Need monochromatic light sources? Noticing the fact that many LEDs have very narrow spectra, I wanted to make a multi-narrow-spectra light source for various scientific purposes such as for botanical specimens under the microscope. This project was a test run to see how well it works, and it seems to work very well. With the exception of the LEDs themselves, it's made entirely from scrap and offcuts. On top, the dial chooses the colour by its central wavelength, in nanometres.
The first part is choosing the LEDs: use a reputable source so you can get the datasheets to find out the spectra. Choose as many and as varied as you like -- these are all quite narrow beams (15 degree), quite bright, and go from ultraviolet to infrared. The graph shows the spectra from the datasheets, except the 400 nm LED which is an estimation.
Make it any way you like: on breadboard, on a circuit board, just-soldered-together. This instructable shows the housing I made with a CNC mill, which gives a nice turnable control, and keeps the light output in one place for easily fixing fibre optics. You could easily make one with laser cut materials, 3-D print one, or just solder it together on a piece of breadboard.
Step 1: Circuit
The circuit is trivial: just the classical LED drive with a current-limiting resistor per LED. One per LED so that we can use different values to suit the individual LEDs. The diagram shows the calculated values for the LEDs I used, for a 12V supply, as they have various voltage drops ranging from 1.3V to 3.2V. Each is calculated as R = (Vsupply - Vdrive) / Idrive. For example, my 470 nm LED lists a typical forward drop of 3,2V, and we'll use the typcial drive current of 0.02A. R= (12 - 3.2) / 0.02 = 440 ohms.
I used the next-larger resistors that were to hand (490R and 680R) rather than getting too precise. In a calibratable one of these I'd use a multi-turn adjustable resistor per LED, but not this time.
I had a handy power cube for 12V available; but you might consider using USB for a 5V supply. Please don't use disposable batteries for a supply unless you really have to.
Use as many LEDs as you want: I picked nine just because they were what I found in the catalogue easily.