This is a simple project i have made as a part of a treasure hunt that organized by our Computer Science teacher Mr. M. Vamvakaris and took place in the junior high school where i am working as a Science teacher.
There are 8 IR leds driven by a 74HC595 shift register. Pushing one of the buttons A or B or C the arduino sends a number to the shift register that it is translated in binary and the corresponding IR leds light up. The students see that pushing the buttons light up the red leds near the buttons but of course they cannot see the light from the IR.
We told them that something was hidden into the light and that the 8 leds were IR. They knew from the science lessons that they can "see" the IR transmission using a telephone camera (not a good quality one like iphone's that has a IR filter) so they managed to see the IR leds, calculated from binary to decimal, found the three numbers, sum them and unlocked a 3 digit number lock to get the clue.
What do you need
1x arduino or any compatible MCU
2x large breadboards
1x 74HC595 shift register
8x IR led
8x 1KΩ resistors for the IR leds
3x Red led
3x 560Ω resistors for the red leds
3x push buttons
3x 560Ω pull down resistors for the push buttons