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Photocell Switch – A complete Guide

writterfellas December 30, 2021

Have you ever wondered how exactly street lights know when to turn on? They are never too early or too late, they will only shine when the sun goes down and the sky turns black. Of course, they are not operated manually. But they certainly can’t work on the timer, because their daily switching times are slightly different? The answer is photocells, also known as dusk-to-dawn sensors.

1. What is a photocell switch?

The phototube switch is essentially a light-dependent resistor, referred to as LDR. Its resistance decreases as the incident light intensity increases. They are used to turn on exterior lights at sunset and turn off exterior lights at dawn. Because they can perceive the surrounding light levels, photovoltaic cells can automatically adjust to seasonal changes in the day and night cycle, regardless of the influence of daylight saving time.

When you are not at home, the photoelectric cell that controls the external lights and the timer that turns on the internal device will create an illusion of occupancy, which can deter intruders.

Other uses of photocells include turning on parking lots or street lights after dark, adjusting indoor dimmers to compensate for changes in natural light levels, or turning on or off illuminated commercial signs. Photocells control the entire circuit, which is an ideal way to manage safety lighting or landscape lighting.

2. Photo sensor switch: how it works

Using a photocell working on the principle of light-dependent resistance, when the light intensity is low (dark), the resistance value of the LDR will decrease, when the light intensity is about 500 ohms, the resistance value of the LDR will decrease because the resistance value (resistance) on the LDR changes Small, causing current to flow, when the lamp is turned on, and then when exposed to large light intensity (bright), the resistance value will increase. When the dark light irradiates the resistance value of about 200 kiloohms, the resistance value (resistance) on the LDR becomes larger (increase), which causes the voltage of the lamp to be blocked (cannot flow) and the lamp is turned off.

3. How to Wire the photocell Switch

The photocell used for lighting has three terminals, marked as follows:

  • Load line (Lo)
  • Neutral line (N)
  • Power supply or live line (LI)

In most photovoltaic cells, the load wire is red, the neutral wire is white, and the power wire is black. This colour-coding is not universal. Change to other brands of photocells may change. The picture of a certain brand of photocell terminal is as follows:

The wiring and installation of the photocell are very simple, as shown in the figure below:

As shown above, the load wire (Lo) is connected in series to the lighting device, and the neutral wire (N) is connected to all lights through a circuit breaker. The photocell is powered by the power supply line of the circuit breaker.

4. How to install a photocell sensor for outdoor lighting

The following steps will guide you to install the photocell sensor. This project requires some electrical work, so if you feel unsure or safe to perform these tasks, you should contact an electrician to install the photocell for you.

Turn off the circuit breaker of the outdoor light. If you don’t know which switch is supplying power to your lights, turn off all the switches in the building to ensure that the power has been cut off. Turn on the outdoor light switch, check again whether the power is off, and make sure it is not turned on.

Remove the housing containing the external light source. You may want to use photos to record how it was broken down so you can easily reassemble it.

The photocell should have two black wires. Those black wires need to be connected to the black wires that connect the lamps to the main power supply of the building. Disconnect the black wire from the house to the light fixture.

Connect a black wire from the photocell to the black wire from the building. Be sure to tighten the bare copper wire to make the connection tight.

Connect the second black wire on the photocell with the black wire on the lamp, making sure that the copper wires are completely twisted together.

Cover your new connection with an electric cap. Make sure that the cover is tightly wound on the wire.

Use electrical tape to fully glue your connection. Copper wires should not be exposed.

To test the photocell, turn on the power again at the circuit breaker. Make sure that the light switch is on. Cover the photocell with your hand-when the photocell is closed, the light turns on, indicating that the photocell is working properly.

Put your lamps back together to complete the installation of the photocell.

5. Troubleshooting Guide for Photocell Sensor

Outdoor lighting enhances the appearance of the residence, acting as a guide, guiding visitors down the path. Many lighting systems use photocell sensors to automatically activate the lighting, but photocells sometimes fail and require simple troubleshooting procedures.


The photocell sensor is an electronic component, usually, a resistor used to detect the level of light. When the sun goes down, the photocell feels the light gradually diminishes. As the light is reduced, the photocell activates the lighting system.


A common problem that affects the function of the photocell is incorrect or loose wiring between the photocell and the main circuit of the lighting system, It can happen from a bad photocell switch manufacturer. The wires connecting the photovoltaic cell to the lighting circuit need to have a strong solder connection. In addition, the system needs to have proper power. Securing all power connections should be a priority.

Over time, small cracks will appear in the assembly of the photovoltaic cell. These cracks can cause intermittent lighting, or even no light activation at all. Check the photoelectric tube for distortion. If it breaks, replacement is the only option.

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