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Project Notes

#617 Weevil Eye Kit

Building a simple but fun light-sensitive DIY kit, a perfect gift for someone learning to solder.


Here’s a quick demo..



I saw this kit on aliexpress for a few dollars, and the nicely designed PCB convinced me to buy a few.

The circuit is simply a pair of light-sensitive LED “eyes”. Surprrisingly fun to play with, especially for children.

The circuit is nicely laid out with generous, perfectly prepared pads, so it also makes an ideal kit for learning through-hole soldering.

The kit comes with standard 5mm red LEDs. I replaced these with slow and fast flashing RGB LEDs to spice up the effect.

The Kit


Packing list:

  • 1 x WeevilEye PCB
  • 2 x Red LEDs
  • 1 x 47k Ohm Resistor
  • 1 x Miniature Photocell (LDR)
  • 1 x 20mm Coin Cell Battery Holder(Batteries not included)
  • 1 x 2N3904 Transistor
  • 2 x 220 Ohm Resistors



It is a simple circuit, with the photocell/LDR controlling the low-side NPN transistor:

  • when exposed to light, the LDR resistance is lowered pulling the NPN base to ground and cutting the current for the LEDs
  • when in darkness, the LDR resistance rises, allowing the 47kΩ to feed base current to the NPN transistor and turning on the LEDs
  • how hard the NPN is turned on will be proportional to the ambient light on the LDR

Here’s my transcription of the schematic:



And a simple breadboard verification:


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.