Using LED As A Light Sensor


Posted on Feb 7, 2014

This circuit shows how to use an ordinary LED as a light sensor. It makes use of the photovoltaic voltage developed across the LED when it is exposed to light. LEDs are cheaper than photodiodes and come with a built-in filter, which is useful when the application involves colour discrimination. The photo-voltage of a red LED (its bandgap voltage)


Using LED As A Light Sensor
Click here to download the full size of the above Circuit.

is typically about 2V. The source impedance of this voltage is about 800M © in daylight, rising to infinity in darkness. A TL071 JFET input op amp is used to amplify and buffer this extremely high impedance signal. Resistor R1 ensures that the op amp "sees" a 0V input when the LED is in total darkness. To avoid undue loading of the signal, R1 would ideally be a 100M © or larger resistor but since such high values are rare and expensive I used a smaller value and increased the gain of the op amp to compensate for the voltage loss. To avoid the need for a second variable resistor to set the op amp`s input offset to zero, R1 must be large enough for the reduced voltage across the LED to swamp the op amp`s input offset voltage. With a 30M © resistor for R1, the voltage at the op amp input when the LED is exposed to bright light is reduced to about 60mV. This is just over four times the 13mV maximum input offset of the TL071 op amp. R1 can be three 10M © resistors in series. Alternatively, I have found that a reverse-biased 1N4148 diode has an impedance of about 30M © (connect it in the circuit with the anode to ground). The output of the circuit is about 0V when the LED is in darkness. VR1 sets the gain of the op amp and it should be adjusted to give the required output voltage when the LED is exposed to bright light.




Leave Comment

characters left:

Related Circuits

  • New Circuits

    .

     


    Popular Circuits

    Stepper Motor Driver Circuit
    R/C On/Off Motor Controller with Brake
    Nokia 7610 MMC Corrupt Problem Picture Help
    The ICL7135 Integrating A/D Converter Functional Diagram and Datasheet
    darkness activated alarm
    Radiation Detector and Meter DT-590A/PDR-56F Scintillation Probe
    Door Lock on Go
    10W Audio Amplifier with Bass-boost
    555 Monostable Multivibrator



    Top