Infrared certainly has its uses, but if you’re trying to locate objects, ultrasonic detection is far superior. It’s contact-less, undetectable to the human ear, and it isn’t affected by smoke, dust, ambient light, or Silly String.
If you have one ultrasonic sensor and a microcontroller, you can detect plenty of useful things, like the water level in a rain barrel or the distance traveled by a tablet along a rail. If you have two sensors and a microcontroller, you can pinpoint any object within a defined range using trigonometry.
[lingib]’s dual sensor echo locator uses two HY-SRF05s, but the cheap and plentiful HC-SR04s will work, too. Both sensors are arranged for maximum beam overlap and wired up to an Arduino Uno. One sensor’s emitter is blocked with masking tape, so all it does is listen.
When the system registers the object, it shows up as a red dot on a grid inside a Processing sketch along with a bunch of details like the object’s coordinates, its distance from each sensor, and the area of the triangle formed by the two sensors and the object. [lingib] reports that the system is quite accurate and will work for much larger playgrounds than the 1 meter square in the demo after the break.
Don’t want to detect objects? Ultrasonic sensors are cheap enough to hack into other things, like this one-way data communications module.