#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:

Notes

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.

Comparator

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.

Behaviour

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

Construction

The Breadboard

The Schematic