DataSlicer

#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.

Credits and References

• Designing an Op-Amp Circuit to Detect LDR Pulses
• LM358N Datasheet
• PIC® MCU Comparator Tips ‘n Tricks