Intro: Printed Circuit Board (PCB) Sprayer Machine
How would you like to make a single or multiple Printed Circuit Boards in less than 2 minutes?
Hey again. Hope everyone enjoyed my last Instructable on how to make a grappling hook launcher. Today I go down a slightly different path of creation.
This project has been in my mind for quite some time and I thought it would be a good time to finally build it. The idea began when I was tired of spending 25 minutes sloshing a Printed Circuit Board (PCB from now on) around in a plastic tray and waiting for it to finish etching. It was messy and wasted a lot of time. I began to search for an alternative and did not get much farther than a bucket with a fish tank bubble machine and perhaps a heater. Sprayers seemed like the most practical solution but the commercial available ones cost thousands of dollars and there was no guide that allowed someone to create one to use in their own shop. That is, until now.
I would like to present to you my Printed Circuit Board Spraying Machine that anyone with basic tool skills can build. Plenty of people have published and built CNC machines. Even some 3D printers have started to pop up. Countless tools and devices for creating projectes have been published on Instructables and across the internet. All of these devices use circuit boards and most can be created using a PCB. However, gone are the days where one needs a messy tray and needs to spend almost a half hour making them. The PCB Sprayer produces them in less than 2 minutes, can produce multiple PCBs at a time, continuously produce them, and then clean them afterwards. It is like a little factory in your workshop. This machine is a great addition to any maker's shop and is an absolute blast to build. You can produce PCB's on demand with ease and not have to spend thousands on an industrial machine. Where else can you improve your wood, plastic, electrical, plumbing and sketching skills?
I have organized each step of this Instructable into 5 sections to try and make it as clear and easy to follow as possible:
1) Step Aim: where I discuss what we will accomplish/create in this step.
2) Tools: which tools will be required to perform this step.
3) Materials: what materials from the part list will you need in this step.
4) Directions: step by step directions for this particular component's construction.
5) Tips: where I make note of any problems I ran into and ways to avoid them during your building process.
Of course, there will be countless photos, diagrams, drawing, and links to supplement each step and avoid any confusion (you may need to view full size to see all the dimensions and notes properly).
I have also included the Sketchup files (.skp) if you would like to view those. Sketchup is a google 3D design product can be downloaded for free here.
As for the ShopBot contest, if I can build a grappling hook launcher with a fire extinguisher and a Printed Circuit board Machine from a few sprinkler heads, who knows what I could do with a ShopBot?
Without further ado, I present to you the internets first do-it-yourself, complete step-by-step guide, to building your own PCB Factory.
As always, have fun building and feel free to email/message/comment with any questions you may have. I recommend looking at the whole intractable first before beginning building so you can get the full picture of what we are creating. Enjoy.
Again, my name is Christian Reed and I am a Mechanical Engineering student at MIT. My blog with other creations can be found here.
Step 1: Materials and Tools
To make things easier, I made the part list into a PDF. Do not let the long list intimidate you. A lot of the parts you may have lying around or may not need if you choose to leave out features. I have included everything I am using to give the broadest possible list. Keep in mind you may not want to go as crazy as I did with the features. I included what materials you will need on each step so you can pick and chose which you would like to use as easily as possible. You can obtain all the parts for about $300 completely new with the RV pump being the most costly (at $100) with the majority of the parts coming from Home Depot. You can also, like I did, repurpose acrylic or other plastic from anything you can thing of to reduce the cost even greater. The only clear sheet you need is the front and everything else could be solid if you want or you could even use a mix of acrylic, MDF, and PVC liner to create you own custom tank. Again, this is the part list I used so do not let that limit you in your building. Message me if you have trouble finding/understanding anything I included on the list and I will be happy to help.
Also, I have included all the Sketchup plans, diagrams and other resources in the Zip file here (if its easier to download them all at once) as well on each step.
See attached PDF
See attached zip file (collection of all the files for the entire Instructable, although they will be found again on each of their respective steps)
- Drill with assortment of drill bits
- Spring Clamp
- Rotary Tool
- Circular saw (with fine tooth blade for cutting acrylic; radial arm saw also will work)
- Exacto Knife
- Screw Drivers (flat and phillips)
- Soldering Iron
- Heat Gun
- Dremel Multimax (optional)
- Corner clamp (optional)
Step 2: Box Construction
ALL OF THE PHOTOS AND DIAGRAMS ARE LARGE, HIGH RESOLUTION FILES. VIEW FULL SIZE TO SEE ALL THE DETAILS IF SOMETHING IS UNCLEAR AT THE NORMAL RESOLUTION.
