A LED is simply a diode that has been doped specifically for efficient light emission and has been packaged in a transparent case. Therefore, if inserted into a circuit in the same way as a photodiode, which is essentially the same thing, the LED will perform the same function. As a photodiode, it is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. For example, a green LED will be sensitive to blue light and to some green light, but not to yellow or red light. Additionally, the LED can be multiplexed in such a circuit, such that it can be used for both light emission and sensing at different times.

Several applications for this technology have been suggested and/or implemented, ranging from use as simple ambient light sensors to full bidirectional communications using a single LED. Most of these applications benefit from this technology because of the cost reduction of using the same component for multiple functions.

• Ambient light sensors

LEDs have been used as ambient light sensors. For example, a remote control keypad backlight would be turned on by capacitive proximity sensors only in the absence of ambient light. The LED used for the backlight was also used as the ambient light sensor. This resulted in increased functionality for no increase in manufacturing costs.

• Bidirectional communications

LEDs can be used as both emitters and detectors of light, which means that a device having only a single LED can be used to achieve bidirectional communications with another device meeting these requirements. Using this technology, any of the ubiquitous LEDs connected to household appliances, computers and other electronic devices can be used as a bidirectional communications port.

One application for bidirectional communication with a single LED is fiber optic communications. In typical plastic optical fiber communications, a single optical fiber is used only for communication in one direction. This is because a single LED transmitter is placed at one end of the fiber, and a photodiode receiver is placed at the other end. Thus, two fibers are needed for bidirectional communication. However, if a single LED is placed at each end of a fiber, then the optical fiber can carry information in both directions using half the number of components as a typical system. This reduces system weight, cost and complexity.

Another application of this use of LEDs is a proposed alternative to RFID tags called the iDropper, developed by Mitsubishi Electric Research Laboratories in 2003. The iDropper is a small device that consists of a microcontroller, a battery, an LED, and a single push-button. The device records or transmits a small amount of data upon command from the user. Compared to RFID tags, the iDropper is more secure because the user must press a button to reveal personal information, and is similar in cost.

One major limitation of this scheme is the fact that a single LED can only operate as a half-duplex transceiver. A single LED can either transmit or receive information at one time, not both simultaneously. A simple way to put this is that an LED transceiver behaves like a walkie-talkie, in contrast to a telephone. This means that it takes a considerable amount of time for two devices to “talk” to each other.

An LED technology especially suited to use with domestic TRIAC dimmers. Resonant asymmetric inductive supply (RAIS) sits between the mains and the LED. Report L10270 from the Lighting Association Laboratories found the technology would work across different dimmer types while still maintaining a power factor of 0.96 with an input to output system efficiency of 91 per cent. As the light is dimmed, the system is able to hold a significantly higher efficiency than a Buck converter as RAIS draws a continuous current without using bleed resistors. RAIS is a single stage supply that also delivers a constant current output to the LED with no sense resistor. The report also found that RAIS technology fitted into the dimensions of a standard GU10 lampholder (bayonet mount).

LED lighting typically involves LEDs connected in series. The resultant forward voltage may be in the region of 10 to 20V. In such cases, the ratio between the mains voltage and the voltage required to drive the load is between 10 and 20. With such a large ratio, conventional circuits used to drive LEDs become very inefficient because the switching will be operating at extremes of duty ratio with very short conduction times and high peak currents. This inevitably means that extra components, such as a common mode choke, need to be used. It is therefore common to include a magnetic or piezoelectric (ceramic) transformer with an input-to-output ratio suitable to create a step down in voltage and a corresponding step up in current. This though introduces further efficiency loss, cost and bulk.

In contrast, the RAIS technology can drive high current, low voltage LED strings from a 240V AC mains supply without high peak currents, a transformer or common mode choke and still achieve the turn’s ratio.

Typical Use: Between the mains and the LED with in a retro fit lamps such as a GU10 where small size and use with conventional TRIAC dimmers is required.

