 Photoresistor

A photoresistor, also called a light-dependent resistor or LDR, is a two-terminal component whose resistance changes with incident light. A common photoconductive implementation uses an illuminated semiconductor between contacts; an external circuit converts its resistance change into a voltage or current signal. A manufacturer’s CdS datasheet explicitly identifies these names and describes resistance decreasing with visible illumination.^[ref]

The photoconductor mechanism is a change in conductivity through generated mobile carriers. A photoresistor’s resistance is consequently not a calibrated unit of optical power by itself. Material, wavelength distribution, temperature, illumination history and the measuring circuit help determine the observed response.

For an original illustrative model over a limited range, let $R(P)=R_0(P/P_0{)}^{-\gamma }$ with $\gamma >0$, fixed spectrum and geometry. Choose $R_0=20\mathrm{k\Omega }$ and $\gamma =0.7$. Four times the reference optical input gives $R\approx 7.579\mathrm{k\Omega }$, rather than one quarter of the original resistance. This stipulated power law illustrates a nonlinear response, not a fitted specification for the cited part.

In a divider with supply $V_s$ and fixed resistor $R_L$, measuring across $R_L$ givesAt $V_s=3.3\mathrm{V}$ and $R_L=10\mathrm{k\Omega }$, the model output rises from $1.10\mathrm{V}$ at $P_0$ to about $1.877\mathrm{V}$ at $4P_0$. The logarithmic sensitivity is greatest when $R(P)=R_L$, where it is $\gamma V_s/4$. Choosing a resistor therefore selects an operating region; it does not remove the material’s response law.

The electrical input must also be characterized. The divider seen at its output has Thevenin resistance $R(P)\parallel R_L$. A finite-input-resistance voltmeter or an analog-to-digital converter with a switched input can change the reading. Self-heating depends on the detector’s actual voltage and resistance, while photoconductor response time can make a rapidly sampled value differ from its settled value.

Photoresistor spectral calibration is essential when interpreting an illumination reading. Lux is photometrically weighted, so equal illuminance from different spectra need not produce equal absorption or equal resistance. The cited manufacturer specifies source conditions for light-resistance measurements, a waiting time for dark resistance and light-adaptation preconditioning before its tests.^[ref] Those conditions describe what was tested; they should not be silently omitted from a comparison.

A proposed characterization uses a stable monitored light source, fixed detector position and a controlled spectrum, stepping illumination in both directions and recording the settled resistance and transient. Measure actual bias and temperature, then test the intended readout separately. A component can work well as a light/dark threshold sensor while requiring substantially more characterization to estimate optical power quantitatively.