Thursday, June 20, 2019

Replacing a B&L Microscope Lamp with an LED

I've recently been restoring some old microscopes for an organization called "Milwaukee Area Science Advocates" who have been gathering them from teacher's closets where they have been unused for a long time.  Many of them just need a cleaning to get them into usable condition, and a few need parts to be swapped.  Eventually the scopes will be used to promote science education by letting kids observe live microorganisms with them.

A couple of the scopes I've been working on are old Bausch & Lomb units with identical illuminators that slide into a bracket under the stage.  The illuminators are nothing fancy- just a 15W incandescent light bulb in a bakelite box with a blue tinted filter glass that probably absorbs IR and prevents the specimens on the slide from drying up.

One of the B&L microscopes with the funky illuminators.

The problem I ran into was that one of the two had a burned out light bulb.  I'm sure someone has a big stash of these bulbs somewhere, but not anywhere I could find, and even if I managed to find one, how much will it cost, and what's going to happen to the scope when the bulb burns out again?

The light bulb used in the substage illuminator.  15W 120V.
The substage illuminator opened up.  The blue glass blocks IR and helps keep specimens on the slide from drying out.

The light bulb base and switch.

I checked into adapting the E26 bulb socket to a candelabra (E12) bulb, but all the E12 bulbs and LEDs I could find were too long to fit into the enclosure.  After spending a while searching a few other possibilities, I decided I'd have to solve the problem myself, taking the easiest approach I could think of- I designed a printed part to screw into the light bulb socket in the bakelite enclosure, that holds an LED and a resistor, powered by a 6V wall wart, using the original power cord minus the AC plug.

I know from previous experience that a single, 5mm diameter, white, 20 mA LED easily throws enough light for a microscope, even at high magnification.  So I started working on the design for the light bulb replacement.  I quickly realized it would have to be done in two parts- one that screws into the E26 socket in the illuminator and the other that will hold the LED and allow it to be aligned with the optical path in the microscope.

The Base

The first thing I needed to do was duplicate the E26 light bulb base.  It's 26 mm diameter, but what are the specs for the threads?  A few minutes research revealed that E26 lamps have 7 threads per inch - that's 3.63 mm pitch - ugh!  I couldn't find a 7 tpi thread built in to Fusion360, so I created it by making a helix and subtracting it from a 26 mm diameter cylinder.

I needed to make electrical connections between the LED and the lamp socket, so I added a hole in the side of the base through which I could lay a wire in the threads.  The wire makes contact with the bulb base threads.  The other connection is made by looping a wire through the bar on the bottom of the base.  When the base is screwed into the socket, the wire will touch the center contact in the socket.  It probably won't get UL or TUV approval, but it works.

The base rendered in Fusion360.  The bar at the bottom holds the wire to make electrical contact with the lamp socket.

The LED Holder

The original bulb socket is mounted horizontally so the LED has to be turned 90 degrees off the axis of the base to point it up the optical axis of the microscope.  I couldn't predict exactly where it needed to be located, so I made the LED holder position adjustable to allow for alignment with the microscope optical axis.  The LED holder just friction fits into a hole in the base that allows it to slide in and out and rotate.  It grips tightly enough that it should stay aligned once set, though a drop of glue could be added to ensure stability.

Here are the two pieces used to make the LED holder.  The cylinder on the left fits into the hole in the base (middle).

Putting It All Together

I tested the 6V wall wart with the LED and resistors to get the current through the LED to about 20 mA.  I ended up using a 270 Ohm resistor.  If you try something like this you'll have to test it to see what value of resistor to use- unregulated wall-wart's voltages can be all over the place with a small load like a single LED.  I attached wires to the LED, fed them through the hole in the LED mount, and then attached them to the base to make electrical connections.

The LED in the base.  The wire will contact the threaded part of the lamp socket.  The bottom contact is also made by looping a wire around a plastic bar in the bottom of the base.

I cut the plug off the illuminator's power cord and wired the wall-wart to it, screwed in the LED mount and it lit up.  Then I put it into the microscope to make sure the light from the LED was aligned with the optical axis of the microscope.  The LED can be rotated and slid in and out of the base so it is easy to position it.

The illuminator with the LED replacing the incandescent lamp.  The blue glass is not really needed because there's no IR coming from the LED so it isn't going to heat up and dry out the specimen being viewed with the microscope.
That's it!  A pretty simple project making great use of a 3D printer.  The CAD file for the E26 to 5 mm LED adapter is located here.

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