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Distance alert? Answered

I need to create a device that alerts me, via fob or whatever, when I am five feet from source.


Like most things how accurate does this need to be.

A lot of fairly cheap robotic sensors, IR, Ultra sound, Visible light can detect objects up to 3 meters away fairly consistently.

The SRF005 ultrasonic range sensor detects objects in it’s path and can be used to
calculate the range to the object. It is sensitive enough to detect a 3cm diameter
broom handle at a distance of over 3m."


For example.

hard to do precisely - a GPS receiver can give your actual position and you can calculate distance from a give point.

A 5 foot bit of string tied to source will also work.

A bright small source of light can be monitored by 2 light detectors set in long tubes set at a slight angle to each other.. They will only both see the light when at a set distance from the source.

If a radio transmitter sends out a signal that is then sent back you caould calculate the distance by the time difference between the 2 signals

None of this is as good as buying a baby watcher - these use a radio signal - this would be too complicated to make at home.


My vote is for the bit of string. ;)

Attached to the eyeball with a safety pin.

That's probably a bit too intrusive.

I worked out a way that WILL do it, using RF and ultrasound, but its complicated. Neat, but complicated.

Speculating: One device sends out an RF signal and starts counting until it hears the first ultrasound pulse. The other recieves the RF and sends out an ultrasound pulse. Subtract the time needed for the circuitry to request and respond, and you get time-of-flight; multiply that by speed-of-sound to get distance. By looking for the _first_ pulse you will mostly screen out reflections if there's a direct path, but around corners you'll have to expect some false additional distance.

Basically, a one-way ultrasonic rangefinder.

Yeah, that could work. One could also try doing something GPS-like -- send out a radio stream of pseudorandom bit sequences with a sufficiently long duration between repeats, have the other device send them back, figure out how much shift is needed to get the two to align with each other and get time of flight from that. Needs faster circuitry, since radio is faster than sound, but would work over longer distances.

Ooh, Pretty well spot on Sherlock ! ;-)
Other approach would be some sort of phase/frequency measurement, like the mini laser distance measurement things. Still needs co-operation though,


Super-conductor powerful magnet, and a reed-switch.
(Not an easy thing...)


That's difficult to calibrate well for a single source. With two sources in known locations, you can theoretically use the sort of phase triangulation that GPS systems rely upon... though that brings up the question of "why not just use GPS and avoid having to design/implement it yourself."