In this step, we will construct the housing for the tank and all of the components in the machine. It will be made out of MDF and contain a PVC liner to prevent any minor (hopefully) spills from ruining it.
-Drill w/ bits
-Band Saw(or similar tool-jigsaw)
-1/2" MDF Board
1) Begin by drawing the appropriate shapes that you will cut out on the MDF board. (Be sure to draw each shape independently-don't use a common line for two shapes as cutting errors will cause problems).
2) Cut out shapes using circular saw. When you reach the corner for the side panel, do not overlap with the circular saw. Finish cut using bandsaw. (Make sure you cut out two sides).
3) Once all shapes are cut out, assemble in place making sure everything is the right size. Once you ensure everything is the right dimensions, glue in place using wood glue and clamps where appropriate. Once glue is dry (no need to rush) drill pilot holes (very important or MDF will crack) and insert screws to firmly secure structure together.
4) Cut tank support strips and glue and then screw into place within the box.
5) Spray paint entire structure, not much else to say here.
6) Once box is fully assembled, cut out PVC liner to the appropriate shape. Make sure it fits in the box and then apply glue to the box and insert liner. Once glue has dried enough so that it does not wobble, use Goop in the corners to ensure no liquid can slip through.
-Drill pilot holes in the MDF before inserting the screw or it will crack.
-Wear latex (or nitrile) gloves when spreading the Goop to seal the corners
- I did this out of order so the liner is in when I had to touch up some paint and install the risers, the liner should be the last thing you do so you do not get any paint on it.
-Measure twice, cut once
Step 3: Main Tank
Here, we set out to construct the basic skeleton of the acrylic tank which will be the centerpiece of our project. It may be possible to use an old fish tank but drilling would be very hard (I'm not sure it can even be done).
-Circular Saw (or radial arm saw)
-Plastic Tray (to mix epoxy)
-Something to spread Epoxy with
1) Measure out and mark lines for acrylic tank (just sides and bottom, do not worry about anything else yet).
2) Cut each piece out and ensure they are the correct dimensions.
3) Drill the three holes in the back sheet of acrylic before gluing anything together.
4) Begin by mixing epoxy in a plastic tray. Spread on edges on the 2 side pieces and front and back piece (making the square first then attach to the bottom). Use the corner clamps to ensure everything stays in place. If you have a limited number of corner clamps, you can do one corner at a time. If no corner clamps are available, hold in place using standard clamps and blocks of wood.
5) Once dry, attach, using epoxy to the bottom.
6) Seal all corners and edges inside the tank using the Goop ensuring everything is covered (we would not want any leaks now, would we?)
-The epoxy is used for its strength of holding the tank together. The Goop ensures that there is a liquid-tight seal and ensure nothing leaks through.
-Corner clamps make this very easy but it can be done without them using wood to keep the sheets in place while drying.
-Make sure your saw blade can cut acrylic or your chose material
-Epoxy is usually workable withen a few hours but check the specific directions
-A lot of my acrylic is not clear because I sanded it down as it was repurposed from another project.
Step 4: Drain Plate
We will construct the the drain plate which will hold the PVC drain tubing in place and prevent twisting from ruining the seal we will make later in this step.
-Circular Saw (or radial arm saw)
-(3) PVC 1/2" Couplings
-(3) #10 Bolts
1) Begin by cutting the drain plate to the right size. Use a drill to cut the three holes and ensure they match up exactly (enough for a pipe to fit through) with the holes you previously cut on the tank.
2) Once you ensure holes are aligned, cut the screw holes that go into the acrylic on the side.
3) Insert 1/2" PVC couplings 1/4" (or whatever the thickness of your acrylic is) past the hole and secure in place with the screw.
4)Use a rotary tool or hack saw and cut off the top of the bolt heads
5) Attach to corresponding holes in tank using epoxy to secure the plate.
6) Seal around drian using Goop once epoxy dries
-Make sure the holes will align with the ones you make in the tank. The Dremel can help you greatly here, using the sanding bit to widen any holes that are not perfectly aligned and the Goop can seal a hole that is slightly larger than the pipe without any problems.
