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Project Notes

#675 LDR Data Slicer Trigger

Using a data slicer to extract a clean trigger signal from a light dependent resistor (LDR).

Build

Here’s a quick demo..

clip

Notes

LEAP#674 LDR Differential Comparator used a differential amplifier to extract a trigger signal from a light-dependent resistor.

There is a simpler way that just uses a single op-amp unit: a technique called a “data slicer”. It is described in PIC® MCU Comparator Tips ‘n Tricks.

The circuit is based on electronut.in’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.

The essential idea:

  • the op-amp is configured as a comparator between the input signal and a sliding reference derived from the input signal
  • the “sliding reference” is just the input signal passed through an RC low-pass filter (R3 + C1)
  • R4+R5 bias the reference low (a single resistor would have been fine - the pot just makes it convenient to experiment with adjustments)

bb

schematic

Test Results

bb_build

For the scope trace below:

  • CH1 (yellow): measuring the voltage at the base of the LDR
  • CH2 (blue): sliding reference
  • CH3 (red): output

A very nice solid pulse when the LDR is initially obscured.

scope

Credits and References

About LEAP#675 SensorsOpAmp
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, 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 (IMHO!).

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Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.