• Arduino Pro Micro board
  • https://www.amazon.com/dp/B01MTU9GOB - 3 for $22.49
• A phototransistor
  • https://www.amazon.com/dp/B00M1PMHO4 - 20 for $5.79
• Some shrink tubes
  • https://www.amazon.com/dp/B072PCQ2LW - a lot for $6.99
• Some sticky Velcro
  • https://www.amazon.com/dp/B000TGSPV6 - 4 for $3.47
• Box of matches or a lighter
  • https://www.amazon.com/dp/B00CKFHYHU - 10 for $5.69

You should be able to make 3 devices for $44.43, or just under $15 plus tax per device...

• Cut a piece of shrink tube about 2 lengths of the sensor and and insert the sensor into it. Align the edges.

• Use a lighter (or matches) to shrink the shrink tube (don't apply the flame directly - just hold it near the tube). Bend the leads as shown - make sure that the longer lead is on the right.

• Insert the light sensor into contacts marked 4 and 6 , bend and solder as shown. Pay attention that the longer lead is still on the right. Do not cut the leads just yet!

• Cut only the left lead. Add some solder to the stub to make it nicer. Bend the right lead onto the stub - this will serve as an activation button. The wire should not be touching the contact - there should be a tiny gap.

• Note: if you soldered the phototransistor incorrectly (i.e. in reverse) - don't worry, there is a "#define REVERSE_PHOTOTRANSISTOR" in the code that can solve the problem

• Add a piece of the loops part of Velcro (the soft part) to the back of the board. I suggest to add a little piece of it to the USB connector as well - it's not really needed there, but it can serve as a reminder that the connector is fragile and to NOT apply torque force to it (when unplugging the cable) - otherwise it WILL break off...