Introduction: Grass Catcher Full Indicator

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First of, I saw this on a commercially available lawn mower a while back. The one I have now does not have it, and I found that this makes me ignore when the bag is full way too often. So I designed this!

The basic idea is that the lawn mower produces a lot of air flow as it propells grass into the catcher. While there is empty space, that air flow will keep the flap up. As soon as the space inside the box is filled with grass, the air flow will not be enough to keep the flap raised, and as it drops you can tell that the box needs to be emptied.

This is meant to go on the plastic lid or top of your lawn mower's grass catcher, and it will notify you whenever that box is full and needs to be emptied.

You will need either a 3D printer (and the experience to use it) or some fiddling with wood and stuff to make your own (outside of but adjacent to the scope of this 'Ible).


Here are the materials you'll need:

  • 3D filament - to print with. Like I said, you can use wood, or plastic, but that means to take the general idea and run with it, away from the direction of this 'Ible. At least for a little bit. Side note, I used PLA. Because that's how I mow (sorry).
  • More 3D filament - to act as a hinge pin. You can substitute this with any thin cylindrical object, like a nail or a skewer.
  • mesh - I used a metal mesh (can be seen in step 2) because I had it on hand. The idea is to keep grass from clogging up the mechanism. You can easily print some, but I did not include it in this because it would prevent me to put the lid in.
  • screws and nuts - to fasten the display to the lawnmower. I used M4 (I think), but anything goes as long as it fits the holes.

And these tools are required, although there are variations:

  • 3D printer - to turn the filament into the shapes coded in the files below.
  • drill - generally a power tool, but a hand crank works as well if you are into that kind of thing.
  • drill bit sized for the big hole - that would be a forstner or spade bit of 40mm diameter.
  • drill bit sized for the screw holes - in this case that's 4mm, and I like using brad point bits for plastic.
  • Alternatively: a pointy piece of metal and a lighter - beware, there's a risk of burning yourself or setting things on fire with this, albeit a small one. The idea is to heat up the tip of something like an awl and gently melt your way through the plastic. That is what I did, but you do you!
  • something to fasten the hardware - pliers, wrenches or spanners that fit the dimensions of your fastening hardware of choice (see above).

Extra note on printing these: You do not need the drill guide to assemble the display, but it helps aim the big drill right in relation to the smaller holes. You can just drill a big hole first and then align the case.

Step 1: Make Holes

You need five holes to mount this on your lawn mower grass box - one for air flow and four for the screws. There are two ways to go about this (three if you count flying by the seat of your pants, hoping things will align anyway. They won't. Believe me.) The only requirement is that the spot you screw this to is flat - no bents, no dents, as they would limit the functionality of this gauge by allowing air to take different ways out.

The first way: You can print the drill guide from the previous step, place it on the grass catcher, and use it to mark the positions of the holes using the appropriate drill bits. Then use these drill bits to create the corresponding holes. Be careful when drilling through plastic. Going too hard might cause breaks and overall headache. If necessary, place a piece of wood on the other side of where you are drilling for additional support and safety.

The second way: You do not need the drill guide for this one. Use the large bit to drill the hole in a position where you imagine the other screws will fit around it. Eyeball it, but I recommend checking your eyeballing skills using the case. Drill that hole, then place the case and align it. Then you can use the holes in the case itself as guides for the remaining four holes.

As mentioned in the "supplies" section, remember that you can make the mounting holes with a piece of metal heates with a lighter, at least in most plastics.

Step 2: Assembly

Sorry for the lack of pictures - I installed this "off the grid" and might have forgotten that I had my phone on me most of the time. Let me know whether you need additional pictures!

But this one is quite easy! Insert the lid into the case and push a piece of filament through the hole. Make sure the flap can move easily in its place.

Then place whatever mesh you want to use against the bottom of the case to make sure your screws can pass through it. For a metal mesh like the one I used, you might need to use an awl to widen the holes. For a plastic screen, like fly screen, you can just punch them through.

Place the case with the mesh on box and align the holes. Insert the screws and tighten the nuts on the other side (or insert them from the inside and tighten the nuts outside. Works both ways, but this way might cause less grass to catch on the inside.

And that's all!

Step 3: Mow!

All that's left to do is to spin up the blades and fill the bag with some grass. As soon as it is filled up, the flap should come down and tell you to empty your grass catcher. You might have to tweak the flap, either by adding a (very tiny) weight to it, or filing the edges down with some sandpaper to make sure it drops easily.

Also, there's a secret! There's "STOP" embossed into the inside of the thing, so in theory you should see that once the lid drops. Since I printed mine in black that doesn't work, and I have yet to go over it with some kind of visible paint to make it stick out. But guess what - seeing the flap go down has been enough for me to tell when to empty the box!

Thanks for checking out this Instructable, I hope it brings you joy and inspires you! Let me know in the comments, and share when you make this!

As always, remember to Be Inspired!

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