# Short distance detecting device

I'm looking for a sensor that would activate at a certain distance from the magnet or whatever would be signaling the location. The distance would range from 2-30cm and there would be obstacles (fabric, human parts,...) in between so that sound and light sensors would probably not work. I would want to be able to change the distance that would activate it. So if I'd set it to 25cm it'd activate when the two parts would get closer than that.

• Magnetic field sensors are called Hall effect sensors. Mar 3, 2015 at 21:29
• sounds like your range requirement is too far for magnetic sensors to work properly - you'd need huge gain/very powerful/sensitive stuff to detect properly, and still have control over detection range as you request Mar 3, 2015 at 21:33
• Do the two parts have any means to communicate? A sideband electrical communication channel? Or are they completely independent?
– M D
Mar 3, 2015 at 21:55
• The two parts could be connected with a wire that would still allow full range of motion if that's what you're asking.
– MKA
Mar 4, 2015 at 3:12

I'd stick with electromagnets aka inductive loops for doing what you want. Whilst not in the presense of big chunks of metal you can get pretty consistent fields generated inversely proportional to distance cubed. Use an oscilator to excite a central loop and use smaller loops tuned to the resonant frequency for measuring the field strength.

Careful though this only works in the one plane - if you rotate a loop thru 90 degrees it will likely go thru a position where no field is detected. So then you use diversity if you need 3D distance detection - two transmit loops at right angles operating at different frequencies.

• The movement would be in one plane and it would determine the angle that the arm is suppose to go to, at a certain distance it would stop it. The angle of the arm would range from 20-180 degrees.
– MKA
Mar 4, 2015 at 3:17
• I think you need to show a diagram. Determining angle is more complex than your original question implied. When you say " at a certain distance it would stop" do you mean " at a certain angle it would stop"? Mar 4, 2015 at 8:24
• Yes, I meant at a certain angle, but at the same time it can be at the a certain distance. Let's say you have a triangle ABC. A = joint, B = sensor and C = magnet. The distance between B and C can determine the angle of a joint.
– MKA
Mar 4, 2015 at 14:30

I just came across a reed sensor. Does anybody know how far I could stretch the distance with a normal AA battery? And if I would be able to manipulate with the distance.