#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..
Notes
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 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.
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.
