General Motors Corp. says it will debut, by 2005, a motion sensor embedded in headliners that will detect if a child, pet or vulnerable adult is left inside a dangerously hot vehicle.

The low-energy sensor, being developed using the automaker's minivans, uses technology similar to Doppler weather radar. It measures the temperature in the vehicle and how fast the mercury is rising. The sensor is delicate enough to identify the slightest movement, such as the blink of an eye, says a GM spokeswoman. When movement is detected in a hot vehicle, the sensor sounds an alarm.

GM is considering several alert signals, including a distinctive sound for the vehicle's horn and/or a vibrating key fob.

GM research shows at least 135 children have died of heatstroke in parked vehicles since 1996. In most cases, caregivers forgot the child was in the vehicle or thought they'd only be away a few minutes. Some deaths occurred when children gained access to a vehicle.

However, the company has yet to decide if the doors should unlock, as a child could be kidnapped or wander into traffic. GM also is concerned that a parent may misuse the device, figuring it is safe to leave their children in the car until the key fob vibrates.

The current set up has the sensor powered by the vehicle's battery. If the battery goes dead, the sensor doesn't work.