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Project Notes

#664 Light Control Switch Kit

Building an LDR-controlled relay kit with hysteresis/delay for on and off transitions (LM393 comparator with RC-dampened inputs).


Here’s a quick demo..



Another cheap little kit from aliexpress. I love’em!

This circuit demonstrates how to use a light-dependent resistor to switch a heavy lead (via a relay), but with a dampening delay implemented with an RC circuit squashing the inputs to an LM393 comparator.

The Kit

From aliexpress, but probably widely available with similar specs.




Ref Item Qty
R1 4.7kΩ 1
R4,6 10kΩ 2
R3 10kΩ pot 1
D3 1N4007 1
Q1,2 805 2
C1 104, 100nF 1
C2 10µF 1
J1,2 2 x 2pin headers 4p
J4 KF301-3P, 3pin terminal block (missing/not included in the kit) 1
R2 510kΩ 1
R5,7 2.2kΩ 2
D1 Red LED 5mm 1
D2 Green LED 5mm 1
U1 LM393 1
U1 DIP8 socket 1
J3 DPDT switch 1
K1 relay 1
J2 LDR 1
  PCB 1

Circuit Design



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.