ThingsyoushouldknowaboutLEDsandphotodiodes.pdfVIP

5V Things you should know about LEDs: The intensity of light that a LED produces is directly proportional to the current. Usually the current is set with a resistor. Ex: At 10mA the diode drops about 2.05V. With a R1 300 5V supply, this leaves 3V across the resistor. Therefore R = 300? (3V/10mA). Note: You dont need to know the exact voltage drop of the diode. If its not bright enough lower the D1 LED current limiting resistor. The maximum DC current should be listed in the data sheet. You may be able to pulse the LED GND with 10 times its rated current if the duty cycle is low enough (say 1% on time or less). Interesting tidbits: Different color LEDs drop different voltages (Red 1.7-2.5V, yellow 2.0-2.3V, green 2.0-2.6V, blue white 3.1-3.7V). The LED voltage increases slightly with increasing current. The LEDs lifetime varies exponentially with current. It may last 100,000 hours at 5mA, 10,000 hours at 20mA, and 100 hours at 100mA. The reverse voltage is very low (sometimes only a few volts) so dont put the LED in backwards. The LED data shown above is for a SLA-370LT3F (a random red LED). Things you should know about photodiodes: Photodiodes produce a leakage current that is directly proportional to the intensity of the light. This leakage current flows in the opposite direction to current in a normal diode or LED. Ex: As more photons hit the photodiode the current increases causing a voltage across R1. As the voltage across the diode (and resistor) increases the linearity decreases. D1 PHOTODIODE A current to voltage converter (or transimpedance amplifier) is an easy way to convert the photodiode current to a voltage and keep the diode voltage at zero (circuit to the right). The output current vs. incident light can be linear over 6-9 orders of magnitude. When light hits the photodiode a current is generated that flows through R1 to the output (no current flows into the op-amp). Note: The output voltage will be negative. If the diode polarity is reversed the out