A photocell is a light-sensitive component that is attached to an electrical circuit or device for a range of applications. They are particularly useful for dusk to dawn lighting that automatically switches on when light levels are low, but they also have applications in other areas as sensors for things like automatic doors or intruder alarms. Learning more about how photocells work and what they're used for helps you understand how they enable dusk to dawn lighting and can alert an alarm system to the presence of an intruder.
What Is a Photocell?
Photocell is a shortened version of photoelectric cell, but the shortened term is much more commonly used. A photocell is a small electronic component with a face that contains a resistor, electrodes (along with wire terminals so you can attach it to a circuit), and a transparent cover over the top that allows light to pass through. The face is usually about one-fifth of an inch in diameter, although they may be larger. You can find photocells that respond to any type of light, including visible light and infrared.
Photocells have a high resistance in the darkness but a much lower resistance when they're exposed to light. Because they respond to light hitting the surface of the cell, they can be used as a switch that is responsive to light levels in the environment. This makes them useful for a wide range of devices.
How Does a Photocell Work?
A photocell works based on the photoelectric effect and the phenomenon of electrical resistance. If you want to know more about resistance, the ohm Lumen Learning page has a detailed explanation, but basically, it's like friction except for the flow of electrical current. In short, when the voltage is the same, a higher resistance leads to a lower current and vice versa.
Photoelectric effect is the name given to the production of a current when light is shined onto a conductor because light is a form of electromagnetic energy just like an electric current. Combining these concepts, a photocell has a high resistance but also responds to light as described by the photoelectric effect. So, the resistance is high except for when the resistor is exposed to light. Then the photoelectric effect generates additional current as if the resistance has become lower. This change in apparent resistance can be used to trigger various effects when it's incorporated into a device.
Use in Dusk to Dawn Lights
The variation in resistance produced by the presence of light in a photocell is used in dusk to dawn lights. During the day, a sensor on the light is continuously exposed to light, and the resistance is lowered. As the sun goes down at dusk, the light level decreases, and the resistance increases. This change in resistance triggers the light to switch on, meaning that dusk to dawn lights turn on at night without the need to estimate the time of sunset with a timer switch or turn them on manually. When the sun comes back up at dawn, the photoelectric effect starts playing a role again. The resistance lowers, and the light switches off.
Other Uses for Photocells
Photocells can have other uses when incorporated into devices. For example, if you have a photocell that is sensitive to infrared light in a device with an infrared light source shining at it, there is a constant current in the device unless something blocks the light shining on the detector. This basic idea is used for automatically operating faucets in bathrooms and to detect intruders in home security systems. In both cases, something blocking the beam of light triggers the change in resistance and the intended behavior from the device.