## Introduction: Spectrum Light Source

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.

## Step 2: The Circuit Board

The circuit board was milled on a CNC mill, but any method of making one will work fine.

The pattern was designed to be milled: if you're etching a circuit board you might design it a little nicer.  Without something like a mill, laser cutter or 3-D printer, I'd use a sliding, rather than turning, design, and make it on stripboard or a simple sawn pattern.

We're going to have a wiper, made from an old relay contact, contact the circuit board on one of the pads at the inner ring of the resistors.  The ground contact will touch the inner ring, joined by a tiny blue wire buried in a milled slot, to the outer ring, which completes the circuit to the LEDs.

NB: My drawing shows the LEDs with a similar construction internally but you can see in the photo they vary.  The most reliable way to determine which way round to connect them is that the flattened side of the 'skirt' is the cathode, and is connected to the ground side of the circuit.  I strongly recommend just testing on a breadboard before soldering just to make sure you have them the right way round.

## Step 3: The Housing

The base is made of a milled block of acrylic, 10mm thick.  These images show the design.

*Mill out the main area where the circuit will sit
*Then mill out the shelf that the circuit board will rotate on
*Then out the aperture for the light to come out
*Then the slot to feed the wires from the power socket, and holes to mount the connector
*A pilot hole in the centre

Drill out the pilot hole to 3.3mm, then tap with M4 tap.

## Step 4: Contacts

The circuit board picks up the supply from contacts attached the base.

This is the only tricky part of the design, getting these to work well is a bit fiddly.

The central collar is simply made by soldering a circle in tinned copper wire, which is then fed down a channel of the central column, along the base, and into the ground of the supply.

The positive supply is fed through the blue wire to the wiper, made from the contacts of a broken relay.  It fits into a little slot milled into the base.   You have to measure carefully to get this part right so that it lines up with the diamond-shaped patch on the circuit board when the LED is pointing directly out of the aperture.

Ideally you'd have something like a spring washer on top.

Note the cable-tie holding the power connector as you don't want pull from the plug disrupting things.  Rubber feet keep the bottom from getting scratched and give some clearance for the bottom screw.

## Step 5: The Top

You don't really need a top as you can just put the circuit board onto the base and label it with a pencil.  But it looks nice, especially if it's engraved with the central wavelengths like this one.

Choose some nice clear lettering, mill with 0.5 mm bit to a depth of 0.2 to 0.5 mm.  These are filled with white error correcting fluid, scraped off afterwards with an old bank card.  The fluid didn't mar the surface at all (test on scrap first, of course) but I think some kind of paint might be better, as correction fluid has a tendency to flake.

The top is stuck to the circuit board with just a drop of glue.

## Step 6: Assembly

The circuit board and engraved top fit on the central pillar on top of the collar connected to ground.  The 12V is fed through the wiper and needs to be just slightly springy when closed.

The top is held to the pillar by a cap head screw, leaving it free to rotate.  The screw from the bottom locks the position.

Rubber feet keep it from being scratched underneath.

That's it!