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In optoelectronics we deal with 2 types of electronic devices.

Light emitting electronic devices: ones that generate electromagnetic energy under the action of electrical field. Example: light emitting diodes (visible and infrared light).

Light detecting devices: ones that transform electromagnetic energy input into electrical current/voltage. Examples: photoresistors, photodiodes, phototransistors, etc.
Light-Emitting Diodes                                        Infrared detector

Light-Emitting Diodes (LEDs) 

• 2 lead semiconductor device.

• Light emitting PN-junction diode.

– Visible or infrared light.

• Has polarity.

• Recall diodes act as a one way gate to current flow.

– A forward-biased PN-junction diode allows current flow from anode to cathode.

• An LED conducts and emits light when its anode is made more positive (approx. 1.4V) than its cathode.

– With reverse polarity, LED stops conducting and emitting light.

• Similar to diodes, LEDs are current-dependent devices.

– LED brightness is controlled by controlling current through LED.

• Too little current through LED > LED remains OFF.

• Small current through LED > dimly lit LED.

• Large current through LED > brightly lit LED.

• Too much current through LED > LED is destroyed.

• A resistor placed in series with LED accomplishes current control

  • Let Vs be the supply voltage
  • Let Vf be the required forward bias voltage for the LED
  • Let I be the desired current flow through LED.

Then, the current limiting resistance R is sized as follow:



If R is chosen smaller than the above value, a larger current will flow through the LED.

– LEDs can handle only limited current (varies from 20mA to 100mA).

– If current through LED is larger than the maximum allowed value, than the LED will be damaged.

Visible-Light LED

• Inexpensive and durable.

• Typical usage: as indicator lights.

• Common colors: green (~565nm), yellow (~585nm), orange (~615nm), and red (~650nm).

• Maximum forward voltage:  1.8V.

• Typical operating currents: 1 to 3mA.

• Typical brightness levels: 1.0 to 3.0mcd/1mA to 3.0mcd /2mA.

• High-brightness LEDs exist.

– Used in high-brightness flashers

Tricolor LED

• Two LEDs placed in parallel facing opposite directions.

• One LED is red or orange, the other is green.

• Current flow in one direction turns one LED ON while the other remains OFF due

to reverse bias.

• Current flow in the other direction turns the first LED OFF and the second LED ON.

• Rapid switching of current flow direction will alternatively turn the two LEDs ON

giving yellow light.

• Used as a polarity indicator.

• Maximum voltage rating: 3V

• Operating range: 10 to 20mA

7-Segment LED Display

• Used for displaying numbers and other characters.

• 7 individual LEDs are used to make up the display.

• When a voltage is applied across one of the LEDs, a portion of the 8 lights up.

• Unlike liquid crystal displays (LCD), 7-segment LED displays tend to be more rugged, but they also consume more power.

The above modern numerical scheme wasn't always followed in the past, and various other schemes could be found as well:

The seven-segment display decoder chips 7446/7447/7448/7449 and 74246/74247/74248/74249 and the Siemens FLH551-7448/555-8448 chips used truncated versions of "2", "3", "4", "5" and "6" for digits A–E. Digit F (1111 binary) was blank.

Latin alphabet

Most letters of Latin alphabet can be reasonably implemented using seven segments. Though not every letter is available, it is possible to create many useful words. By choosing better synonyms, it is possible to work around many shortcomings of seven-segment alphabet encodings. Some letters ('O', 'I', 'S') look identical to numbers, though use of lower-case 'o' and 'i', or putting 'I' on the left as shown here, could be used instead. In addition, seven-segment displays can be used to show various other letters of the Latin, Cyrillic and Greek alphabets including punctuation, but few representations are unambiguous and intuitive at the same time.When all letters need to be displayed on a device, sixteen-segment and dot matrix displays are better choices than seven-segment displays.

How LED Works

• The light-emitting section of an LED is made by joining n-type and p-type semiconductors together to form a pn junction.

• When the pn junction is forward-biased, electrons in the n side are excited across the pn junction and into the p side, where they combine with holes.

• As the electrons combine with the holes, photons are emitted.

• The pn-junction section of an LED is encased in an epoxy shell that is doped with light scattering particles to diffuse light and make the LED appear brighter.

• Often a reflector placed beneath the semiconductor is used to direct the light upward.


• Phototransistor is a light sensitive transistor.

• In one common type of phototransistor, the base lead of a BJT is replaced by a light sensitive surface.

• When the light sensitive surface @ the base is kept in darkness, the collector-emitter pair of the BJT does not conduct.
• When the light sensitive surface @ the base is exposed to light, a small amount of current flows from the base to the emitter. The small base-emitter current controls the larger collector-emitter current.

• Alternatively, one can also use a field-effect phototransistor (Photo FET).

• In a photo FET, the light exposure generates a gate voltage which controls a drain-source current

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