Stereo Microscope Illuminator

A cost reduced design for an Illuminator.

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The old stereo microscope that I use for building surface mount PCBs and other fine pitch work came with an incandescent illuminator that was disappointing at best. Before it could be used at all, I had to make a new illuminator. The new version used the Super Bright (for 2012) LEDs with a linear current regulator. The LED placement is OK, but could be better. I wanted better LED placement, so I designed a new illuminator that had LEDs all around the lens. My first thought was to make it all on a single piece of PCB, but the cost exceeded my desire for better lighting. Being that I only need to build 1 of these, and time is something that I have a surplus of right now, I traded ease of assembly for part fabrication time. The new design has an aluminum heat sink/mounting plate and LEDs on small PCBs and one larger PCB for the power supply.

The first iteration of this illuminator had all the parts on a single PCB which would straddle the lens of the microscope. The physical size was set by the microscope.

Initial Mechanical Design, too expensive to fabricate.First Iteration Re-Design

The completed design was on a 2 layer board. Fabrication costs for the board alone were a little over $80US. This exceeded my interest in the project, so I re-designed it with an aluminum plate to serve as a mount and heat sink. There are 4 small PCBs with an LED and two mounting holes on them and one larger PCB with an LED in the center and the power supply on it. It made sense to me to put the small boards on a panel with the power supply board, and cut it up before building it. The cost reduced version PCB costs were about $20US, which is a lot more acceptable.

Cost Reduced PCB (Second Iteration of Re-Design)
Cost Reduced PCB (Second Iteration of Re-Design)

The white silkscreen lines for cutting the board up are visible on the board layout shown. Cutting the board was done with a vertical band saw. I thought about assembling the board before cutting it up, but was concerned that the board would flex too much while cutting and damage the boards or components.

The heat sink/mounting plate can be seen here:

Heat Sink / Mounting Plate
Heat Sink / Mounting Plate

It is fabricated from 1/4" aluminum, with mounting holes for the LED boards and main board tapped for machine screws. The heat sink outline was rough cut with a vertical bandsaw and then cleaned up and the holes drilled on a mill. Tapping was done by hand on the work bench.

The assembled illuminator looks like this:

Completed Assembly with bare PCBs for Reference.
Completed Assembly with bare PCBs for Reference.

I included the bare boards to show how the unfinished boards look. Soldermask near the LEDs on the back side of the PCB is masked to give better heat transfer to the heat sink. The entire back side of the PCB is a ground layer in addition to the heat sink. The blue LED wires will be glued down to the heat sink to prevent snagging them on stuff in use.

The LEDs have a heat sink pad that runs through the middle of the package that does not have an electrical connection. This pad was tied to the ground plane. Clusters of vias can be seen around the LEDs to conduct the heat from the top of the PCB to the back side. Osram OSLON Square 3000K LEDs were used. They are capable of handling up to 1800mA, but initial tests look like under 50mA will suffice. At 250mA, this thing would give you a sun tan.

The power supply design is just a re-use of the design used previously on the goose-neck microscope illuminator shown in a previous project. Because the back side of the PCB is a solid ground sheet and needed to be flat for good contact to the heat sink, I included two jumpers in the initial design rather than going to a 4 layer PCB.

Here is the first LED illuminator that I built back in 2013.

First LED illuminator. Replaced the Incandescent one.

It uses a simple linear constant current regulator. It was designed on a napkin, and worked well enough to use for 7 years. Because it was built on perf board it needed to sit on standoffs, 1/4" below the plane of the mount plate. The large LEDs stuck farther down into the work area under the microscope lense.

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  • Installed on the Microscope

    Bharbour05/11/2020 at 19:18 0 comments

    Over the weekend, I installed the illuminator on my microscope. This scope is a pretty old Bausch Lomb scope that only had a provision for one of those round "flashlight" style lights in the mounting bracket. The first version of my illuminator needed a flat surface to screw to, so I made the bracket shown here:

    Side view of microscope, mounting bracket and new illuminator.

    A "leg" on either side of the scope is attached to the mount frame for the scope by a pair of machine screws, in holes that I drilled and tapped. From the dust, it is obvious that this bracket was made and installed a long time ago (2013). Two of the mounting holes in the original plate were used to attach the aluminum heat sink on the new illuminator to the bracket. The first version of the illuminator only had LEDs as far forward as the edge of the bracket, resulting in some shadows in the image.

    The new illuminator has a pair of LEDs out to the end of the aluminum heatsink, resulting in much better illumination at the "lower" edge of the image.

    View looking up at the illuminator and microscope lenses.

    Now, the light is very even from top to bottom and side to side. 3000K LEDs were used for this illuminator, while the previous one used 4000K or higher. The colors are a closer to natural now. The light output from this illuminator is overkill. I used LEDs that were left over from my other illuminator project #LED Microscope Illuminator  which needed to operate from 6" to 12" ranges, and only had 2 LEDs. Nothing suceeds like Excess!

    5/20/2020 After using this illuminator for building a number of boards, I really like it. The 3000K LEDs make the colors a lot more natural and better distribution of the light is easier on my eyes. This is definitely a win.

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Gerben wrote 04/22/2020 at 12:06 point

1800mA! that's insane. I though that was a mistake, so I checked the datasheet. And it's indeed 1800mA. You'd need some serious heatsinking when they are running at 5W each.

Clever idea to create separate panels. No need to waste FR4 when you already have an aluminum backplate.

  Are you sure? yes | no

Bharbour wrote 04/22/2020 at 12:42 point

Pretty much my reaction on the 1800mA.

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Bharbour wrote 04/22/2020 at 14:08 point


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