LEDs and Lasers

LEDs, or light emitting diodes, are exactly like every other semiconductor diode in the world, except, instead of just getting hot and emitting a sporadic photon here and there, they are doped to release many more photos. The physics of how diodes works is a fascinating topic, but not one for this place.

Indicator LEDs

Chip on Board (COB)

Other LED Packaging

Driving LEDs Correctly

Driving LEDs safety is an important job, but not super complicated. There are three possibilities, one easy, and two just a little more difficult. Just using a series resistor will work, but a little more effort and you can use a constant current source, or you can also use an IC dedicated to driving LEDs.

Series Resistor

One resistor per LED, not just one for a group. TODO: Schematic comparison

Some diodes do have built-in series resistors, but if they don't explicitly say they do, they very much don't.

Calculating using Ohm's law

where:

• \(V_{in}\) is the input voltage. This is the value of your power supply, battery, etc.
• \(V_f\) is the forward voltage drop. This should be available from the manufacturer's data sheet. If it isn't, there are tools to measure it, or you can just wing it with something like 1.7V for a red LED and 3V on any other color. There's more details based on color and materials available.
• \(I_f\) is the desired forward current. Remember, LEDs care about current most of all. Too much current releases the magic smoke.

So, if we want to drive this blue Cree LED properly, we can check the data sheet (see how to read a data sheet) and on the 2nd page we will find an \(I_f\) of 35mW (typical) to 100 (max), and a \(V_f\) of 3.4V (typical). If we want to power this with a 5V supply, We can stick those into our equation:

Great, now we just need to find a 45.71428571 ohm resistor. Wait, there isn't one, and while we could certainly construct a resistor arrangement that would achieve it, we also need to take into consideration that most hobby grade resistors are 1% tolerance at best. Since a higher resistor value will result in a lower current, and therefore more safety, we should choose a 47 ohm resistor (more about the E-series idea later).

If we plug that back into our equation, we'll get 0.0340426 watts, or 34.04mW. Close enough. With tolerance taken into account, that's 33.7-34.3mW of current. Perfect.

Constant Current Source

LED Driver IC

Laser Diodes

Laser diodes are mostly like regular diodes, except in order to exhibit the textbook definition of a laser (coherent, collimated, and on a single wavelength), they require some slight changes in their construction, and integrate collimating optics instead of diffusion. This video provides a good introduction to them.

If we look at the breakdown of what's inside of a typical TO-5 can laser diode, we can understand better what's going on.

The pieces that make up the photo diode are, from top to bottom:

• Protective can. Keeps dust and also outside air out of the can. Typically, the can is actually held at a medium vacuum.
• Window. The window allows the light to leave the can while maintaining the seal.
• Laser diode. The main part of the show.
• Heat sink. Removes heat from the laser diode itself and transfers it to the metal protective can.
• Monitoring photodiode. Used to monitor the output of the laser diode and adjust the current to it. Like all LEDish diodes, they need constant current to behave their best.

For these configurations, you'll have 3 pins so that you can access to laser and photodiode separately and use the photodiode to help provide feedback and drive the laser diode at a constant power. Because of the complexity in driving a laser diode, I'd strongly recommend using laser diode modules, which integrate the driver circuit into them directly. Adafruit carries a few of them.

Laser diodes are available in a bunch of colors, but some typical ones and their uses are:

For typical hobbyist uses, laser diodes range in power from 1mW to around 500mW. You will find more powerful laser diodes in things like hobbyist laser cutter/engraver where they can reach from 5-20W typically. Note that most (all?) laser diodes above 5W are actually composed of multiple 5W laser diodes focused together to the same spot.

Also, like all LEDs, once you get about a few mW of power, you will need to begin thinking about heat dissipation and (potentially) active cooling. Laser diodes are, perhaps, 10-50% efficient (10% is probably accurate for most cheap models), which means that 500mW laser diode needs to dissipate several watts of energy as heat. Oh, and they generally want to operate in the 25-50C (77-122F), which requires substantial considerations if you intend to operate them at full power continuously (called CW, or continuous wave). Obviously operating them pulsed averages things out and reduces the amount of heat that needs to be dissipated.

3rd Party Material

• An introduction to laser diodes