Light Detector

Posted on Feb 6, 2014

With the comparator output driving an LED, the next step will be to sense the intensity of the LED. This lab will build a light detector circuit using an op amp and an everyday light emitting diode (LED). It`s common knowledge that when a current flows through an LED it emits light, but you may not have known that they also work in reverse. When a

Light Detector
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n LED is exposed to light, it will create a small current from cathode to anode due to the photovoltaic effect. Using this property of LEDs, we can use them not only to produce light, but to detect it as well. This small current then can be amplified into a large voltage through the use of a transimpedance amplifier. However there are some limitations to this approach. First, LEDs best detect light at wavelengthsshorter thanthe light they normally emit. This circuit will first be performed using the same type of blue LED that is used in the PWM Generator  lab. This circuit will output a voltage that is proportional to the intensity of ambient light. The higher intensity the incident light the LED is exposed to, the higher the output voltage of the ambient light sensor circuit will be. The following schematic will be used to sense ambient light. Build the circuit in the schematic below. Connect V_out to one of the analog input channels so that it can be observed on the oscilloscope. This circuit can be described very simply. When light shines on a LED there is a corresponding current that flows backwards through the LED (direction shown by red arrow in Figure Z. 1). Also, by characteristics of ideal operational amplifiers, both input terminals equal 0 volts. The blue arrows represent the direction of current that will be used to solve for output voltage in terms of resistance. This means that a very small current can...

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