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31 July 2007

LED Light Emitter and Detector

Dr. Clark Radcliffe, Professor Mechanical Engineering Michigan State University East Lansing Michigan 48824 Reference: R. Stojanovic and D.Karadaglic, "Single LED Takes On Both Light Emitting and Light Detecting Duties", Electronic Design, Vol. 55, No. 16, 7.19.07, www.electronicdesign.com LEDs can be both emitters and detectors of light. When biased forward, the emit light with a characteristic "diode drop" in voltage across the diode at a current of 10-20 mA. When subjected to light, they generate a backwards biased current proportional to the light striking the diode. The photocurrent generated by a typical photodiode LED is about 50pA. The light emitted and the detected are at nearly the same frequency. A green-yellow LED emits at a wavelength of about 555nM while is more sensitive to light at 525nm. Although a photodiode designed for light emission has a small photocurrent as a detector, this current is readily detected by a microcontroller like the BasicStamp using the RCTIME function. The LED used as a photodetector (Figure 1) has an analytical model consisting of a parallel combination of a current source represented by the LED's response to external light and a very small internal capacitance of the LED structure. The photo current ir is about 0-50 pA, the internal capacitance Cr is about 10-15pF and the maximum BasicStamp load current i L is less than 1 A and is ignored in the model.. The voltage Vcr across the LED is

Vcr (t) = Vcr (0) -

1 Cr

i (t) dt

r

(1)

ir

Figure 1: LED Photodetector Circuit and Analytical Model (from: Stojanovic and Karadaglic, 2007)

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31 July 2007

LED Voltage Vcr vs Photocurrent ir (Cr = 10 pF)

6.00E+00 5.00E+00 4.00E+00 3.00E+00 2.00E+00 1.00E+00 0.00E+00 0 ir=25uA ir=50uA

Vcr(t)

0.001

0.002

0.003

Time, t

Figure 2: LED Voltage Vcr versus Time for Different Photocurrents i r . Note that the LED voltage crosses Vcr = 1.4 volts at ~14 ms for ir = 25uA and ~7 ms for ir =50 uA If the LED is initially "charged" to Vcr (0) = 5 volts, the time required for the voltage to drop to 1/0 threshold voltage (1.4 volts) is directly related to the average photocurrent ir. This behavior is evident in the simulated responses (Figure 2) which show the behavior for Cr = 10pF and ir = 25A and 50 A. The BasicStamp can measure the time required for the LED voltage to drop directly using the RCTIME function. In RCTIME, each time count equals 2 sec so that we would expect RCTIME to return values of 3500 and 7000 for the above cases. The circuit used to detect light can also be configured to emit light (Figure 3). In this application, the resistance on pin P1 is used to limit the driving current and 220 Ohms yields a current through the LED of about 10 mA. The use of the LED in both modes is demonstrated in using the circuit in Figure 3 and the PBASIC code attached.

Figure 3: LED Photoemitter Circuit l (from: Stojanovic and Karadaglic, 2007)

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31 July 2007

My office ambient light

My hand shades the sensor

Figure 4: Typical Results for a Red LED on the Board of Education

Figure 5: Red LED Test Circuit on Board of Education

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31 July 2007

'{$PBASIC 2.5} 'file:LED_Emitter_Detecter.bs2 'Clark Radcliffe 'Michigan State University 'July 30, 2007 'Requires an LED in series with a 220 Ohm resister 'between pin 1 and the LED connected to pin 2 'Declarations Pin2 PIN 2 Pin1 PIN 1 light VAR Word time CON 2000 'Code DEBUG "Start test...", CR DO DEBUG "LED is an Emitter (LED ON)", CR OUTPUT Pin2 OUTPUT Pin1 Pin1 = 0 Pin2 = 1 PAUSE time DEBUG "LED is a detecter (LED off and charging)", CR Pin1 = 1 Pin2 = 0 PAUSE 10 RCTIME pin1, 1, light DEBUG "Light measured = ", DEC light, CR PAUSE time LOOP

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Microsoft Word - LED Light Emitter and Detector 7-31-07.doc