I am working on this project to detect anything that is entering the zone of RC helicopter blades (around 10cm diameter) region and stop the rotor. It needs to detect any solid object so metal or plastic or human body.

I was thinking to use ultrasonic sensor such as HC-SR04 placing on top and bottom of the rotor in such an angle that it just scans the region right outside blades (because I don't want the spinning blades to cause any feedback). But since it only has 30 degree angle of accuracy and I need a full 360 degree sweep (because object could approach in any direction), it means I would need more than 8 of them, which is too many. Then I was thinking to let 3 or 4 of them spin at top along with the rotor shaft in a slower rpm. But I am afraid that since it's not guaranteed the same receiver gets from his pair transmitter, the time will be mixed up, which might cause Arduino to think something is close when it's actually far away.

I was also thinking of using maybe infrared sensor such as SHARP 2Y0A21, and place them on top and bottom, but it also needs many of them to cover the 360 degree which is not ideal.

Therefore I am really confused at what is the best approach and what sensor/implementation should I apply? I realise my wording is hard to understand, so please don't hesitate to ask me any question.

Thank you!

PS: at this moment can assume the RC helicopter position is fixed and only consider the rotor system (blade, motor, sensor system). And the system should detect object in advance so the blades can slow down. for example if it's hand approaching, the blades should not injure the hand.

  • \$\begingroup\$ Your requirements are a bit confusing. If you say that you need to detect a "hand approaching" you must specify the relative speed of the hand and the helicopter. Also, you need to tell us how long it takes for the blades to "slow down". Alternatively, you could specify a detection zone (minimum and maximum distance from the blade tips) that is big enough to account for objects moving toward the blades. \$\endgroup\$
    – Joe Hass
    May 8, 2014 at 18:23
  • \$\begingroup\$ Hi, thanks for the question. Yes I am thinking to have a detection zone, which will compensate the time for rotor to stop. In this case the blades can be sampled as strings so they will wrap around the shaft when rotor slows down so it should be fairly fast to contract. So yes what suggestion do you have for the detection zone? \$\endgroup\$
    – user41547
    May 8, 2014 at 18:50
  • \$\begingroup\$ What are the blades made of? What is their rotation speed (RPM)? \$\endgroup\$
    – Andy aka
    May 8, 2014 at 19:29
  • \$\begingroup\$ [[you must specify the relative speed of the hand]] -> (1) 1-2 m/s typical. 5 m/s max. [2] NO. Doing so is useful but hands move at the speed that people wave them. This can be refined and is NOT a "must" at first. Accidental "walk into blades" is a main problem ||| [[... and the helicopter.]] -> The helicopter was specified as stationary - see his text. \$\endgroup\$
    – Russell McMahon
    May 9, 2014 at 0:59
  • \$\begingroup\$ [[ Also, you need to tell us how long it takes for the blades to "slow down".]] -> [1] "Adequate". [2] 0.1s possible. 0.5s more likely . 1s if able to be accommodated .[3] Not so essential as to close the question. Roll with it. Address the main issues. || [[Alternatively, you could specify a detection zone (minimum and maximum distance from the blade tips) that is big enough to account for objects moving toward the blades.]] -> Yes. Assume 100mm 500mm & about 1m from blade when discussing answers. \$\endgroup\$
    – Russell McMahon
    May 9, 2014 at 1:02

1 Answer 1


You might want to take a look at wide angle photodetectors. For example, the OP993 has a viewing angle of 118 degrees and is fairly cheap. If you look deeper on google and on online stores like mouse or digikey you might be able to find some with 180 degrees viewing angles.

This is just a guess, but I would avoid ultrasonic sensors since the vibration of the rotor/helicopter might be in that frequency range. Even if it is not, large non-ultrasonic vibrations could interfere with the sensor.


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