1

I'm looking for a way of detecting if 2 pieces of hardware are within 2 meters of eachother or not. The idea is to build a prototype to lock computers automatically. I just need a recommendation on the sensor if it exists. Geofencing, bluetooth etc are not reliable enough.

2
  • Just point a ultrasonic sensor towards the chair in front of the PC. Or add a force sensor to the chair.
    – Gerben
    Commented Feb 16, 2018 at 17:01
  • i would try an ultrasonic emitter for the wrist, like those "mosquito repelling" things, along with a sensitive mic on the other end. Audio fades very reliably at distance; twice as far is 25% as loud, and the dead-simple "buzzer" can be very low power compared to more complicated emitters.
    – dandavis
    Commented Feb 16, 2018 at 20:05

1 Answer 1

1

You can use a lot of different methods with advantages/disadvantages:

  • Use an IR LED/receiver and calculate the distance of travelling of the lights. Disadvantage: you need a very precise timing mechanism (light is fast), also take latency into account for non-light parts, and hardware needs to be in visible range and pointing to each other with LEDs)
  • Use an ultrasonic sensor like SR-HC04. Disadvantage: devices must point to each other in a small precise angle.
  • Using an RF method and sending/receiving e.g. via nRF24L01+ 2.4 GHz trasnceivers, to check the difference in time between sending and receiving when very close or a few meters apart. However, there might be aspects that might distort the result (other receivers/transmitters, what kind of material is in between, nRF antenna placement/direction etc). Using lowest speed (most reliable), highest power.
7
  • I was thinking more of a bracelet like design, which will not work with both of these sensors. What are they using in these ankle bracelets for criminals? ;)
    – Thomas
    Commented Feb 16, 2018 at 14:27
  • 2
    "What are they using in these ankle bracelets for criminals?" GPS
    – Juraj
    Commented Feb 16, 2018 at 14:44
  • I doubt they will use GPS, since you don't have GPS in house and you can cover/circumvent GPS signals easily. So my bet is a signal that is sent and received and a time difference (travel) time is performed calculating distance. Commented Feb 16, 2018 at 14:44
  • no need to monitor a position of the bracelet in the house
    – Juraj
    Commented Feb 16, 2018 at 14:46
  • 1
    I would do some tests starting with nRF24L01+ 2.4 GHz (if you have them) and check if you notice a difference in sending time and receiving time when very close or a few meters apart. However, there might be aspects that might distort the result (other receivers/transmitters, what kind of material is in between, nRF antenna placement/direction etc). Using lowest speed (most reliable), highest power. Commented Feb 16, 2018 at 15:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.