For several years, technology companies have promised the dream of the connected home, the connected body and the connected car. Those connections have proved illusory.

But in the last year app-powered accessories have provided the mechanism to actually make the connections. That is partly because smartphones have become the device people never put down. But it is also because wireless sensors have become smaller, cheaper and ubiquitous.

Big companies with strong brands have been heavily promoting the new uses for these gadgets. General Motors advertises its Chevy Malibu Eco with a man showing his parents how he starts the car with a smartphone. A major selling point of the popular Nest thermostat is its ability to turn up the furnace from miles away with a cellphone.

“Now that, increasingly, consumers have a device with them to monitor virtually anything they do with the Internet, why not offer that capability to monitor and remote control?” said Ross Rubin, an analyst at Reticle Research.

The idea of turning off the lights with a smartphone may seem gimmicky, but consumers are warming to applications, said Bill Scheffler, director of business development for the Z-Wave Alliance, a consortium of companies that make connected appliances. The situation resembles the time when power windows started catching on for automobiles, or when television makers started offering remote controls, Scheffler said.

At the recent International Consumer Electronics Show dozens of companies showed off connected accessories they can hook up to their home appliances to make them work with smartphones, and many also displayed wearable devices that can help people monitor their health on their phones. Some of these products were being provided by large companies.

AT&T, the wireless carrier, said that in March it would begin selling a wireless security system called Digital Life that will allow people to use tablets or phones to monitor cameras, alarms and even coffee pots.

If a burglar trips a motion sensor in the house, for example, a user can receive a text message, then call the police. Customers can choose to expand AT&T’s wireless service to appliances like lights, door locks, thermostats and security cameras, which can be controlled and monitored through the AT&T mobile app.

Ralph de la Vega, chief executive of AT&T Mobility, said in an interview that home security was a big opportunity to increase revenue. Only 20 percent of homes have security systems, he said, leaving millions of homeowners as potential buyers.

Ingersoll Rand, which makes industrial products, offers a starter kit and software for people to connect their homes. It includes a lock, a light and a wireless “bridge,” or base station, to connect the devices to the Internet. They can be controlled with a smartphone or tablet app called Nexia Home Intelligence.

Products by several other companies take advantage of a smartphone’s sensors and connection to the Internet to monitor consumers’ health. IHealth sells monitors for people to track their blood pressure with an app. At the electronics show, it introduced a wireless glucose meter, called the Smart Glucometer, that lets people with diabetes determine their blood sugar. A user puts a blood sample on a test strip, pops it into an accessory attached to a smartphone, and an app gives a reading of the blood sugar level.

Adam Lin, general manager of iHealth, declined to say how many products the company had sold, but he said it was in the “six-figure” area. IHealth products have appeared at Apple, Target and Best Buy.

A small start-up, AliveCor, has created an iPhone case that, when grasped, records an accurate electrocardiogram on the iPhone screen via its app.

Nike, Jawbone and Fitbit sell wearable electronic devices for people to track their movements with smartphones. Fitbit, based in San Francisco, sells a pocket pedometer called the Fitbit One, which can track a user’s steps and floors climbed, and also monitors sleep patterns.

Its newest product is due in spring, the Fitbit Flex, a step counter and sleep tracker that is worn around the wrist. It synchronizes with a smartphone app to give users updates.

Woody Scal, chief revenue officer of Fitbit, said one reason that wearable fitness gadgets had become popular was that the sensors had shrunk and battery life had improved.

Scal said wireless fitness devices were becoming popular because they addressed basic needs for consumers, unlike another trend seen at the show, enormous televisions.

“In the end, I don’t wake up in the morning, look myself in the mirror and ask whether my TV has enough pixels,” he said.

“But I do wonder how I’m going to get enough exercise, eat better, sleep well or manage my weight despite all the other things going on in my life.”