-Ignore the black strip at top as we will put this on in the next step.
-I have two bolts in the second hole because I didn't drill the original hole properly and needed to fix it, you should not (hopefully) need to do this.
Step 5: Finishing the Tank
In this step, we will make a top support for the tank using an old fish tank ( simple strips of acrylic at top will work too if you do not have an old tank lying around), insert the divider into the tank to separate the etching and washing partions, and finally make the covers for the top of the tank
-Rotary Tool with cutting wheel
-Heat Gun (acrylic bender if you have one)
-Old fish tank
1) Using your rotary tool with a circular cutting attachment, make two cuts into the fish tank black plastic top as close to the end as possible. Using a flat piece of metal (flathead screwdriver?) try to carefully remove it from the tank. A box cutting blade can help too.
2) Once you have enough pieces, cut to the right size and use epoxy to attach to the top of the tank. You may have to make several cuts in the side strip to get it aligned perfectly with the front and back strips.
3) Cut the divider to size and epoxy and then Goop into place. Use clamps on top to make sure it stays where it should. Once dried, take an extra strip of black plastic from the fish tank top and epoxy onto the top of the divider for the covers to rest on.
4) Cut the two covers to size.
5) Cut out the handles to their size and make the mark on them
6) Using a piece of wood, align the acrylic for the handles with the edge of the wood (the mark we just made should be aligned with the edge of the wood.
7) Place something heavy on the back to hold in place, use a heat gun to bend overhang to a 90 degree angle, repeat for the other side and then the other handle
8) Wrap top of handle with electrical tape (or whatever you prefer) for grip and then epoxy handles onto each respective tank cover.
-If you do not have a old fish tank available, use little strips of acrylic and glue them to the top edge of the tank. Their future purpose will be to hold the covers in place so make them accordingly.
-The drain plate should already be installed at this point (I did it out of order).
-Do not keep the heat gun in the same place or the acrylic will bubble as just the surface is being heated. Move in a side to side pattern over the top and bottom to ensure an even heating.
Step 6: Sprayers Assembly
Here we will assemble another critical component, the sprayers, which will distribute the etchant solution on one side of the tank and water to clean the processed board off on the other side of the tank. On the water side, we will place sprayers on both sides to make sure all of the etchant solution is gone from the board and the holder.
- Circular Saw
- Rotary Tool
- Drill (or drill press)
- Exacto knife (scissors work too)
- Small cable ties
- Acrylic sheet
- (15)Toro 3 GPH Spray Head
- (3')Sprayer tubing
- Sprayer Tees
- Sprayer Elbows
- (7) Orange 180 degree sprayer heads
1) Let us begin with the etchant sprayer first. Cut sprayer acrylic sheet ( 9 1/2" x 10 1/2") Mark out the holes that you will later drill (but not yet) on the acrylic sheet (I made one row or sprayers to help decide where to mark the holes).
2) Assemble the three identical strips of sprayers. Ensure the holes that we are planning to drill on the acrylic sheet line up with the assembled sprayer strips, otherwise adjust accordingly.
3) Drill the holes in the acrylic.
4) Using the rotary tool with some sort of small routing bit, route out little channels connecting each pair of holes on the opposite side of the acrylic. These are there so the cable ties dont stick out the back which would not allow us to glue it to the tank later on.
5) Insert cable ties through each pair of holes, lay down the sprayer assemblies and secure each into place and cut off excess cable tie.
6) Coat the back of the plate with epoxy and secure to side of the tank (make sure its the right side-etchant-bigger partition)
7) The water sprayers are a similar process but we just use a strip of acrylic as they go in a line. Follow the same procedure and secure both to the cleaning side of the tank.
8) Once all sprayers are in the tank, drill holes for the tubing that will supply the sprayer with liquid. Attach a 90 degree corner at the and so that there is no excess tubing and it is flush with the edge of the tank.
You may notice my water side of the tank sprayers are both different in design and number of sprayers. Ignore this and go with the design I have outlined above as it works better. You can use the same number of sprayers on either side if you would like but i did not really see a need to as the back ones were just to clear off any extra solution on the back so I decided on having 5 in the front and two in the back.
