Typically, an LED is used as a light emitter, but it can also function as a light detector (a photodiode). When light is shone on an LED, it generates a small amount of current due to the photovoltaic effect. This effect is similar to how photodiodes operate, although LEDs are not optimized for this purpose. One can use light sensor like photodiodes as illustrated in the tutorial Photodiode Light Detector with Arduino and LDR which was demonstrated in the tutorial Light Dependent Resistor (LDR) Light Detector Alarm with Arduino, however in dire situation you can also use a typical general purpose LED, which is explained in this tutorial with light sensor circuit diagram.
Using an LED as a Light Sensor in the Circuit
Photovoltaic Effect in LEDs:
- When light falls on the LED, it generates a small photocurrent. The LED operates in reverse bias mode, which allows it to detect light and produce an electrical signal.
Current Amplification with Darlington Pair:
- The small current generated by the LED is usually too weak to be useful on its own. Here, a Darlington pair made of two 2N3904 NPN transistors is used to amplify the current significantly. This amplification makes it possible to detect even low levels of light effectively.
Signal Output to Microcontroller:
- The amplified signal can then be connected to an analog input pin of a microcontroller, allowing you to read the light intensity as a voltage level. The output LED (D2) provides visual feedback, illuminating when sufficient light is detected by the sensor LED.
Circuit Limitations and Considerations
While this approach can work, it’s important to understand its limitations and factors that might affect its performance:
1. Sensitivity:
- Low Sensitivity: LEDs are less sensitive than photodiodes or phototransistors, meaning they require more intense light to generate a detectable current. The Darlington pair helps increase sensitivity by amplifying the small current.
- Light Wavelength: LEDs are more responsive to specific wavelengths of light, usually those close to their emission spectrum. For example, a red LED is more sensitive to red or infrared light than to blue or green light.
2. Signal Strength:
- The generated current is typically very small (in microamperes), so it is crucial to minimize any noise or interference in the circuit. The Darlington pair effectively boosts this weak current to a more usable level.
3. Environmental Conditions:
- Ambient Light: The circuit's performance can vary significantly with changes in ambient lighting conditions. It may require calibration or adjustments, such as shielding from ambient light, for consistent results.
- Temperature: LEDs can exhibit varying sensitivity at different temperatures, which might affect the consistency of light detection.
4. Calibration:
- You may need to calibrate the circuit to ensure accurate readings. This involves adjusting the resistor (R1) values or the gain of the Darlington pair to match the expected lighting conditions.
Circuit Schematic
Here's a simplified schematic of the light detection circuit using a common LED:
Practical Considerations
Component Choice:
While 2N3904 transistors are used here, any general-purpose NPN transistors suitable for your voltage and current requirements will work. If you're looking for higher gain, consider more sensitive transistors.Sensitivity Tuning:
Experiment with resistor values to find the best sensitivity level for your application. A potentiometer can also be used in place of R1 for easier adjustment.
Conclusion
Using a common LED as a light sensor is a cost-effective and creative approach, especially in DIY projects and prototyping. While it does have limitations in sensitivity and response speed compared to dedicated photodetectors, it serves well in applications where precision is not critical.
For applications requiring high sensitivity and accuracy, consider using dedicated photodiodes or phototransistors. However, for hobbyist projects and educational purposes, this LED-based light detection circuit provides a fascinating exploration of unconventional electronics applications.
Resources
# Light and Darkness sensor Circuit with LM393
# Light Dependent Resistor (LDR) Light Detector Alarm with Arduino