Hello. I've been thinking about this for months and haven't figured out how to do it... Don't be frightened by the wall of text, it's a VERY entertaining read! You'll fall down laughing! ______________ The basics: Imagine two systems: A and B. Both A and B use the Arduino as their "heart". I want A and B to know, at all times, the distance between them. The distance is 3-dimensional, ie, it doesn't matter if A is in front of B, if A is on top of B, etc. The distance ranges from about 10cm to 2m. It's not that strict, though. Something around that... The final project (which is not the sensor; the sensor is just a piece of it, just like the Arduino or an LED or a resistor) is a gift, so it has to be 'good-looking'. Because of that, the sensor has to be as inconspicuous as possible. Problem 1: Linear solutions can't be used, like Infrared LEDs or Ultrasonds. Besides being too linear (which makes sensing in 3 dimensions too hard and expensive because of the use of arrays of LEDs or sound generators), the only way (that I came up with) they could be used is this (because it can't be based on reflections): 1- A emits signal (IR modulation or Ultrasonic frequence) and starts timer (microcontroller function). 2- B receives signal. B waits 2 milliseconds. B emits signal (different from the signal emitted by A, so they don't get messed up). 3- A receives signal and stops the timer. A then calculates the distance through a simple formula. That's how A would know the distance between itself and B. B would do exactly the same thing to know the distance between itself and A, but with signals different from those used by A (so they wouldn't get messed up). This is infeasible because: Using IR, every millisecond (the smallest unit of time a general microcontroller can measure) is equivalent to 30,000,000cm (or 300,000km or 186,400 miles). Using sound, every millisecond is equivalent to 34cm (or 13.4 inches). With such a short distance range (10cm to 2m), 34cm is too much, which makes the sensor incredibly inaccurate. Problem 2: Using the sensor with the Arduino The sensor would have to output something to let the Arduino know the distance so that I could then do stuff like: If distance is between 0 and 20cm, do this. If distance is between 21 and 50cm, do that. If distance is 51 and 200cm, do that. Else do nothing. etc. :) Problem 3: It has to be simple (and cheap) I'm no programmer nor do I know a lot about electronics. I'm a simple hobbyist with the dream to finish this project, so it can't be overly complex. :) So, after reading this wall of text, what do you think I could use? Do you know any wireless technology that lets me do this? Oh, sorry about claiming it would be a fun read.. It was the only way I found to make you ACTUALLY read this. :p THANK YOU! :D