Build a Light Sensor Circuit with an LED and Op-Amp

Description

This schematic illustrates a method for employing a standard LED as a light detector. It leverages the electrical potential generated across the LED when it’s illuminated. LEDs represent a more economical alternative to photodiodes and incorporate a built-in filter, offering benefits when color discrimination is a requirement in the application. The photovoltaic potential of a red LED – its bandgap voltage – is generally around 2V. The input resistance of this potential is approximately 800MΩ under daylight conditions, escalating to infinity in the absence of light. A TL071 JFET input operational amplifier is utilized for amplifying and bolstering this exceptionally high impedance signal.

Circuit diagram:

Resistor R1 is configured to ensure that the operational amplifier “sees” a 0V input when the LED is in complete darkness. To minimize signal loading, R1 would ideally be a 100MΩ or larger resistor, however, due to the relative scarcity and expense of such high-value components, a smaller value was selected, and the gain of the operational amplifier was increased to compensate for voltage loss. To circumvent the need for a second adjustable resistor to calibrate the operational amplifier’s input offset to zero, R1 must be sufficiently large to allow the reduced voltage across the LED to overwhelm the operational amplifier’s inherent input offset voltage. With a 30MΩ resistor for R1, the voltage at the operational amplifier’s input when the LED is exposed to bright light is lessened to approximately 60mV.

This represents a figure more than four times the 13mV maximum input offset of the TL071 operational amplifier. R1 can be realized as three 10MΩ resistors connected in series. Alternatively, it has been discovered that a reverse-biased 1N4148 diode exhibits an impedance of approximately 30MΩ (it is incorporated into the circuit with the anode connected to ground). The output of the circuit is approximately 0V when the LED is in darkness. VR1 regulates the gain of the operational amplifier, and it should be adjusted to deliver the required output voltage when the LED is exposed to bright light.

TL071 Operational Amplifier

The TL071 is a high-input-impedance, rail-to-rail JFET input operational amplifier. It’s valued for its excellent performance characteristics, including low input bias current, high open-loop gain, and low noise. Its rail-to-rail output allows for full-scale signal swing, making it suitable for a wide range of applications, including instrumentation, sensor amplification, and general-purpose analog circuits. The 13mV maximum input offset voltage allows it to handle very small signals effectively. This IC is frequently used in applications needing precision amplification and buffering of weak signals, especially when dealing with high input impedances. The TL071 is a robust and reliable component, often found in both hobbyist and professional electronic circuits.