Step 7: Control Panel
In this step, we will completely assemble the control panel which is greatly important as it will control the function of the entire machine so make sure to do a nice and neat job. We will also make some slight modification to the thermometer/timer unit
- Circular Saw
- Rotary Tool
- Drill (or drill press)
- Exacto knife (scissors work too)
- Soldering Iron
- Acrylic sheet (.093") or thinner
- (3) illuminated SPST Switches
- (3) SPST switches
- (5) Push Button Switches
- Telephone wire or any other thin gauge you have lying around
1) Take apart the timer unit back.
2) The part of the casing attached to the screen, leave it alone as it is necessary for the screen to work.
3) Solder Jumper wire for F/C selection depending on your preference and another one on the temperature alert which should always be on.
4) Solder wires to switch pads and label ends so they are easier to identify later.
5) Inset plastic or small acrylic plate in between panels and glue two panels together.
Part II-Control Panel
1) Print out the attached PCB Control panel file.
2) Assemble into one piece (unless you can print on a single sheet of paper if you have a big enough printer) ensuring scaling is 100% so the length should be 17". You may also laminate it if you want (all of this can be done at a staples)
3) Cut 17"x4.5" sheet of thin acrylic.
4) Placing print out of control panel underneath the acrylic sheet, Mark out the appropriate location of buttons. Sizing will depend on what switched you choose to purchase/ have on hand. You just want to make sure they are centered (which, as you can see, I messed up).
5) Once marked out, begin making cuts using a drill press for the round switches and screws and a rotary tool for the switch holes.
6) Use exacto knife to make holes in sheet for control panel that will accomodate the switches.
7) Install switches and LCD screen (which was secured using hot glue).
8) Solder timer/thermometer wires to appropriate pushbutton switches.
9) Do not screw into box yet.
-Make sure your switches line up with the holes you cut or it will look sloppy.
-Drilling small holes for the wires in the timer circuit board may make it easier to solder.
-Staples or Kinkos can probably print and laminate the whole thing if you just bring them the file.
Step 8: Plumbing-PVC
We set out to make the PVC drains and the return line to supply the pump with solution during the etching process.
- Pipe Cutters (or whatever you normally use to cut 1/2" PVC)
- (2) 1/2" PVC Valve Slip x Slip
- (1) 1/2" PVC Valve FIP x FIP
- (3) 1/2" PVC 90 degree elbow Slip x Slip
- (1) 1/2" PVC 90 degree elbow Slip x MIP
- (1) 1/2" PVC Tee
- (1) 1/2" PVC Female adapter
- (1) 1/2" Hose barb x MIP
- (1) 1/2" 90 Degree elbow Hose barb x MIP
- About a foot of 1/2" SCH 40 PVC for connecting parts
- PVC Primer and Cement
1) Assemble the valves and elbows before putting them on the tank.
2) Follow the pictures as a reference as there is nothing particularly tricky or hard in this step.
3) Look here if you are unsure of how to use PVC cement or primer.
- Make sure the two drain lines fit together and are aligned properly so they will fit into the tank
- Use teflon tape for threaded connections on the pump return line to ensure no leaks
Step 9: Plumbing-Etchant and Water
We will connect all the internal plumbing for the water line and etchant line and connect the pump and solenoids to the plumbing.
- Exacto Knife
- Screw Driver (Flat and Phillips)
- Teflon Tape
- Shurflow pump
- 1/4" Sprinkler Line
- 1/4" Tubing
- 1/2" Tubing
- Loop Clamps
- BreakAway Series 1/4" Black I.D. Inline ShutOff Hose Barb insert
- BreakAway Series 1/4" Black I.D. Panel Mount Hose Barb Body
- (5) 1/4" Polyethylene Hose 90 degree elbow Hosebarb x Hosebarb
- 1/4" Polyethylene Hose Tees Hose barb
- 1/4" Polyethylene Hosebarb x MIP
- (2) 1/2" Polyethylene Hosebarb 90 degree Elbows
- (2) 1/2" Polyethylene Hosebarb x FIP
- 1/2" x 1/4" Polyethylene Hosebarb x Hosebarb
- 1/4" x 3/16" Polyethylene Hosebarb x Hosebarb
1) This is done much easier visually so I have included a lot of photos to help guide you through this process as well as a simple drawing I made which will help you better visualize what everything is being connected to.
2) I assembled the water lines first, then connected the pump system.
