💡 Light-Sensitive Relay Module: Understanding the Photoelectric Switch
The 1V DC Light-Sensitive Relay Module is a standalone optoelectronic switch designed to automate devices based on ambient light intensity. It integrates a photosensor, a comparator circuit, and a mechanical relay to convert an analog light measurement into a binary (ON/OFF) switching action. This is the foundational technology for systems like automated streetlights and dark-activated security devices.
I. Core Principles: Sensor and Comparator Circuit
The module's intelligence lies in converting the continuous change in light into a clear digital command based on a preset threshold.
1. The Light Dependent Resistor (LDR)
The sensor used is typically a Light Dependent Resistor (LDR), also known as a photoresistor. The LDR operates on the principle of photoconductivity:
Darkness: In low-light conditions, the semiconductor material (e.g., Cadmium Sulfide) has high resistance (up to several MOmega).
Light: When exposed to light, photons excite electrons into the conduction band, decreasing its electrical resistance (down to hundreds of ohms).
2. The Voltage Divider and Comparator
The LDR is connected in a voltage divider circuit, which converts the change in resistance into a corresponding change in voltage. This analog voltage is fed into one input of a Comparator IC (often an LM393).
The Potentiometer: The other input of the comparator is connected to the potentiometer (the adjustable sensitivity screw). This potentiometer sets a stable reference voltage (the threshold).
The Switching Action: The comparator constantly compares the analog voltage from the LDR with the fixed reference voltage from the potentiometer.
If the LDR voltage exceeds the reference, the comparator output goes HIGH or LOW (depending on the circuit's design).
This output state (the result of the comparison) drives a transistor, which then switches the main relay coil.
II. The Relay and Output Pinouts
The relay is the final output stage, electrically isolating the low-voltage control circuit (12V DC) from the high-voltage load (up to 250V AC).
| Terminal | Full Name | Description | Default State (Relay OFF) |
| COM | Common | The central pivot for the switch. This is where the power source for the load (e.g., the light bulb) is connected. | Connected to NC. |
| NC | Normally Closed | The terminal that is connected to COM} when the relay is de-energized (OFF). | Conduction (Load ON) |
| NO | Normally Open | The terminal that is disconnected from COM when the relay is de-energized (OFF). | No Conduction (Load OFF) |
Application Examples:
Night-Activated Light (Dawn-to-Dusk): Connect the light between COM and NO. The relay is OFF during the day (light present), meaning the NO contact is open. When it gets dark, the relay activates, NO closes, and the light turns ON.
Day-Activated Device: Connect the device between COM and NC. The relay is OFF when dark, so the NC}$ contact is closed, keeping the device ON. When the sun rises, the relay activates, NC opens, and the device turns OFF.
III. Configuration and Protection
Sensitivity Adjustment: The potentiometer must be calibrated in the target environment. To set the relay to trip at a specific lighting level, the operator brings the ambient light to the desired threshold, and then slowly turns the potentiometer until the relay clicks (BLUE LED changes state).
Anti-Reverse Protection: The module includes a series diode at the input power pins. This diode prevents current flow if the 12V DC source is accidentally connected with reversed polarity, thus protecting the sensitive comparator IC and the relay coil from damage.