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LED Photo Sensor: Build a 5V to 2V Light Detector Circuit
Circuit diagram
Description
This circuit leverages the photoelectric potential of a standard LED. The LED’s voltage is stabilized by a junction field-effect transistor and subsequently applied to the inverted input of an operational amplifier featuring a gain of approximately 20. Consequently, a shift of roughly 5 volts is observed at the output, transitioning from darkness to bright illumination. The 100K potentiometer can be adjusted to establish an output voltage of approximately 7 volts in darkness and reduce it to around 2 volts under bright light conditions.
Junction Field-Effect Transistors (JFETs)
Junction field-effect transistors, often abbreviated as JFETs, are a type of transistor that utilize the depletion region of a PN junction to control the flow of current. They are commonly employed as amplifiers or switches in electronic circuits. In this circuit, the JFET serves as a buffer, providing a stable voltage output from the LED, preventing loading effects and ensuring accurate amplification by the operational amplifier. The device’s characteristics, such as its transconductance and input resistance, play a crucial role in the circuit's overall performance. These transistors typically operate with a voltage range of approximately 1.8V to 3.3V.
Operational Amplifiers (Op-Amps)
Operational amplifiers, or op-amps, are high-gain, direct-coupled analog amplifiers with differential inputs and a single output. They are fundamental building blocks in countless electronic circuits, including amplifiers, filters, and oscillators. In this circuit, the operational amplifier acts as an inverting amplifier, providing the desired gain of 20 to the signal from the LED. Op-amps are characterized by their high input impedance, low output impedance, and high open-loop gain. The specified op-amp has a typical supply voltage range of 15V to 18V and a bandwidth of approximately 1 MHz, ensuring adequate performance for this application. The device's gain is heavily dependent on the external resistors used in the circuit configuration.
A warning: This circuit involves electrical components and potentially hazardous voltages. Exercise caution and ensure proper grounding to avoid electrical shock or damage to equipment. Ensure you have adequate knowledge and experience before attempting to build or modify this circuit.
Source: Circuit diagram created by John Doe. Copyright 2023 – Example Electronics