Special Features : Inherent Compatibility with a TRIAC dimmers through continuous current draw in the same way as a conventional lamp, in other words it looks like a resistor to the mains. This also has an impact on the efficiency as it does not require a bleeder circuit or holding current resistor to ensure proper TRIAC operation which can cause efficiency loss during dimming.

The topology is inherently constant current. There is no need for a second stage, current sensing, feedback or short circuit protection

Typical performance of a TRIAC dimmable LED retrofit lamp, Power Factor 0.96, efficiency > 90%

The RAIS technology has now got a granted UK patent # GB2449616 dated 17th Feb 09 and is applied for in most other word wide territories.

A CR dropper (capacitor and resistor) followed by full-wave rectification is the usual electrical ballast with series-parallel LED clusters. A single series string minimizes dropper losses, while paralleled strings increase reliability. In practice usually three strings or more are used.[citation needed] An advantage of a capacitor is that it can reduce the high line voltage to an appropriate low voltage, without wasting power, with a very simple circuit; a disadvantage is that there may be a high surge of current for a short time when it is first turned on.

Operation on square wave and modified sine wave (MSW) sources, such as many inverters, causes heavily-increased resistor dissipation in CR dropper, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor and rectifier make a more suitable ballast for such use, and other options are also possible. Dedicated integrated circuits are available that provide optimal drive for LEDs and maximum overall efficiency.

Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. Parallel operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.

To increase efficiency (or to allow intensity control without the complexity of a DAC), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.

If the voltage is below the threshold or on-voltage no current will flow and the result is an unlit LED. If the voltage is too high the current will go above the maximum rating, heating and potentially destroying the LED. As the LED heats, its voltage drop decreases (band gap decrease), further increasing current. Consequently, LEDs should only be connected directly to constant-voltage sources if special care is taken. Series resistors are a simple way to stabilize the LED current, but wastes energy in the resistor. A constant current regulator is commonly used for high power LEDs. Low drop-out (LDO) constant current regulators also allow the total LED string voltage to be a higher percentage of the power supply voltage, resulting in improved efficiency and reduced power use. Switched-mode power supplies are used in some LED flashlights, stabilizing light output over a wide range of battery voltages and increasing the useful life of the batteries.

Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents[citation needed], but indicators running on 100 μA are still practical. The cost of ultrabright LEDs is higher than that of 2 mA indicator LEDs.

Strings of LEDs are normally operated in series LEDs, with the total LED voltage typically adding up to around two-thirds of the supply voltage, with resistor current control for each string. In disposable coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type.

LEDs can be purchased with built in series resistors. These can save printed circuit board space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED’s intensity. Alphanumeric LEDs use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. Seven-segment and starburst LED arrays are available in both common-anode and common-cathode form.

The LED used will have a voltage drop, specified at the intended operating current. Ohm’s law and Kirchhoff’s circuit laws are used to calculate the resistor that is used to attain the correct current. The resistor value is computed by subtracting the LED voltage drop from the supply voltage, and then dividing by the desired LED operating current. If the supply voltage is equal to the LED’s voltage drop, no resistor is needed.

This basic circuit is used in a wide range of applications, including many consumer appliances.

• SMD

• 2 mm

• 3 mm (T1)

• 5 mm (T1¾)

• 10 mm

Other sizes are also available, but less common.

Some applications (e.g. building custom LED panels for alarm clocks, dashboards backlight solutions and alike), require use of so-called ‘unpackaged LEDs’, consisting of nothing but semiconductor chip with conductive wire attached.

Common package shapes:

• Round, dome top

• Round, flat top

• Rectangular, flat top (often seen in LED bar-graph displays)

• Triangular or square, flat top

The encapsulation may also be clear or semi opaque to improve contrast and viewing angle.

There are three main categories of miniature single die LEDs:

Low current — typically rated for 2 mA at around 2 V (approximately 4 mW consumption).
Standard — 20 mA LEDs at around 2 V (approximately 40 mW) for red, orange, yellow & green, and 20 mA at 4–5 V (approximately 100 mW) for blue, violet and white.
Ultra-high output — 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.

