BeginnerTopic 8 of 104Electronics & Embedded
LEDs and photodiodes
Light-emitting diodes convert current to light; photodiodes convert light to current.
In depth
LEDs emit photons when electrons recombine across the band gap, with colour set by material (GaAs red, GaN blue). They need current limiting. Photodiodes generate current proportional to light in reverse bias or photovoltaic mode. Used in displays, optocouplers, remote controls and fibre links.
Key points
- Forward voltage 1.8–3.3 V
- Efficiency in lm/W
- Optocoupler isolates circuits
ExampleA TV remote uses an infrared LED; the TV has a photodiode receiver.
LEDs and photodiodes convert between electricity and light, which is the basis of indicators, displays, optical sensors and fibre links.
What is LEDs and photodiodes?
Light-emitting diodes convert current to light; photodiodes convert light to current.
Why is it important to understand LEDs and photodiodes?
LEDs and photodiodes convert between electricity and light, which is the basis of indicators, displays, optical sensors and fibre links.
How do I choose a resistor for an LED?
Subtract the LED forward voltage from the supply, then divide by the desired current. Use a resistor rated for the resulting power.
Why do photodiodes respond to light?
Absorbed photons create electron-hole pairs in the depletion region, which produce a current proportional to the light level.
How is LEDs and photodiodes used in practice?
Typical applications include driving an indicator LED with a series resistor on a control panel, reading a light level with a photodiode in a camera meter and designing an optical link for an industrial encoder.
What should I learn before LEDs and photodiodes?
Study "Zener diode regulators" first (topic 7 of 104 in Electronics & Embedded). It covers the groundwork this topic builds on.
What are the key points of LEDs and photodiodes?
Forward voltage 1.8–3.3 V. Efficiency in lm/W. Optocoupler isolates circuits.
Can you give an example of LEDs and photodiodes?
A TV remote uses an infrared LED; the TV has a photodiode receiver.
What comes after LEDs and photodiodes?
The next topic is "Bipolar junction transistor", which builds directly on this one.
How difficult is LEDs and photodiodes?
It is classed as Beginner in Electronics & Embedded (topic 8 of 104). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.