Introduction: Proxxon MICROMOT Drill Stand MB 140/S Light
Some time ago I purchased the MICROMOT drill stand MB 140/S and I noticed that when I use the drill on my desk with the overhead lights I cannot drill precise holes because the drill bit is in the shades of the stand. I decided to make a LED ring light to fit to the drill stand and put some light on the item I'm drilling.
Step 1: Designing the Circuit
I've ordered 10 SMD led diodes. The rest of the circuit is built from the parts I had around. The circuit is designed in the KiCad.
KiCad is a free software. For full description check the link.
This light ring is designed to work with 9V voltage. As the forward voltage drop of white LEDs is about 3.3 volts and I used 2 in series this means that the voltage must be above combined voltage drop of 2 diodes (6.6V)
Step 2: Design the Hardware/PCB
After designing the circuit we must check the dimensions of the board which can fit under the drill stand. This will be made from 1.5 mm PCB single layer board. Measure the dimensions in your favorite CAD program.
Unfortunately KiCad cannot import DXF or similar files with PCB shape so you have to draw the outline yourself. There is a hack for importing the PCB outline but I've tried it and it didn't work so I just drew the outline.
Step 3: Drawing PCB
Once you placed all components and traced all tracks and you are satisfied with the layout print the board on plain paper, cut it and check does it match the drill stand.
Step 4: Toner Transfer
Once you have your outline drawn correctly place components on PCB. Export GERBER file and you are ready for printing.
I develop my board using toner transfer method and Press-n-peel Blue PCB Transfer Paper Film bought from dipmicro store on eBay:
First I print the board on a plain paper and check dimensions. Usually home laser printers do not scale the printout correctly. I had to use 1.05 for X and 1.04 scale for Y axis in order to get the dimensions of the printed board right.
Once the dimensions are OK put Press-n-peel paper in your printer. I used the top copper layer. If you use this layer for PCB development you need to mirror before printing. KiCad has an option for this in the print dialog. Once I have my pattern printed I cut out the PCB to dimensions approx 1 cm bigger that the final board. Clean it with 600 grit sandpaper and then rub it with acetone. You don't need to spend much time cleaning. I usually sand and clean board this size in few minutes.
Once the PCB is clean and dry do not touch it. Place it on heat resistant mat, place the printed Press-n-peel paper on it with printed pattern touching the copper. Cover with a sheet of paper or paper towel and iron it on acryl setting for about 3 minutes. Iron temperature should be about 150 degC. In any case check the instructions for your toner transfer paper. If you use photo paper you need to experiment until you get good results. This should't be too complicated. I used several glossy photo papers and they all worked fine.
Step 5: Peeling Off Press-n-peel and Etching
When you are done with ironing put the board under a stream of cold water and peel of the Press-n-peel paper. Remaining pattern should be clean and sharp. If there are errors you will need to clean the board with acetone to remove toner and iron newly printed pattern again.
Once the pattern is transferred dip the board in the etching solution. I use Ferric chloride heated to 60 degC approximately.
When etching is completed wash the board and clean it with acetone to remove all traces of toner. You can test it for conductance between the ground copper pour and tracks. If you find conductance etch it some more until all the tracks are isolated. Be careful not to overetch and brake the traces.
Step 6: Preparing Pattern for Solder Resist
When your board is clean and if it is etched correctly you can now start preparing for the solder resist.
Export GERBER file from KiCad. Open GERBER view program in KiCad and print solder resist layer. I have noticed that the KiCad did not export solder resist for the few vias that I have put on bottom copper. Didn't want to bother with this now. Next time I'll try to change the properties of vias used so they will show on both layers.
Print your solder resist pattern on a plane paper to check is it a perfect match to your PCB. Most probably you will have to apply scaling in printing dialog. I have applied here same factors as I used for first print on Press-n-peel paper. I have Samsung Xpress M2022W printer at home and for A4 paper factors are X:1.05 and Y:1.04. You will have to find correct factors for your printer. That's why I first print on a plain paper.
Once your factors are OK put the transparency sheet into your printer and print 2 copies of solder resist. Check again is the scaling correct. If it is cut 2 patterns and paste them over each other for better contrast.
Step 7: Pasting Solder Resist Paint
I use solder resist paint off eBay. I used 2 different kinds. Both worked perfectly. I just ordered first one that popped in eBay search. This one is blue.
Put the small amount on the PCB cover with cellophane (I used one from chocolate box wrap). It is important that the cellophane is clear. Spread it with the credit card until nice and even.
WARNING: this paint will stain anything so be careful when working and use rubber gloves.
Once you satisfied with the solder resist spread it is time to develop.
Step 8: Exposing the Solder Resist
I don't have UV exposure device at home so I just tied a lightbult to a stool. This is not ordinary light bulb. This is UV lightbulb. WARNING: UV lightbulb might damage your eyes so you need to expose your board somewhere away. I switch on UV lightbulb and leave the room.
Before exposing align the solder resist pattern with your PCB. This is critical. As the solder resist paint is dark I use strong light to help me align. Cover all with piece of glass. I have used piece of 2 mm Plexiglas from other project.
I have exposed this for 20 minutes. (In my experience the time is not so critical). After exposing peel carefully corner of the cellophane covering the paint and check is it cured. If it is not continue exposing until cured. Remember the time for next time. Once cured, peel of the cellophane and swab the uncured parts gently (and I do mean gently) with acetone to remove uncured paint. Once your remove uncured paint return to UV lamp and expose some more now without anything covering the PCB to complete the curing process.
Step 9: Cutting the Board
Board is now complete so you can cut the outline. Preferably with CNC. I just drilled the holes with drill press and cut the rest with Dremel rotary tool.
Once the outline is cut, drill the holes for trough hole components. In my case it is push switch and power plug. Clean the board again with acetone and you are ready for soldering.
I usually test the board with multimeter at this point. Just in case that board was damaged during cutting and drilling.
Step 10: Soldering
And now the fun part.
Soldering SMD components with the soldering iron is not so complicated as it seems. Check out the videos.
Don't forget to clean the flux after soldering.
Step 11: Testing
Now it's time to plug it in and Murphy to kick in.
As this is first time I'm working in KiCad I didn't notice that I got the pinout of the BJT transistors wrong so I had to flip them over and resolder. (Check out step 12 for corrected PCB)
Lights are working and drawing about 120 mA of current. They are brighter than I expected and working fine.
Now it's only to left to fix this permanently to drill stand and to pass a cable somewhere.
Step 12: Files
I've been asked to email KiCad and CAD files to some interested folks so I uploaded to Google Drive all KiCad files used and PDF with the dimensions of the board. Google Drive folder contains KiCad files only. If you want GERBERs send me a message.
Also, I've been asked why did I put 2 capacitors in the soft latch circuit. This is basically because i didn't breadboard this so I put two different footprints of capacitors just in case I needed to change something when board was already complete. I do this often if I do not simulate or breadboard the project and if I have space on the PCB of course.
This is corrected version of the PCB with correct pinout of the BC847 BJT transistors used. I've got a load of them at home so they are BJT's of choice in my projects.
For the guys complaining that their circuit cannot be switched off or behaves erratically most probable culprit is the P channel MOSFET leakage current switching Q2 on very fast. The solution is bodging a 100K resistor between base and emitter of a Q2 BJT or making a new PCB. I've updated the files in the link with the R10 resistor. This should solve the issue.