Electronics > RF, Microwave, Ham Radio

"Distance Alert", looking for inspiration for project.

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thinkfat:
I have a project on my mind that I'm looking for inspiration for.

The background of it is my son. He was born with chromosomal anomaly, Trisomy 21, aka "Down Syndrome". He's eight years old now but due to his condition he's quite a bit different from "standard issue" kids of the same age. He tends to get lost in his own world quite a bit, which is sometimes stressful especially when we're out and about in places where it's difficult to keep an eye on him. He tends to just wander about and get lost and is then unable to find us or ask for help. Think mall or grocery store. I have another kid (6y) and keeping track of them both is a challenge.

There are parents who put they kids on a leash out of desperation, which is a pragmatic approach but I would like our "bond" to be a bit more subtle. So I would like to make a "distance alert".

The idea is to have two transceiver boxes that "ping" each other every couple of seconds, and when they cannot hear each other any more, an audible alarm would sound from both. I would be carrying one of the boxes, my son the other one. When he gets out of range, the alarm from my box would alert me and the sound from his box would allow me to track where he went. 

The range should be no more than 10 meters. The RF frequency should be high enough to allow for small antennas yet at the same time be a manageable task to develop with hobby-grade equipment. 2.4 GHz ISM band would be suitable, I think. Battery autonomy should be a couple of hours from a full charge. The total size should make it wearable, maybe with a wristband or on a belt.

I can pretty much cover all of the topics, except for the RF platform.

I'm guessing something like a TI CC1352P could be used, but maybe there is something simpler?

MK14:
You could consider the esp (esp8266/esp32) range of devices. If their range is sufficient.

Interestingly, for this application. is, the latest/newer ones (possibly not released yet and/or delayed by chip shortages), have a new range finder feature (time of flight). Which uses the speed of light, to detect the exact distance between the units (if I remember/understand it correctly).

EDIT:
Example mentioning it, trying to measure exactly 10 metres (like you, by coincidence), mentioned later in the posts:

https://github.com/espressif/esp-idf/issues/5059

DrG:
I would suggest that you evaluate a BLE beacon and a phone app as the receiver. Look into RSSI, which you would need to specifically evaluate programmatically.

I am not saying that this approach will absolutely work for you, but I think it is worth investigating because the hardware and software are out there and relatively cheap. Battery life would not be a problem.

Some searching, including; RSSI, ble beacon, programmable advertising interval and so on will give you a good idea of usability.

Let us know how this project progresses - ok?

thinkfat:

--- Quote from: DrG on December 23, 2021, 05:39:35 pm ---I would suggest that you evaluate a BLE beacon and a phone app as the receiver. Look into RSSI, which you would need to specifically evaluate programmatically.

I am not saying that this approach will absolutely work for you, but I think it is worth investigating because the hardware and software are out there and relatively cheap. Battery life would not be a problem.

Some searching, including; RSSI, ble beacon, programmable advertising interval and so on will give you a good idea of usability.

Let us know how this project progresses - ok?

--- End quote ---

Thanks for the suggestion. My use case would involve both sides being aware of the maximum distance violation, though. So both boxes need to transmit and also receive. But a BLE beacon could still be used.

The 10m distance doesn't need to be exact. It is a sensible value that allows a good degree of personal freedom for my son and at the same time peace of mind for me because I should still be able to see him and hear the alert from his box.

The solution can not be too simple, as both boxes need to sound the alert as soon as one of them loses contact. So, when one of them doesn't receive the transmission from its partner any more, it would change it's own signal from "I can hear you" to "I cannot hear you". This should counter asymmetric transmit/receive scenarios.

I will keep this thread alive, though when I start the actual development I will likely create a new thread.

jpanhalt:
User on-the-fly programmable GPS might be a better option.  See: Invisible Fence.  Of course, you would not use a shock.

My best friend in elementary school had a slightly younger brother with Trisomy 21.  We were all great friends.  His brother used to visit me frequently and unannounced on his own.  That was no problem for me, of course, but it involved him crossing a busy street in LA.  Everyone worried about that.  Good luck is finding a solution.

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