Photoconductive Sensors & Photoresistors

A photoconductor absorbs photons that create mobile carriers, increasing conductivity. Unlike a photovoltaic detector, it does not generate a standalone output: bias is required and the readout observes current or divider voltage.

Transfer law and operating point #

The resistance-versus-illuminance relation is commonly nonlinear and device-specific. A divider converts resistance to voltage,

$$V_o=V_{exc}\frac{R_{sensor}}{R_{fixed}+R_{sensor}},$$

but sensitivity peaks near $R_{sensor}=R_{fixed}$ and collapses near either rail. Choose the fixed resistor from the illuminance range of interest, not a single headline dark resistance.

Bias power warms the detector and may change dark current and sensitivity. Larger active area can collect more optical signal, while capacitance and noise constrain response. In low-frequency infrared systems, optical chopping and synchronous detection distinguish modulated source light from background and drift; useful chopping frequency lies above the detector's $1/f$ region and below its response limit.

Persistent and environmental effects #

Photoconductors can show slow rise/fall, hysteresis-like history, and dark recovery after bright exposure. Temperature changes both dark resistance and sensitivity. Spectral response means equal optical power at two wavelengths need not produce equal current.

When to choose another detector #

Use a photodiode when linearity, speed, calibrated optical power, or low memory matters. A photoconductor remains attractive for simple threshold and ambient-light sensing when its material, temperature range, response time, and environmental restrictions are acceptable.

Verify dark output, response at several calibrated levels, rise/fall after over-range light, temperature shift, source spectrum, and noise over the intended bandwidth.