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I'm very new to this. Please bear with me. I want a simple circuit which beeps when something is moved apart farther than a certain distance. I'm thinking of a sensor and something that's sensed (sensee?). They would be about 3 inches apart, if they are moved farther apart than that I need a beep. I need this to be somewhat configurable, between 2 and 4 inches. My initial thought is NFC, but I don't want to use a smart phone, and ideally I want something really fundamental and simple, with a passive sensee (no power needed on that) and very low power draw on the sensor and beeper circuit.

What are my options?

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  • \$\begingroup\$ "Moved apart" can be in a different ways. It can be just a literal translation, or it might be some angular displacement as well. And how should it react if something is placed between the two parts? \$\endgroup\$ – Eugene Sh. Jun 2 '15 at 17:29
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    \$\begingroup\$ Magnetic perhaps. I've used magnets for ~1-2" with hall sensors and they work well. Can change the magnet to adjust sensing distance. Laser would work if you don't put anything between the objects and don't turn them at an angle. \$\endgroup\$ – I. Wolfe Jun 2 '15 at 17:50
  • \$\begingroup\$ Only translation, angle doesn't matter. Nothing will be placed in between. \$\endgroup\$ – naveed Jun 2 '15 at 18:15
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    \$\begingroup\$ I've got the feeling that if you clearly state what you want to do you will get better help. \$\endgroup\$ – Vladimir Cravero Jun 2 '15 at 20:32
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    \$\begingroup\$ kjmagnetics.com/… I use KJ magnets and they have a handy magnet calculator. This article explains how to use it. 4 inches distance seems to require a sizable magnet though. \$\endgroup\$ – I. Wolfe Jun 2 '15 at 21:15
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Aside from Hall sensors mentioned in the comments, there are two widespread types of proximity sensors which may have the range you need:

  1. Ultra-sonic sensors
  2. Infra-red / laser based sensors

Ultra-sonic sensors provide good range and coverage (think of car park-aid systems) but they usually have a minimum range which is too big for your purposes. Laser-based sensors only cover one particular direction, but are more precise. Infra-red sensors provide wider coverage. I believe you can find one which will suit you.

Depending on the circuit you need to build, you can choose a sensor with digital interface (I2C, SPI etc) or some sort of analog signal (usually PWM).

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You can use a Proximity sensor for your purpose. Sensors come with the I2C interface which is simpler to build. The range of the Proximity can also be configurable in few sensors. You can configure the beeps based on the output data of the proximity which is directly proportional to the sense distance.

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