Modulated Light Beacons for Orientation in a Room

Christof Eb. · 2025-12-13 15:57

Hi perhaps someone has done something similar or has some other comments? - Thanks in advance!

Idea is:

A robot is moving inside a room.
Goal is to have position and orientation.
It has a fototransistor mounted similar to the robots having a ping sensor, so that it can be turned horizontally with a servo motor. Actually the idea is to use the same servo. The fototransistor is sitting in a tube, so that it's angle of vision is limited. - A 1 pixel camera.
In the room there are at least 2 light beacons, which each have a light (preferable visible). These lights are modulated as square wave blinking on/off but the two (or more) beacons each have their own frequency, to be able to distinguish them.
The signal from the fototransistor is high pass filtered, amplified and fed to a P2 ADC then filtered again either with the Goertzel Smart Pin Mode or with Goertzel Software working on a sampled buffer.

I had done something in this direction using a camera some time ago. It's frame frequency was low and the blinking frequency was also low. Now I am more thinking of something like 10 kHz.

The method seems to be interesting, because a beacon can be build cheaply and the hardware on board of the robot seems also to be very simple, if the P2 can do a lot.

Of course wondering about resolution and possible range.
Cheers Christof

banjo · 2025-12-13 16:14

While not same as you are planning, at least it might give you some ideas: Earlier this year I used modulated IR for navigation in a straight line, see this discussion (you were also participating): https://forums.parallax.com/discussion/comment/1566360/#Comment_1566360
I got a lot of help from @JonnyMac so kudos to him.

Rayman · 2025-12-13 18:08

Hmm... Maybe one would want to use a camera to localize the beacon? In that case would want to use very low frequency, <<60Hz...

Christof Eb. · 2025-12-14 09:03

@banjo said:
While not same as you are planning, at least it might give you some ideas: Earlier this year I used modulated IR for navigation in a straight line, see this discussion (you were also participating): https://forums.parallax.com/discussion/comment/1566360/#Comment_1566360
I got a lot of help from @JonnyMac so kudos to him.

Thank you! There are indeed some valuable hints in your thread!

Christof Eb. · 2025-12-14 09:35

@Rayman said:
Hmm... Maybe one would want to use a camera to localize the beacon? In that case would want to use very low frequency, <<60Hz...

Thanks! Yes, a camera is a possibility. I actually have done some experiments. https://forums.parallax.com/discussion/175897/thoughts-about-robot-navigation-using-blinking-beacons-and-camera-s#latest
In my opinion P2 is not really matching to cameras. Not enough RAM. Also I had problems controlling the camera when brightness was low. The camera then switched to a very slow frame rate.

Idea now is to use the onboard possibilities of the microcontroller with minimal external hardware.

P2 has an ADC, quite good for AC voltage,
a core can be dedicated to analyse the signal.
A Goertzel filter for one frequency does need only minimal RAM.
I already learned, that P2 can indeed receive a long wave (77kHz) radio signal in all of this other stuff in the aether.

Christof Eb. · 2025-12-14 11:32

Made a funny first measurement. Wanted to know if a LDR D=5mm might work here. LDRs are slow but I don't know how slow.
So I varied frequency and up to 500Hz it gives reasonable signal.
Sender is a bright yellow 5mm LED.
At a distance of 7cm I can see the 500Hz at some 3mV.rms in the FFT of my scope.
This seems to work better than my selfmade fototransistor which is a BC107 with a metal cap that was sawn off many years ago. :-)

Perhaps I should try a small solar cell as area seems to help?

Modulated Light Beacons for Orientation in a Room

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