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Ask HN: How do you "ping" a modern car keyfob?

3 points by MarkusWandel ↗ HN
This has no malicious intent. But in my household, keys sometimes disappear, in one case not to be found again until six months later.

Now the new car (a Honda) can tell whether a keyfob is nearby. This means the keyfob is either constantly transmitting a low power "I'm here" signal, or it is constantly listening at low power for an interrogation signal.

This means it should be possible to "sniff" for the keys' presence. Only presence! Not whatever rolling unlock code would allow the car to be used. But this means one could have some help locating them in the house by just repeatedly transmitting the interrogation signal and listening for a reply.

Is there any knowledge out there to do this? With an RTL-SDR or a Flipper Zero or whatever? That is not tied up in criminal car thieving contexts?

9 comments

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orbz
Absolutely something you can do with a SDR usb dongle: https://web.stanford.edu/class/ee26n/Assignments/Assignment5...

Realistically though just put an AirTag on your keychain and don’t worry about it. You’ll get the benefit of finding it anywhere like in a jacket you left at a coffee shop.

mikewarot
I would assume you could use an SDR dongle to find cell phones (that are on) in the same manner, along with anything using WiFi. (At different frequencies, of course)

Back in the 1980s, we amateur radio operators (I'm KA9DGX) used to call it "Fox Hunting", and did it on a much larger scale, on the 2 meter band.

MarkusWandel
One of my kids was (still is to some extent) a "runner". It did occur to me to go low tech, put an oscillator kind of thing on her, and then use one of those helical antenna thingies to DF her. I'm sure schematics for that kind of analog fox hunt stuff would be easy to find. But times being what they are, if you showed up with a rig like that at the playground, it would at minimum be a "conversation starter" and at worst be "call the cops" kind of thing. So instead you have to get the SIM card and security hole equipped kind of thing and do it with your smartphone. Nothing anyone does with a smartphone raises eyebrows...
tamimio
If that’s your purpose, just attach an airtag to the car keys keychain. However, you can definitely use an SDR to capture the signal, and to send it you need the SDR to be able to transmit, rtl-sdr won’t work.
KomoD
just attach an airtag or tile to it
MarkusWandel
The Airtag thing did occur to me, since they beep when you look for them (right?) Otherwise "it's in the house" won't help much when you know that anyway. An SDR solution could presumably help locate it based on signal strength. Also we're not an Apple household.

One thing against a locator gadget is that these (Honda) keyfobs have the keyring on the detachable part (with the physical emergency key). Frequently we detach that and just take the keyfob, in a securely zipped pocket, when going, say, grocery shopping. That makes the fob easier to misplace, but the locator thing would still be on the keychain.

appplication
AirTags have much better fidelity than “it’s in the house”. You can directionally track items down to within a meter or so.
simonblack
I have a similar problem. Our kids left a gate-opener fob with us while they went away. But do you think we can find it now they have returned?

We have a defined place (a box marked "keys") where we put things like that. but for some reason or other, it's not in there.

Any suggestions?