A Lawnmower That Looks Where You’re Going

As a kid, one of the stories my dad told me was about mowing a fairly large field of grass on the farm with a gas-powered push mower. One day, some sort of farm tool was left in the field and the old industrial mower shredded it, sending a large piece of sharp metal hurtling toward his leg. Luckily for my dad, the large plastic wheel managed to stop the piece of metal, destroying the wheel. My grandfather was frustrated that he needed to repair the lawnmower but was grateful that my dad still had both feet attached.

Of course, this story was used as a lesson for me not to gripe about having to mow the lawn when it was my turn, but there was also the lesson that lawnmowers can be dangerous. [DuctTape Mechanic] took it upon himself to see if he could prevent that sort of accident altogether and has created an automatic safety shutdown mechanism for his family lawnmower. (Video embedded below.)

This uses an inductive sensor that can detect metal before it gets sucked into the mower itself. The sensor trips a relay which forcibly shuts the mower down by grounding the ignition coil. While it doesn’t physically stop the blade like other safety mechanisms, it does prevent a situation from escalating by turning off power to the blade as soon as possible. Getting to the ignition coil wasn’t easy as it required getting deep into the engine itself, but now [DuctTape Mechanic] has a mower that could be expanded further with things such as with a capacitive sensor or more smarts to determine if it is detecting underground or above ground metal.

Someday we’ll have robotic mowers, but until then, we laud the efforts of hackers out there trying to make the world a little safer.

Thanks [wahmad530] for sending this one in!

21 thoughts on “A Lawnmower That Looks Where You’re Going

  1. I am not sure that you will have much time between it detecting something and it getting sucked in. It also needs to hit the middle of the sensor fairly close to even trigger it to begin with. Interesting concept even if this also isn’t exactly IP rated either but probably not likely to help all that much there either for long? One kind of typically wants safety sensors to be fairly robust.

  2. Thats quite a nice hack! In my opinion a RS latch would be a nice addition. Just to make shure the mower stays off until the user wants it. Just imagine driving over a screwdriver at a fast pace. Yes, the ignition coil would get shorted out for a few hundred milliseconds. But I bet the motor would have enough inertia to start up again with the screwdriver in the blades.

  3. Good idea, but the inductive sensor has a too narrow range, it is easy to miss an object which is not centered. A loop sensor could cover the full width of the mower. Two additional sensors, one on each side ,could detect the metal object before the mower comes over.
    Electronics should be placed in a waterproof box
    System should detect motor start (hall sensor) and start /stop automatically. Mower should not be allowed to start if batteries are empty.

    1. This is a good prototype. I had the same thoughts as you. It would be a good fail-safe if the mower couldn’t turn on without battery power. Might even be a good idea to include an on/off switch so you aren’t wasting battery life between mows.

  4. A full 3+ seconds for that motor to slow down and stop… How far do you go in 3 seconds pushing a mower? Surely far enough to pass over that metal.

    If you’re driving a coil (the relay) anyway, why not just drive a solenoid to drop a flap on the air intake? It will stop the engine much quicker.

    It’s also very clear that as soon as the metal object clears the sensor the relay opens up again immediately: The motor will instantly ramp back up to full power, right about the time that bit of metal will hit the blades.

    1. A timed relay would solve that potential issue. Where the relay stays actuated for 10 seconds after its triggered therefore preventing the engine from starting up again. I like the air intake idea but that would require powering the solenoid as well. Anyway it was just a prototype I came up with. Thanks for the feedback.

    2. I think he should do us a demonstration,

      Put a few hundred ball bearings nuts and bolts on the ground on some grass near some expensive cars and demonstrate how he “will detect” the stones in time.

      No one said anything about stopping in time or doing anything useful

      1. If I use a timed relay that stays actuated after being triggered it would cause the motor to shutdown/ at the very least slow down by the time you hit the object (so your blade would hit at a lower RPM and cause less damage). Even the saw stop saftey mechanism causes some injury. If this prototype garners enough interest I will do a remake with modifications including a automatic flywheel brake to physically stop the blades.

    3. The yellow loop coming off the handle pulls a cable to a flywheel brake that is normally closed and grounds the coil and will stop the blade in about half a second. he just needs to have a quick release grafted into the pull cable and it will be 3x as safe.

  5. In the early 70’s my parents had a house built and sodded the lawn. I wanted my dad to buy a gas mower but he didn’t like the idea of having a can of gas in the garage, probably thinking I’d start a fire, and he was probably right. He bought an electric mower and a 100 foot extension cord. I hated that damn power cord.

    I had severe allergies in those days and my father used to make me mow the lawn with that awful corded electric mower. One day I was mowing our unevenly sodded lawn and suddenly had trouble pushing the mower. I figured the cord was stuck on something as it would always do about 20 times before I finished mowing, but what had happened was one of the front wheels had dropped into a slot between the sod rolls. Suffering from allergies, pissed off that I had to mow the grass, and then getting the damned cord hung up, I was very impatient and gave the mower an extra hard push. As I pushed I stepped forward and the back of the mower lifted up and I put my foot right into the blades. It chopped through my leather shoe and caught the corner of my big toenail. I jumped up and down on one foot for a few minutes, swearing like a stevedore. I eventually recovered, but refused to ever mow the lawn again.

    Slightly off-topic, I have a friend who is a hand surgeon. He does great business in summer and winter- lawn mower and snow blower seasons…

    1. Completely unrelated to this (great) project, but … when I was a kid I had to mow our lawns with a two stroke mower. I used to get pissed off with it cause it was a cow to start and it constantly cut out. One day after unsuccesfully trying to start it for a while, I noticed it had run out of gas. In my rage I spilt a couple of hundred mls of petrol all over the mower (it was impossible to fuel it without spillage due to our fuel can design). This time when I started it, it started immediately and immediately caught fire. Out runs dad with the fire extinguisher saying I’d never mow the lawns again.

      Bonus! … or so I thought until a week later when Dad got our self propelled Briggs and Straton Drum mower going again and attached a stand on trailer to it. I only ever got to ride that baby once. Of course, as soon as Dad got fed up keeping that old behemoth going, the olds had bought modern and much heavier 4 stroke mower & I was put back on lawn duty. In hindsight I miss that two-stroke masport mover. It was bloody light and easy to push around.

  6. I hold two patents covering a proximity detection system that prevents bystanders, children, or pets from being impacted by all types of agricultural equipment. There are many types of debris that can become projectiles, not just metal. So I think this concept is too limited. Flying debris are an issue with mowers, however the biggest problem I see is that mowers are the #1 cause of traumatic childhood amputations. One of every five mower fatalities is a child. Roll over accidents are also very significant. Visit my Facebook page, Lawn Mower Accident Support and Prevention.

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