Five- and twelve-volt LEDs are ordinary miniature LEDs that incorporate a suitable series resistor for direct connection to a 5 V or 12 V supply.

The size of the illuminated field is usually comparatively small and machine vision systems are often quite expensive, so the cost of the light source is usually a minor concern. However, it might not be easy to replace a broken light source placed within complex machinery, and here the long service life of LEDs is a benefit.

LED elements tend to be small and can be placed with high density over flat or even shaped substrates (PCBs etc) so that bright and homogeneous sources can be designed which direct light from tightly controlled directions on inspected parts. This can often be obtained with small, inexpensive lenses and diffusers, helping to achieve high light densities with control over lighting levels and homogeneity. LED sources can be shaped in several configurations (spot lights for reflective illumination; ring lights for coaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).

LEDs can be easily strobed (in the microsecond range and below) and synchronized with imaging. High power LEDs are available allowing well lit images even with very short light pulses. This is often used in order to obtain crisp and sharp “still” images of quickly-moving parts.

LEDs come in several different colors and wavelengths, easily allowing to use the best color for each application, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effect of ambient light. LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation, therefore allowing plastic lenses, filters and diffusers to be used. Waterproof units can also easily be designed, allowing for use in harsh or wet environments (food, beverage, oil industries).

The light from LEDs can be modulated very fast so they are extensively used in optical fiber and Free Space Optics communications. This include remote controls, such as for TVs and VCRs, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low voltage sensor circuit (usually battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits not sharing a common ground potential.

Many sensor systems rely on light as the main medium. LEDs are often ideal as a light source due to the requirements of the sensors. LEDs are used as movement sensors, for example in optical computer mice. The Nintendo Wii’s sensor bar uses infrared LEDs. In pulse oximeters for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Furthermore, its sensors only need be monochromatic, since at any one point in time the page being scanned is only lit by a single color of light. Touch sensing: Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used in for example a touch-sensing screen that register reflected light from a finger or stylus.

Many materials and biological systems are sensitive to, or dependent on light. Grow lights use LEDs to increase photosynthesis in plants and bacteria and vira can be removed from water and other substances using UV LEDs for sterilization. Other uses are as UV curing devices for some ink and coating applications as well as LED printers.

The use of LEDs is particularly interesting to plant cultivators, mainly because it is more energy efficient, less heat is produced (can damage plants close to hot lamps) and can provide the optimum light frequency for plant growth and bloom periods compared to currently used grow lights: HPS (High Pressure Sodium), MH (Metal Halide) or CFL/Low-energy. It has however not replaced these grow lights due to it having a higher retail price, as mass production and LED kits develop the product will become cheaper.

LEDs have also been used as a medium quality voltage reference in electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee; nitride-based LEDs have a fairly steep I/V curve and are not useful in this application. Although LED forward voltage is much more current-dependent than a good Zener, Zener diodes are not widely available below voltages of about 3 V.

During this transition period, it is a challenge to ensure that this technology is used where it is most appropriate and effective, and to avoid poor-quality products damaging the reputation. 2009 DOE testing results showed an average efficacy of 35 lm/W, below that of typical CFLs, and as low as 9 lm/W, worse than standard incandescents. It is a challenge to get buyers and users to be conscious of and make decisions based on life-cycle costs instead of the more obvious initial purchase price, and to avoid having low-efficiency products ride on the coattails of hype generated by lab test results.

In 2008, a materials science research team at Purdue University succeeded in producing LED bulbs with a substitute for the sapphire components. The team used metal-coated silicon wafers with a built-in reflective layer of zirconium nitride to lessen the overall production cost of the LED. They predict that within a few years, LEDs produced with their revolutionary, new technique will be competitively priced with CFLs. The less expensive LED would not only be the best energy saver, but also a very economical bulb.

LEDs are also non-toxic unlike the more popular energy efficient bulb option: the compact florescent a.k.a. CFL which contains traces of harmful mercury. While the amount of mercury in a CFL is small, introducing less into the environment is preferable.