Imagine small, cheap, low-power sensors attached to all sorts of stuff people want to monitor: dog collars, rental scooters, irrigation devices, air-quality sensors, even water coolers.

Now imagine those devices communicating over a new, longer-range network — a low-cost, decentralized alternative to the big cellular networks maintained by companies such as AT&T and Verizon — created by regular people who place gadgets on their windowsills that sensors connect to, in exchange for a tiny cut of the revenue the network generates.

That’s the vision of San Francisco startup Helium, which last week started selling its $495 Helium Hotspot in Austin, Texas. The book-size devices are coming to San Francisco, Seattle, Portland, Ore., and other cities in the fall. Helium says its LongFi technology has 200 times the range of Wi-Fi, so just 100 to 200 hotspots can cover an entire city.

Like Wi-Fi, LongFi operates in unlicensed spectrum, so it does not need to buy pricey airwave licenses, Helium said. AT&T, T-Mobile, Verizon and others spent $2.7 billion on 5G licenses in a recent government auction.

“We’re building what we call the people’s network,” said Helium CEO Amir Haleem. “Everyone can participate, acting as a wireless network operator.”

Here’s the catch: The rewards aren’t in cash; they’re in a bitcoin-like cryptocurrency Helium is issuing.

The concept raises lots of doubts: Will regular consumers buy the expensive hotspots, which don’t provide them any immediate benefit? Will the cryptocurrency scheme work out? Will components be available for hardware makers that want to adopt LongFi? Haleem notes that people initially said Airbnb’s peer-to-peer approach to lodging was insane. “In the early days, it’s a big leap of faith,” Haleem acknowledged.

But Helium has attracted serious interest and $51 million in backing from the likes of Union Square Ventures, GV, Khosla Ventures and Salesforce CEO Marc Benioff.

It has also signed up some impressive customers and early testers. Lime, the San Francisco scooter startup, wants to better track its vehicles. Nestle wants to monitor office water coolers to know when they need refills. Agulus wants to get data from its irrigation valves and pumps. A pharmaceutical company would like to build trackers into the caps of medicine bottles, Helium said. They all see value in untethering devices.

Haleem joined forces with Napster founder Shawn Fanning in starting Helium.

“We started out trying to help friends solve the problem of connecting small, low-powered devices, like baby monitors to the internet,” Fanning said in an email. “Early on, we realized that it’s very hard and prohibitively expensive to connect those kinds of tiny gadgets, especially if they’re outside. After a lot of trial and error, we realized that ... we had to find a way to create a global ubiquitous network without expending billions of dollars like traditional telcos.”

That led them to the idea of a distributed, peer-to-peer structure — like Napster’s music sharing, Fanning said.

Jessica Groopman, industry analyst and founding partner at Berkeley’s Kaleido Insights research firm, said distributing ownership of a longer-range wireless network makes sense.

“Helium taps into a number of trends (such as) shifting governance, power and control back to consumers,” she said. “It’s power to the people.” The corollary: “It’s probably inevitable that Helium will face extreme challenges from big telco providers.”

Smart devices connected to the internet are a major market, predicted to hit $1.7 trillion in 2020, according to IDC. Helium is far from the first to tackle low-power wireless networks for such devices, which require just a trickle of bandwidth. Sigfox, LoRa and Nodle.io are among others trying to develop such a network.

But many smart devices need a network that’s ubiquitous, not one in process of rolling out. That’s why Whistle Labs, which makes a smart dog collar using GPS, relies on cellular for its users to monitor their pets’ activities, said Kevin Lloyd, chief technology officer of the company, now owned by Mars Petcare. Customers buy its $99 device and then pay $7.99 or $9.99 a month for the cellular connection.

“I needed something so I know my devices would work 99% of everywhere,” he said. “Today that’s traditional cellular. I look forward to Helium reaching that kind of penetration.”

Oakland’s Gregory Gotts likewise wanted to create a small device called InvisiLeash for people to puts on their pets’ collars to track their whereabouts and monitor their activities. He quit his job as a vice president at Dell to start InvisiLeash, but found existing technology to be cumbersome and pricey. Then he learned about Helium.

“It’s a breakthrough, disruptive technology,” he said. “We couldn’t do it otherwise,” Gotts says a national pet retailer with 2,000 stores is potentially interested in selling Helium Hotspots packaged with the InvisiLeash, and might even install the hotspots in its stores. If that deal happens, that could help solve the chicken-and-egg problem that stalls so many nascent networks.

Carolyn Said is a San Francisco Chronicle staff writer. Email: csaid@sfchronicle.com Twitter: @csaid