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Project Notes

#047 Stereo Low-light Trip Detector

Monitor two LDR sensors for stereo light-level triggering effects.

The Build

Here’s a quick video of the circuit in action:

LDRStereoTripDetector demo


This demonstrates two circuit concepts:

  • a light sensor comprising a light-dependent resistor (LDR) and Wheatstone Bridge.
  • a threshold detector using an OpAmp comparator.

Calibrating the Wheatstone Bridge

The particular LDR used has a range of about 200Ω (bright light) to 20kΩ (dark). However the ambient light conditions used for testing the circuit exhibited a typical range of 2kΩ-5kΩ, hence 4.7kΩ is selected as the common resistor value in the bridge so that we can drive positive and negative voltages from the bridge. A 50kΩ variable resistor opposes the LDR, so that the actual threshold level can be trimmed.

The voltage differential measured across the bridge - from the base of the pot to the base of the LDR - will tend to rise in lighter conditions as the LDR resistance drops. As conditions get darker, the LDR resistance rises until a point there the voltage differential will go negative.


This voltage differential is fed to an OpAmp comparator (an LM324 Low Power Quad Op-Amp). By feeding the LDR end of the differential to the negative input of the comparator, it acts as a low-threshold trigger. Reversing the inputs to the comparator could be used for a high-threshold trigger.

Note: the circuit as described does not follow best practices for tethering the unused OpAmp units in the LM324 since this is only a test and it didn’t have a noticeable impact on behaviour.


For plots of received light to triggering, see the LDRComparator project. It is pretty much identical to this circuit except uses a single LDR only and monitors results with an Arduino.


The Breadboard

The Schematic

The Build

Credits and References

About LEAP#47 OpAmp
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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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