A photoresistor is an electronic component whose electrical resistance changes based on how much light hits it. In darkness, a typical photoresistor has very high resistance (often in the megaohm range), and in bright light, that resistance can drop to just a few hundred ohms. This simple property makes it one of the most widely used light sensors in everyday electronics.
How a Photoresistor Works
A photoresistor is made from a semiconductor material, most commonly cadmium sulfide (CdS). In darkness, the material has very few free electrons available to carry electrical current, so resistance is high and almost no current flows through it. When light strikes the surface, photons transfer energy to electrons in the material, knocking them free from their fixed positions in the crystal structure. These newly freed electrons (and the “holes” they leave behind) can now carry current, so the material becomes much more conductive.
The key requirement is that the incoming light must carry enough energy to push electrons across what’s called the band gap, which is essentially the energy threshold the material needs to release those charge carriers. For cadmium sulfide, this threshold lines up well with visible light, which is why CdS photoresistors respond strongly to the same wavelengths your eyes can see.
Spectral Sensitivity
CdS photoresistors have a peak spectral response between 520 and 650 nanometers, which covers the green-to-red portion of the visible spectrum. This closely approximates the sensitivity of the human eye, making CdS sensors a practical stand-in for measuring light the way people actually perceive it. Outside the 490 to 700 nanometer range, response drops below 50% of the peak, and sensitivity falls off sharply beyond 320 nm (ultraviolet) and 770 nm (near-infrared).
Resistance Range and Light Response
The relationship between light intensity and resistance is not linear. It follows a logarithmic curve, meaning that on a log-log plot, resistance drops in a roughly straight line as light increases. A common approximation converts resistance directly to illuminance in lux using a power-law equation. In practical terms, this means a photoresistor is very sensitive to small changes in dim light but less sensitive to equivalent changes in bright light.
In complete darkness, resistance can climb into the megaohms. Under bright room lighting or direct sunlight, it drops to hundreds or even tens of ohms. This enormous range (roughly 1,000:1 or more) is what makes photoresistors useful for distinguishing between “light” and “dark” conditions, even if they’re not precise enough for scientific-grade light measurement.
Response Time and the Memory Effect
Photoresistors don’t react instantly. When light hits a photoresistor that’s been sitting in total darkness, resistance drops to its new level in about 10 milliseconds. But the reverse is much slower: when light is removed, it can take up to 1 second for resistance to climb back to its dark value. This asymmetry is called the resistance recovery rate, and it creates a kind of “memory effect” where the sensor lingers at a lower resistance after the light source disappears.
For most applications, like turning on a street light at dusk, this lag is irrelevant. But it means photoresistors are a poor choice for detecting rapid light fluctuations, such as high-speed optical communication signals or fast-blinking indicators. Response delays in those scenarios sit around 0.1 seconds, which is far too slow.
How Photoresistors Are Used in Circuits
On its own, a photoresistor just changes resistance. To turn that into a useful signal, it’s almost always wired into a voltage divider: a simple circuit where the photoresistor sits in series with a fixed resistor, and you measure the voltage at the point between them. As light changes the photoresistor’s resistance, the voltage at that midpoint shifts proportionally. A microcontroller or comparator circuit can then read that voltage and make decisions based on it.
Choosing the right fixed resistor matters. Ideally, it should be close to the photoresistor’s resistance at the light level you care about most. If you’re building a circuit that triggers at dusk, you’d pick a fixed resistor in the tens-of-kilohms range, roughly matching the photoresistor’s resistance at twilight light levels. This maximizes sensitivity right around your switching point.
Common Applications
Photoresistors show up in a surprising number of devices:
- Automatic street lights that switch on at dusk and off at dawn
- Camera light meters that help determine proper exposure
- Clock radios that dim their displays in dark rooms
- Smoke detectors where smoke blocks a light beam aimed at the sensor
- Laser security systems that trigger an alarm when a beam is broken
- Audio compressors in professional equipment, where an LED and photoresistor pair controls signal gain based on volume levels
The theme across all of these is the same: the circuit needs to know whether light is present, absent, or at a certain intensity, and it doesn’t need to measure that with laboratory precision or at high speed.
Cadmium Restrictions and Alternatives
The most common photoresistor material, cadmium sulfide, contains cadmium, which is classified as a hazardous substance under Europe’s RoHS (Restriction of Hazardous Substances) directive. This means CdS photoresistors face manufacturing and import restrictions in many markets. An exemption once allowed their use in analog optocouplers for professional audio equipment, but that exemption expired at the end of 2013.
As a result, cadmium-free alternatives have become more common in consumer products. Phototransistors and photodiodes, which use silicon or other non-toxic semiconductors, can fill many of the same roles. They respond faster and avoid the regulatory issues, though they lack the simple two-terminal design and the wide resistance swing that made CdS photoresistors so easy to work with in basic circuits. For hobbyist projects and educational use, CdS photoresistors remain widely available and inexpensive, often costing less than a dollar each.