Backstory #

Ever since moving into the apartment where we live now at the time of writing my girlfriend complained, that the garage door remote for her garage is shit.
It has low range (5 meter maximum) and frequently requires her to get out of the car and walk close to the garage door and hold the remote to the door.

Unsuccessful fixes #

Analysis of old transmitter #

The transmitter looked very old and since it was not mine, but only rented modifying it or the receiver was not really an option.
So I took a closer look at what gets transmitted and on what frequency it runs.

First I tried to measure the transmitted signals with a spectrum analyzer, first at 2.4GHz, then 868MHz, 433MHz and never got anything.
As a last resort I wrapped a loose wire around the transmitter and hooked that up to a fast oscilloscope to hopefully finally getting something.

And of course there was the signal nice and strong.
I’m not telling you the exact frequency, since with that you could probably open our garage door…
But it was not in the GHz range, not in the MHz range but in the low kHz range!

Yes that’s in the audible range.
I never expected to find a transmitter that uses such a low frequency.
Now I have a wav file on my phone which which I can open the door from outside with only the built in speakers of the phone.

Build my own transmitter #

With the signal measured and analyzed I knew it would be trivial to generate that with any microcontroller.

At my workplace we do development for an avalanche beacon which runs at 457kHz.
It uses a big ferrite antenna similar to the ones found in DCF77 Clocks.
After extracting the ferrite rod from a defective device I had most of the antenna for my own transmitter.

I then designed a simple PCB with a simple and cheap STM32G030F6P6.
It has one big button which connects the battery and then the controller starts transmitting for as long as the button is pressed.
Two leds show activity and battery state.

I even designed and printed a housing, which includes the board, antenna and two AAA batteries for easy usage. Unfortunately it was about as “good” as the old transmitter and therefore the problem with the low range persisted.

Next idea #

So now I have a DIY transmitter that can open the garage door with the original receiver and motor unit. But it only has low range. How about activating this transmitter with a different one, that has more range?

Fortunately Aliexpress has cheap ready made remote control units with a sleek one button remote and a compact PCB for the receiver.

In this picture I already desoldered the relay since I don’t need it. After a bit of soldering this module was combined with the homemade transmitter into a repeater.

The new receiver now activates the homemade transmitter whenever the big button on one of the two included remotes is pressed. With a bit of hotglue all the components were fixed to avoid shorts. Then a all that was needed was to mount this repeater unit inside the garage somewhat near the original receiver unit.

The repeater is the black rectangle on the left, right next to the original antenna which is in the short horizontal tube.
A 12V power supply is placed on top of the original garage door controller (blue box) and connected to a nearby outlet.
Some zip ties later it even looked somewhat decent.
Maybe I will add a label to it to describe what it is. (Definitely not a bomb, trust me 😉)

This repeater now has a nice long range of at least 20-30 meters and the necessity to get out of the car because of lack of range is definitely no longer there. It has only been up for a few days now as of writing this, time will tell if this solution is the final one.