And while those sensors may have been connected to a siren or flashing lights, the real benefit to having an alarm has always been that someone will call the police when it goes off. For many years, that was also something the alarm companies were happy to provide — witness the countless television ads that have featured thoughtful and hyper-competent people staffing an alarm company’s command HQ, ready to call the police and offer reassurance to the homeowner.

As King Gillette discovered with razors and cellphone providers have popularized with monthly fees, the real money to be made in alarms is not in the hardware — it’s in the monitoring. The hardware can be subsidized by the security company almost to the point where it is free, but paying an alarm service about $30 a month for years and years to watch over your system more than makes those companies whole.

Newer systems, however, can reduce the total cost of alarm ownership. Instead of relying on installers to rewire the house, new “security systems in a box” use a combination of battery, wireless and cellular technology to make installation simple and quick for most homeowners. Some systems can even bypass monitoring firms directly and contact the owner instead of a third party when the alarm is set off. One such system is sold by InGrid Home Security. The basic package, available for $199 at ingridhome.com, includes an alarm console, a phone that doubles as an alarm control and three window or door sensors. The entire kit fits in a box about as big as a shoebox.

The sensors stick to doors and windows with sticky tape and are completely wireless. They are powered by tiny watch batteries and connect automatically with a few taps on the cordless handset. You can monitor the system online, even taking video and images using an optional video camera. The system took about an hour to install in a two-story home. No tools were necessary but there were a few snags while activating sensors and base stations.

The monitoring service costs $20 a month and discounts are available with a yearly contract. A video camera costs $130 and temperature and water sensors — for basements or unattended summer homes — cost $60 each. The service offers 24-hour monitoring as well as access to video feeds over the Internet. Smoke and siren detectors cost $100 and $50 respectively. LaserShield, another company offering a monitoring and hardware package, focuses more on motion detectors in the home. The starter kit, which costs $200 and is available at lasershield.net, includes a motion sensor and keychain remote. It requires a standard telephone connection for monitoring and for sending alerts; monitoring costs an additional $20 a month. You can control the system through a Web site.