Ask HN: Could we build a cheap contact-tracing device?

When the country reopens and there's a need to track who's been in contact with known cases of Covid-19, there's always been one oversight: some people don't have phones. And others don't have smartphones capable of running the current contact-tracing apps. What would it take to mass-produce a cheap device that doesn't make phone calls or run apps -- a single-purpose device whose sole purpose is to sit in your pocket and recognize when you're within range of another person running a contact-tracing app. Just as a technical problem I find it an interesting question. But the ultimate goal would be to mass-produce something cheap enough to hand out to anyone who needs one. (Pass them out in front of grocery stores, for example...) Thoughts?