Medium

Photodiode Light Meter

A photodiode generates a small current proportional to the light falling on it — amplified through an op-amp, that current becomes a voltage easy to read on a multimeter, like a simple light meter.

Circuit Diagram

+9V −9V / GND Photodiode BPW34 (reverse-biased) pin 2 (−) LM358 pin 3 (+) → GND R feedback (large) pin 1 OUT → multimeter Transimpedance amp — output voltage rises with light intensity

How to Build It

  1. Place the BPW34 photodiode on the breadboard, reverse-biased (photoconductive mode).
  2. Connect the photodiode to an LM358's inverting input, with a large feedback resistor between the output and the inverting input (transimpedance configuration).
  3. Connect the non-inverting input to GND.
  4. Power the op-amp between the positive and negative rails.
  5. Measure the output voltage with a multimeter under different lighting — the reading should increase with light intensity.

Bill of Materials

ProductCodeQty
Integrated circuit — 1
Photodiode BPW34-V 1
Resistor DR47K-8/9 1
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