Does anybody know what is the technology the DUPLO Action Bricks are based on?

It is not any RFID or NFC because I printed out the white one on normal white paper which turns lights on/off. The symbol on the brick is not important - even a white paper with a printed symbol of rain drop turns the light on/off.

I'm not able to make coloured ones - blue, red, green or yellow. Is the exact color shade so important? If so, then how can one make their own Action Bricks?


4 Answers 4


It is about color and length

The Duplo action brick uses a color sensor, so you want to use the correct colors and it needs the length of the brick for the sensor to determine the color.

paper replacement (youtube)

How to 3d print your own


Color sensor for some amount of time.

I was able to make green Lego tiles work, if the area I covered was 2x8 (in LEGO scale, not DUPLO) bumps. I put a standard Lego scale 4x8 brick under the Duplo track piece, and then built up the exposed area to the height of the center of the Duplo track, and then put 2 green Lego tiles (1x8) on top. This worked really well, but is not child safe.

Trans green (even with solid green underneath) does not register.
I ran two tests to verify this.

I cut the center tie out of a piece of straight Duplo track.
I used a standard Lego scale 4x12 centered under the piece of track.
This alone, did not cause the train to reverse (due to distance from sensor).
I then built up the area by putting a Lego 2x8 on top, taking the Lego bump top to the same height as the center tie would have been.
This was a tile less deep than my previous test.
This worked, the train did reverse.
This also worked with a 2x10, but only the length of 8 was needed to trigger the reverse.

I then tried using a Trans Green duplo track (without the center tie cut). I put the standard Lego scale 4x12 underneath and then built up the area on each side of the tie with Green Lego 2x4 bricks.
This did not work. The center tie, where the color was trans-green (on top of regular green), was an interruption.

None of my solutions are considered child safe. But if this is not a concern, I recommend the solution that does not alter the train track piece, and using Lego scale 1x8 tiles.

If choking is a concern, the solution where I cut out the center tie, and then use large standard Lego scale green bricks, is a bit safer. But even this solution, where the smallest brick is 2x10 (standard Lego scale), does not meet child safety requirements.

I bought some extra green Duplo smart tiles.


As seen in this question there is a sensor below the train. This is most likely to be a color sensor which is how it detects action brick.


The following is deduced from looking at the train and its reactions on the tiles. (I know about electronics)

It seem that if no reflections are detected within approximtely 1 second, then the train switches off. In normal driving, the gray track parts reflect light on/off. But if you lift the train, then it sees no alternating reflections - and stops.

The colored tiles: Looking at the bottom of the DUPLO train, you see a tiny window. Inside this is a 3 color LED diode. It looks as if the diode is cycling through the colors black (actually off) - red, blue, green, cyan, magenta, yellow, white. These are the colors possible if each diode is just switched on or off (I dont know if it uses brightness modulation - but I think not)

In the window is also some light detector, measuring the amount of reflected light. So a red tile will reflect a lot when the red LED is on, but a green tile will reflect very little when the red is on.

This provides 7 possible tiles for the coding.

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