of explicitly pushing button on the phone.
Which is also why I asked what other things the OP wanted to do with the button push. Because those other things might have likely been able to be accomplished using the phone location as well.
I have my garage door open automatically for me every time I come home and I never have to pull my phone out to do it.
The first post deals with the other things he wants done.
Personally I find the phone difficult to use from the car, especially in the drivers seat.
He says he wants a button, give the man a button.
But yes some automation could be applied if you have good presence detection. However that is not a given, even with the phone app.
My own house is between two roads and you have to drive on road 1 past the house to turn down road 2 where the entrance is. But most presence detection would get triggered from the road 1. Then the garage would open, even if I am not going straight home. Maybe I am going on down road 1 to go to the pub, and I come home some hours later to an unlocked house and open garage door.