How to Select a Photoresistor: 6 RFQ Checks

Select a photoresistor by its resistance at a stated illuminance, dark-resistance timing, gamma, spectral response, response time, electrical limits and mechanical light path—not by disk diameter alone. A useful RFQ defines the real light source and switching threshold, then asks the supplier to test samples under the same conditions. This prevents a part that looks correct on a bench from saturating, drifting or reacting too slowly inside the final enclosure.

1. Define the light condition before choosing resistance

State illuminance, source spectrum and measurement geometry

“Bright” and “dark” are not engineering test conditions. State the expected illuminance at the sensing face in lux, the type of source, distance, angle, enclosure window and any diffuser or filter. A CdS cell responds differently across wavelength, so two sources that look equally bright to a person may not produce the same resistance. The Advanced Photonix NSL-A603 series, for example, specifies light resistance at 10 lux using a 2856 K source. Those are external reference conditions, not LORIDA specifications.

Include the lowest and highest usable illuminance rather than only the room-light target. If the application is simply “day or night,” define the decision threshold plus margin for dirt, window aging and unit variation. If the system must report calibrated lux, a digital photometric sensor may be more suitable; Adafruit explicitly describes common photocells as basic change detectors rather than precision lux instruments.

2. Compare light resistance and dark resistance correctly

Keep the timing and adaptation history with every value

Light resistance is normally specified at a defined illuminance. Dark resistance can depend strongly on how long the cell has been dark after exposure. The NSL-A603 datasheet defines dark resistance 5 seconds after 10 lux. Its A6031 example lists light resistance from 60 to 130 kΩ at 10 lux and a minimum dark resistance of 1.2 MΩ. These figures illustrate why a purchasing comparison needs both the numerical limit and the test method.

Do not convert a single typical value into a guaranteed threshold. Production spread, temperature, previous illumination and enclosure leakage can shift the divider voltage. Ask for minimum and maximum resistance limits at the agreed bright and dark points, then calculate the worst-case ADC or comparator voltages. A sample should pass both limits after the same light-adaptation and dark-recovery sequence that will be used in incoming inspection.

3. Use gamma to predict behavior between test points

Match slope, not just one resistance value

Gamma describes the slope of resistance versus illuminance on a logarithmic scale. The referenced NSL-A603 variants show typical gamma values from 0.65 to 0.85 over 10 to 100 lux. Two cells can meet the same 10-lux resistance but create different divider voltages at 30 or 80 lux because their slopes differ. That matters when firmware uses multiple thresholds or when the usable range spans more than one decade.

Request the gamma test range and tolerance, or ask for resistance limits at two or more application-relevant light levels. Use those limits to simulate the complete divider. For a pull-down resistor, the measured voltage is Vcc × Rfixed /(Rfixed + Rphoto) or the complementary form depending on placement. Choose the fixed resistor around the region where the decision needs the most resolution; a value suited to dim light can saturate in daylight.

4. Check spectral response and optical construction

Make sure the sensor sees the intended light

The NSL-A603 series lists a typical spectral peak of 600 nm. That does not mean it responds only at 600 nm; it means sensitivity varies with wavelength and peaks there for that family. Describe whether the real source is daylight, incandescent, fluorescent or a specific LED. If a colored lens sits above the cell, provide its material or transmission curve because it can move the effective response away from the laboratory result.

Mechanical details are part of the optical design. Specify active-face diameter, body diameter, height, lead pitch and length, orientation, coating, window opening and distance to the light guide. Avoid assuming that a larger disk always means greater sensitivity. The electrode pattern, material formulation and optical exposure all matter. Use real internal imagery and the LORIDA photoresistor category to align the requested package before sampling.

5. Verify voltage, power and temperature limits

Keep the divider inside absolute maximum ratings

A photoresistor is passive, but the applied voltage and self-heating still require limits. The external NSL-A603 reference gives a maximum peak AC or DC voltage of 300 V, maximum power dissipation of 90 mW at 25 °C, and operating and storage ranges of −30 to +70 °C. Do not apply these values to a LORIDA part unless its own approved datasheet confirms them; use them as examples of fields the RFQ must contain.

Calculate power at the minimum expected resistance and maximum supply. Consider a fault in which continuous bright light holds the cell at low resistance. Include the enclosure hot-spot temperature, not ambient room temperature, and ask whether derating is required. If the board uses mains-referenced circuitry, creepage, insulation and safety approval are system issues that cannot be solved by selecting a high-resistance cell alone.

6. Specify response time and final-circuit validation

Test the transition that controls the product

Rise and decay behavior may be asymmetric and depend on starting light level, final light level and previous exposure. Define how quickly the circuit must recognize light-to-dark and dark-to-light changes, how firmware filters noise, and whether a delayed response is acceptable. A staircase, doorway detector or street-light controller can have very different timing requirements even at the same nominal lux threshold.

Validate samples using the final pull-up or pull-down resistor, ADC reference, comparator hysteresis, cable length and enclosure. Test minimum and maximum supply, hot and cold conditions, clean and partially obscured windows, and repeated transitions. Record raw resistance or voltage rather than only pass/fail. For broader component support, see LORIDA products and send the complete test fixture details through the contact page.

Frequently Asked Questions

Can I select an LDR using only its diameter?

No, diameter alone cannot define electrical or optical performance. Two photoresistors with similar-looking disks may have different 10-lux resistance, dark resistance, gamma, wavelength response, coating and timing. Start with the real light threshold and divider circuit, then specify the mechanical envelope. Ask for resistance limits under stated illuminance and source conditions. Diameter and lead pitch should be treated as fit requirements after the sensing window is defined. Validate at least the darkest, threshold and brightest application points inside the real enclosure rather than comparing loose samples under uncontrolled room light.

Why does the measured resistance change after the light is removed?

Photoresistor recovery is time-dependent, so dark resistance must include a delay and previous-light condition. A reading taken immediately after illumination can differ from one taken several seconds later. That is why the external NSL-A603 reference defines dark resistance five seconds after a 10-lux exposure. For an RFQ, describe the light-to-dark transition and maximum allowed decision time. For incoming tests, use the same adaptation, exposure and wait periods for every sample. Firmware delay or comparator hysteresis can reduce chatter, but it cannot replace verifying the cell’s worst-case recovery behavior.

Should I choose the divider resistor from the nominal LDR value?

Choose it from the worst-case resistance range around the required decision point, not from one nominal number. Calculate divider voltage using minimum and maximum photoresistor limits, supply tolerance, ADC reference tolerance and temperature. A large fixed resistor improves resolution in dim conditions but may saturate under bright light; a small resistor extends the bright range but compresses dark-level resolution. Prototype more than one value if the light span is wide. Confirm the final choice with real optical geometry because enclosure losses can shift the operating point substantially.

What information should a photoresistor sample request include?

Send the illuminance range, light source, switching threshold, circuit, timing, temperature and mechanical drawing. Include supply voltage, fixed-resistor value, ADC or comparator limits, allowable rise and decay time, active-face size, body diameter, lead pitch, lead length, coating and annual usage estimate if available. Photos of the current sensor and enclosure opening help confirm fit, but they do not replace electrical limits. Ask the supplier to identify the datasheet test conditions and any values that are typical rather than guaranteed before samples are approved.

Last Updated: September 9, 2026