#675 LDR Data Slicer Trigger
Using a data slicer to extract a clean trigger signal from a light dependent resistor (LDR).
Here’s a quick demo..
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)
Test Results
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.
