Electronics > RF, Microwave, Ham Radio

"Distance Alert", looking for inspiration for project.

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thinkfat:
I feel I should give closure to this project. It is mostly complete, the hardware is done, the software is mostly functional for the intended purpose, the only thing left is fine tuning of parameters and trying to make it as convenient as possible.

I've attached some photos of the final hardware. The case is 3D printed out of blue PETG, I think it turned out well.

The PCB is 55mm by 36,6mm, the size is mainly dominated by the battery, which is a 40x30x4 mm LiPo pouch cell with 500mAh capacity. The PCB itself could by way smaller still. But I don't want to spend time on it, unless I decide to make more of them to sell: initial feedback from other parents of kids with the same genetic condition were positive.

I was able to characterize the power consumption and it mostly depends on how long the ESP32 can spend in "light sleep", and that in turn depends on the advertisement and scan intervals.

An interesting complication was, and still is, the 32kHz sleep clock. I can't seem to get it quite right. I computed the load capacitors to be around 15pF, based on the xtal datasheet, but when I place them, the oscillation becomes unstable. It's also a couple Hz off. I had best results with two 5.8pF capacitors, or none at all, but on one board the oscillator frequently wouldn't start at all of I don't place the caps. So, something's marginal here and that doesn't instill full confidence. Unfortunately, unlike for the STM32, there is no detailed design procedure for the oscillators and no part recommendations. The reference manual only states to choose the load capacitors to be compliant with the crystal datasheet. Well, that's what I did and it didn't work.

The first "field tests" were successful, though. Another topic I might pick up is writing an Android app so that one of the devices could be replaced by a smartphone.

Thanks to all who tagged along, cheerio for now.

RoGeorge:
Congratz!  :-+

mag_therm:
Congratulations on the project.
My son with DS is now 18, and when younger, crowds alleyways museums  etc were a significant challenge.
I tried a commercial version of what you have built which also had a crude direction indicator,
but it was too troublesome ( not really fit for purpose )  and I only used it about 2 times.

Now he has a an ordinary cell device and uses sms  email , and the video phone.

When I looked a cell phone location apps a few years ago the main item was the costly subscription.
Not sure if same now.

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