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Project Notes

#674 LDR Differential Comparator

Using a differential amplifier to extract a clean trigger signal from a light dependent resistor (LDR).


Here’s a quick demo..



LEAP#034 LDR Comparator used a Wheatstone Bridge to measure changes in ambient light and provide a signal that could be used as a low/high light trigger. While the Wheatstone Bridge allows the output signal offset to be independently set, it is a sensitive signal and prone to “bounce” if used as a trigger.

The circuit I’m testing here filters out much of the noise by measuring the first order derivative of the LDR. It comprises:

  • first op-amp stage configured as a differential amplifier
  • seconds op-amp stage configured as a comparator, to exclude changes below a preset threshold

The circuit is based on’s notes: Designing an Op-Amp Circuit to Detect LDR Pulses

Circuit Design

I’m using LDR 5528 which is specified as dark: 1MΩ, light: 8-20kΩ.



Test Results


For the scope trace below:

  • CH1 (yellow): measuring the voltage at the base of the LDR
  • CH2 (blue): differential amplifier output
  • CH3 (red): comparator output

A nice solid pulse when the LDR is initially obscured.


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.