Electronic – arduino – Detect drones passing through hula-hops


I host some drone races at times and I make gates with hula-hops. I fit some led stripes in there and they glow nicely.

We ONLY run micro drones, F36, tinny whoop and those. And mostly indoors (although something that works outdoors would be nice)

I use small nodeMCU boards for lots of projects and I was wondering what would be the best approach to detect the drone.
I'd like to get some advice before starting my tests.

Some of my ideas are:

  • Infrared emitter and receiver at the other side of the circle:
    I have the impression that the drone can pass by without being noticed, maybe two pairs of emitter / sensors but.. sounds fiddly. And not sure about its performance outdoors.
  • Ultrasounds: I think the drone will pass too fast for it to see it
  • Hall effect: Gluing an small magnet to the drone but … runners will hate it.

What do you think guys?
Are there any better alternatives?

Best Answer

Detecting the drone by having it break a light beam sounds like the way to go. It should be easy enough to build emitters and detectors into the hoop. If the drones are small relative to the size of the hoop, then you need multiple light beams so that any drone within the hoop will break at least one of them.

Three emitters might be enough with sufficient detectors spaced around. You blip each emitter for a short time, then look for the signal from the detectors immediately before each blip, during, and after. There needs to be enough of a rise during the blip relative to before and after to consider the particular beam unbroken.

Each blip shouldn't need to be more than a microsecond or maybe a few, depending on the settling time and therefore noise immunity you want from the detectors. Let's say doing a complete blip with before, after, and during measurements takes 20 µs. Actually I think it could easily be half that, but I'm tyring to start with pessimistic values to see where this is at. With 3 emitters, that means you can check all light beams every 60 µs, or 17 kHz rate. That should be good enough.

To do a sanity check on that, let's see how fast a drone would have to be going to move 10 mm during one scan cycle. (10 mm)/(60 µs) = 167 m/s. That sounds quite fast for a drone, and they should occlude a beam over more than just 10 mm. Note that at the speed of sound, a drone would only move about 20 mm during one scan cycle.