LED As Photodiode


Posted on Feb 5, 2014

In the project LED Touch Sensing by Jeff Han, I saw a magic happen on a LED matrix. That LED matrix not only lighted but also sensed the touch of fingers. At first, I thought there should be some secret behind and the LED matrix he used must be a special one. After discussing and simple trial with Tom Igoe, I found that any LED could be used as photdiode! Actually, the


LED As Photodiode
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LED, as its name (Light Emitting Diode) implies, is electrically a diode and can be used as a detection device similar to a photodiode. But there are some restrictions: LEDs will only detect light of wavelength shorter than the wavelength of light that the LED would emit if it was put in a circuit that forward biased the LED. To use the LED as an optical detector, do not forward bias the LED into quadrant # 1 of the current-voltage (I-V). (Quadrant 1 is when the operating voltage and current are both positive. ) Allow the LED to operate in the solar cell mode, quadrant #4 (operating voltage is positive, current is negative), or in the photodiode mode quadrant #3 (operating voltage is positive, current is negative - ). I am also working on touch panel simultaneously as an input device for my project daPass. However, I prefer to use LED which can be both input and output device for my project if I can conquer hopefully. The test is doing when I cover the poarta. 0 LED dark enough (seria data value <= 2), tehn the LED on portb. 7 will be triggered to light up. Porta. 0 LED is reversed by its pins to get negative voltage. Below is my code: DEFINE OSC 4 TRISA = %11111111 ADCON1 = %10000010 tx var portc. 6 rx var portc. 7 n9600 con 16468 adcVar var word main adcin 0, adcVar serout2 tx, n9600, [DEC adcVar, 13] if adcVar<=2 then high portb. 7 else low portb. 7 endif pause 100 goto main In this step, I used 2x2 LED matrix as...




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