Testing a photocell takes about five minutes with a basic multimeter, or even less if you just need a quick check with no tools at all. Photocells (also called photoresistors or light-dependent resistors) change their electrical resistance based on how much light hits them, so the core test is simple: measure resistance in darkness, measure it again in bright light, and confirm the values change dramatically. If they don’t, the sensor is bad.
Quick Test Without a Multimeter
If your photocell is already installed in an outdoor light fixture, you can test it without any tools. Cover the sensor with a dark cloth, piece of cardboard, or even your hand to block all light. This simulates nighttime. Within a few seconds (depending on the built-in time delay), the connected light should turn on. Then remove the cover and let natural light hit the sensor. The light should turn off.
If the light activates smoothly when covered and shuts off when uncovered, your photocell is working. If nothing happens in either direction, or if the light flickers rapidly during the transition, the sensor likely needs replacement or adjustment.
Testing With a Multimeter
A multimeter gives you a definitive answer by measuring the photocell’s resistance directly. This is the most reliable method, especially for standalone sensors that aren’t wired into a fixture yet.
What You Need
- Digital multimeter set to resistance (Ω) mode
- Something opaque to block light (tape, cloth, a cup)
- A bright light source like a flashlight or lamp
Step-by-Step Process
First, disconnect the photocell from any circuit. Testing a component while it’s still wired in can give misleading readings because other components in the circuit affect the measurement. If you’re testing a sensor pulled from a light fixture, just work with the bare component on your workbench.
Turn your multimeter dial to resistance mode, usually marked with the omega symbol (Ω). Insert the black test lead into the COM jack and the red lead into the VΩ jack. Before you touch the leads to anything, the display should read “OL” (overload), which just means the circuit is open.
Touch each test lead to one of the photocell’s two terminals. It doesn’t matter which lead goes where since photocells aren’t polarized. Note the resistance reading under your current lighting conditions. Then cover the photocell completely to block all light and watch the resistance climb. Finally, shine a bright flashlight directly onto the sensor and watch the resistance drop.
When you’re done, remove the leads (red first, then black) and turn off the multimeter to save battery life.
What the Numbers Should Look Like
A working photocell shows a huge swing in resistance between dark and bright conditions. In total darkness, a standard cadmium sulfide photocell reads somewhere between 200,000 ohms and 10 megohms. Under moderate light (around 10 lux, roughly equivalent to twilight), that drops to about 10,000 ohms. In bright direct light, it can fall even lower.
The exact numbers vary by model, but the pattern is what matters. You’re looking for a clear, dramatic change. A photocell that reads the same resistance regardless of light level is dead. One that changes only slightly may be degraded and unreliable for switching applications.
Keep in mind that photoresistors respond relatively slowly compared to other light sensors. You may need to wait a few seconds after changing the light level for the reading to stabilize, especially when transitioning from bright to dark. This sluggishness (in the range of tens of milliseconds to a few seconds) is normal for this type of sensor.
Testing a Photocell in a Powered Circuit
Sometimes you need to test a photocell that’s already wired into a transformer or lighting controller and you’d rather not remove it. In that case, you’ll measure voltage instead of resistance.
With the circuit powered on and the photocell covered to simulate darkness, use your multimeter in voltage mode. Place one probe on the common terminal and the other on the photocell’s output tap. A properly functioning photocell will pass the full circuit voltage through (for a 120V circuit, you should read approximately 120 volts, give or take 5 volts).
If you get a reading of zero, try this: disconnect the photocell and install a jumper wire between the input and output terminals, then retest. If the voltage reads correctly with the jumper in place, the photocell itself has failed and needs replacing. If you still read zero, the problem is elsewhere in the circuit.
Signs of a Failing Photocell
Before you grab a multimeter, a visual inspection can reveal obvious problems. Look for cracks in the housing, corrosion on the terminals, or any sign of water inside the lens. Moisture ingress is one of the most common causes of photocell failure, especially in outdoor fixtures exposed to heavy rain, snow, or prolonged direct sunlight. A cloudy or discolored lens can also throw off the sensor’s light readings without the internal components actually failing.
Behavioral symptoms are equally telling. The three most common signs of a photocell going bad are:
- Flickering lights: The connected fixture blinks on and off repeatedly, especially around dusk or dawn when light levels are borderline.
- Delayed switching: Lights come on well after dark or stay on long past sunrise.
- False triggering: Passing car headlights, reflections from nearby buildings, or even moving shadows cause the light to switch on and off erratically throughout the night.
Flickering and false triggering sometimes point to a sensitivity or delay setting rather than a dead sensor. Most photocells have an adjustable lux threshold (a small screw or dial) that controls how dark it needs to be before the sensor activates. There’s also typically a time delay setting that prevents the sensor from reacting to momentary light changes. A delay of 3 to 10 seconds is the sweet spot for most installations. Too short and you get flickering from passing headlights; too long and the light feels unresponsive.
Photoresistors vs. Phototransistors
The resistance test described above works specifically for photoresistors (the cadmium sulfide type commonly found in outdoor lighting). If your sensor is a phototransistor, the testing approach is different. Phototransistors produce a small voltage rather than changing resistance, and they have polarity, so which lead connects where matters.
A simple bench test for a phototransistor involves connecting it with a resistor (around 10,000 ohms) and measuring the voltage across it. In darkness, you’ll see a very low voltage (around 50 millivolts is typical). Under bright light, that rises to 100 to 250 millivolts. Phototransistors also tend to respond faster than photoresistors, though Darlington-type phototransistors can be comparatively slow.
If you’re not sure which type you have, look at the component. Photoresistors usually have a visible squiggly pattern on their face and two identical leads. Phototransistors look more like small LEDs with a clear or tinted dome and may have two or three leads.