3) Look at the pictures for reference and assemble the system using clamps where appropriate.
4) It may be advisable to assemble the solenoids with hosing outside then place it once completed as it may be harder to do once inside the box.
5) I have included the drawings for where to make cuts for all of the pipes to leave the box.
-The pump is screwed down if you did not notice from the pictures
- Use teflon tape for threaded connections on the pump return line to ensure no leaks
-You can use sprinkler solenoid valves, they are just bigger than the other ones and will require an adapter to lower the voltage to 24V but this can be accomplished with little trouble.
- Tubes can be cut easily with an exacto knife.
Step 10: Electrical
In this step, we connect all the wires to the power supply and the control panel to allow everything to function properly.
- Soldering Iron
- Screw Driver
- Blue 2 gang box
- Gang box cover
- 5/8" Screws
- Wire nuts
- Heat Shrink Tubes
- (3) Conduit Clamps
1) Secure the gang box so the top of it is flush with the top edge of the PVC liner.
2) Drill hole for the power wire to come through the side of the box (if you have not already done so in the previous step).
3) Insert power wire into gang box through conduit clamp.
4) Insert pump, heater (make sure heater is installed in tank first), and solenoid wires through conduit clamp.
5) Solder wires long enough to reach gang box onto the control panel first before securing the control panel to the tank. Use heat shrink tubes to seal connection once the wires have been soldered to the switches.
6) Connect everything according to the provided schematic PDF file using wire nuts.
7) Once everything is connected, put cover on gang box and screw shut.
8) Put temperature probe in some sort of bag before sealing it into the tank as it is made out of stainless steel and will dissolve otherwise.
- Be very careful not to leave any exposed wires. Everything should be covered in the box or with heat tubing
Step 11: Copper Clad Holder
Here we will construct the PCB holder which will hold the printed circuit boards in place as they are being sprayed down.
-Circular Saw (or whatever you have been cutting the acrylic with)
-Dremel Multimax (optional)
- 1/4" Nylon Bolts
- 1/4" Nylon Nuts
- 1/4" Nylon Washers
1) Cut the appropriate shapes out of acrylic using the provided plans (do not drill the holes in the top piece or the top hole on the side piece if you plan on epoxying it into place)
2) Drill Holes where marked on the plans.
3) Cut the slots for the pieces to move around in. See tips for alternate method if you dont have a Multimax.
4) Assemble the fixture as shown in the picture except epoxy the top piece into place if you do not have any extra nylon bolts (either way if fine, I would probably recommend epoxy as it is a little bit more firm)
5) Assemble the two holders for the PCB Holder (one for the etchant side, one for the water side). For demonstration purposes, I made two different ones with parts I had lying around so you can see which you like better and build it accordingly.
- If you dont have a Dremel Multimax or similar tool, you can simply drill holes at close intervals and then remove whats left with a regular rotary tool
- I originally drilled a bunch of holes and then decided to multimax it later on. You do not need to do this.
- Epoxying the top piece leads to a more form structure but may be a little more hard to clean. Either way is completely fine.
Step 12: Putting It Together
Time to put all the components together to finisht the machine! Also we will build a customized fitting to make things easier in connecting everything.
- Exacto Knife
- 1/4" Sprinkler Tubing
- 1/4" Clear tubing
- (3) 1/4" Sprinkler Tees
- (2) 1/4" Sprinkler 90 degree elbow
- 1/4" 90 degree elbow
- 1/4"x 3/16" adapter (1/4 sprinkler to 1/4" regular)
1) First, we will make the custom fitting. Begin by taking the three sprinkler tees and cut off both of the side portions (look at the picture of what were trying to make to get a better idea of what we are trying to accomplish here.
2) Cut one end off the 90 degree elbow.
3) Connect all of the pieces in line with crazy glue. Once dry, seal all of the seams with epoxy.
4) Place the tank carefully inside the box.
5) Attach the water line to the cleaning tank, the water line to the etchant tank (put a 90 degree elbow on the end), the custom fitting we just make to the spray heads to a 1/4" sprinkler x1/4" regular adapter then the 1/4" etchant line, the pump return line to the hose barb, and finally the pressure relief valve witha 90 degree elbow at the end.
- Again, this step is much more easily done visually so look at the pictures full size for a better idea.
