Putting Together the Mixing Stand

The mixing stand consists of the base of the unit and the components used to control the mixing cylinder.  Those include the valve at the bottom which closes the cylinder, the pump, the electromagnet for spinning the agitator.  The camera arm and central stalk (not shown) are mounted to the stand.  The orientation dots are also part of the mixing stand assembly.  This section will go into how this part of the autotester is assembled.

The first step is to assemble the mixing valve.  For this you will need the 3D printed mixing stopper, a short piece of heat shrink tubing, a short piece of stiff wire such as from a paperclip, 2x 2x3x2 bearings, 1x M3x16 screw, 2xM3x10 screws, and the cylinder from a 10mm syringe.  The pictures below show the build process.

The heatshrink tubing is used to provide a seal for the stopper.  The mixer stopper has a raised circle.  When the heatshrink tubing is shrunk over this circle, it provides a trampoline like area that gives a tight seal against the mixing tube.  The linkage between the solenoid and the mixing stopper should be very loose.  Linear motion is being converted into rotatary motion, so give in the joint is desirable.  The two bearing fit into either side of the mixing stopper and provide for smooth motion.  After assembly make sure everything moves smoothly.  I test that the unit seals properly and no water leaks out when closed by energizing the solenoid with 12V.  Sometimes the syringes have a small nub at the end that is a manufacturing defect.  This can be sanded off, or the syringe can just be replaced.

The valve should freely open and close.  The mounting holes for attaching the solenoid to the frame are oversized to allow for adjustment to ensure that there is no binding in the valve.  Depending on printing tolerances, it might be necessary to put a washer between the solenoid body and the plastic mount (so it moves outward some) if that is a source of binding. The mixing tube should hold water when the valve is closed.

Next we add the rest of the components.

Since no two cameras are identical, it is necessary for the autotester to establish a frame of reference.  2x 3D printed round inserts are necessary.  These should be printed in black for contrast to the mixing stand.  When the autotester starts up, it searches for these two dots and uses those to compute any error corrections need to the camera image.  The dots should simply snap into place in the upper part of the mixing stand.  It is important that they not fall out, as the autotester will refuse to operate if it cannot locate the dots.

Next we want to attach the camera arm.  This is a the 169mm long piece of 2020 aluminum. It attaches to the stand using M5x10mm screws and T-nuts (M5x8mm also works, but is really tight).  If you are using US parts then 10-32 screws 3/8″ long and square nuts also work.

The electromagnet is simply pushed into the slot behind the mixing tube.

The last step is to add the pump.  The tubing out of the pump is only 1/8″ outside diameter.  I used 1/4″ tubing that had a 1/8″ ID.  The pump tubing fit inside the outer tubing.  It was a tight fit, but that assured no leakage.  The output tube is fed through the hole above the mixing cylinder. It extends far enough so that the water coming out of the pump flows into the cylinder, but not so far that it obstructs drops coming from above.

The pump is screwed down with either #4 self tapping screws or M3 self tapping screws.  The pump runs on 12v.  On my pump, the positive terminal was marked with a red dot.

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