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Comment by jcims | Hacker News Reader

The HB100 [0] and CDM324 [1] are very inexpensive little CW doppler radar modules at 10GHz and 24GHz respectively. Feed them 5v and send the mixer output to a small microphone amp/input jack and you can record the frequency beat of most objects in motion.

[0] https://www.amazon.com/dp/B00FFW4AZ4

[1] https://www.amazon.com/dp/B07WH67J9W

This is what it sounds like when you toss a quarter in front of an HB100. https://www.youtube.com/watch?v=8riretP8ylE

The bowtie looking part is the actual flight, the other part right after that is it bouncing off the counter.

Spinning same quarter - https://www.youtube.com/watch?v=5lnYvJoxRak

Shining it at parts of a ceiling fan - https://www.youtube.com/watch?v=tIiFvByf1CQ

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It amazes me how cheap these application-specific chips have become - it opens up so many new potentials. I can only guess that something like this would cost $1000+ roughly 10 years ago?

No, not really, doppler radar based proximity sensors (used in e.g. baths) cost about 40 € more than 10 years ago. It's not complicated technology.

What's an e.g. bath?

e.g. is https://en.wiktionary.org/wiki/exempli_gratia

which you can pretty much read as "for example, [example(s)]"

My mnemonic is ‘eggs ample’. :)

Commonly confused with i.e. (id est), which trasnslates to ‘that is’. The mnemonic there for me is ‘in other words’.

Sorry, I should have guessed my comment would be lost on those who don't know how to use "e.g." correctly.

Just assuming you were being earnest (perhaps learning english!) as there wasn't much to gain from interpreting it another way.

I don't know what they're doing in a bath, but small radar systems have been used as proximity fuses for anti-aircraft shells since WWII.

As light switches and such; PIR sensors don't really work in a shower room, for example.

There actually isn’t much to these tbh, oscillator, mixer and pcb antennas. To use these in an application you would have to still process the analog waveform coming out. Fortunately the frequency is very low (audio range) allowing for all manner of inexpensive detection/measurement circuits.

I'm sure they are cheaper, but I don't think they were ever really expensive. They have been used for automatic doors, like at the grocery store, for quite some time.

Nice teardown https://www.allaboutcircuits.com/news/teardown-tuesday-hb100...

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