Step 13: Etchant Solution
Finally we will make the etchant solution (if you already do not happen to own a gallon of FeCl3
- Measuring cup
- Gas or other plastic container
- Muriatic Acid
- Hydrogen Peroxide
- Acetone (not needed but will clean off copper once it has been etched)
1) Make sure you are in a well ventilated area and are wearing safety glasses and gloves
2) Dump two containers of Hydrogen Peroxide into your container (assuming they are standard 32 oz for a total of 64 oz).
3) Measure out 32oz (4 cups) of muriatic acid.
4)This should make enough to use in your tank (you could always make more just in case following the 2:1 Hydrogen peroxide: Murtiatic Acid ratio)
- Follow the instructions exactly and add acid to water (muriatic acid to hydrogen peroxide)
- This instructable is where I got my information from if you are interested in fidning out more behind the chemistry of it.
- You are welcome to use FeCl3 if you want but I just prefer to make this and like it more as it favors my machine better (as it will be regenerated while you etch your board due to the large amount of aeration)
Step 14: Using Your New Machine
Here I have outlined a simple procedure to ensure that everything goes well with your etching project. The video demonstrates me outlining these procedures on my tank to use as a reference if you need it.
Procedure For Running PCB Machine
1) Check all tubing and connections for any leaks, ruptures or loose connections.
2) Insert water hose into quick connect.
3) Insert drain tube into drain line and place other end into bucket or other drain source.
4) Carefully pour etchant solution into tank.
5) Attach copper plates to be etched onto the PCB Holder.
6) Insert PCB Holder into tank.
7) Plug in Machine with GFCI adapter to outlet.
8) Turn on main power supply,
9) Turn on heater and set temperature set point on thermometer between 85-90 degrees
10) When temperature is reached, set timer to about 2 minutes (depends on your machine and thickness of copper) and turn on etchant pump.
11) Once timer is complete and copper is gone, turn off pump, move PCB holder into water tank, and turn on cleaning tank water.
12) After about 3 minutes, turn off water, remove PCB and either continue to cleanup (step 13) or etch more boards (step 10).
13) Turn off heater.
14) Close pump return valve. Place drain hose in etchant container and open etchant drain valve and put hose back in drain bucket once complete.
15) Turn on etchant tank and cleaning tank water and open all valves. Once enough water has accumulated (may need to close etchant valve), run some water through the pump to clean out any remaining etchant.
16) Once tank has been flushed for a few minutes, turn off power and remove all remaning water.
Step 15: FAQ's
Is this hard to build?
Not at all! The hard part was designing everything to work and now its just a matter of assembling all the parts and putting it together. I did this whole process from drawing to finished product in two weeks so it is just a matter of the time you have each day to spend on the project.
Is this machine even useful?
Absolutely. This will cut down your etch time by more than 20 minutes and allow you to make multiple ones very quickly of different sizes. You can also make multiple ones very quickly and save a great deal of money rather than buying a professional one or sending away for the boards to be etched.
Why does it work so well and greatly reduce the time?
The reason why it takes less than 2 minutes is due to the high turnover rate of etchant solution on the PCB being etched. Also, the pressure relief valve aerates the solution (similar to a fish tank bubbler).
Can I etch faster than 2 minutes?
Yes! My solution was fresh when I used it but it actually etches a little faster when there are more CuCl2 ions in the solution which comes once enough copper has been dissolved. Also, the ideal temperature (maximize etch rate) is reached when the solution is heated to about 95 degrees Fahrenheit. To compare, mine etched in less than 2 minutes at 71 degrees and with brand new solution. You may find this chart helpful.
Where can I learn more about the copper etch rate and the chemistry behind it?
Step 16: Improvements
Of course, as much thought as I gave to this device, there are plenty of ways to improve it. I spent just as much time building this as I did experimenting and trying to get it to function properly. Here I will post any improvements I can think of or the community can provide so future builders will hopefully be able to build the perfect device.
Improvement 1-Add a fan to the enclosure
The pump could possibly overheat depending on the ambient room temperature. A fan would be a nice idea to keep the pump and the overall enclosure nice and cool
Improvement 2-Integrated Electronics
For the simplicity of this Instructable, all of the electronics components are off the shelf and hence are not customized to fully interact with each other (ie heater shuts off when set point reached, water alarm kills the power, etc). This is quite possible to accomplish using relays and other simple electronics.
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