Lawnie the Remote Controlled Lawnmower

Back in 2019 I made a remote controlled lawnmower. It is not a very original design, it follows closely the mower from Make Magazine and many other youtube videos, with some modifications to suit what parts I could get. I got an electric wheelchair and an electric mower second-hand, bought a speed controller and some relays, and made my own RC controller and receiver with arduinos and nRF24L01 radio set. I tried my best to keep the cost low, using as many second-hand parts or things I already had in my shop. This mower has been mowing my lawn ever since, and makes a tedious chore fun! I highly recommend building one if you still push a mower yourself.

Lawnie as originally built

Here is a video of the mower running around. There are a few other videos on my youtube channel if you poke around. This one shows the controller and startup procedure, but the lawn was already cut so you have to trust me that it cuts the grass.

Safety

Surprise: remote control mowers are dangerous! Even with the blade turned off, the mower weighs at least a hundred pounds and is steel, so if you run into something (patio furniture for example) it is likely to cause damage. Do not chase dogs or children! Do not operate close to expensive autos!

My mower has an emergency stop button that cuts the power to the relays that enable the drive and cutting motors. It isn’t rated for the current they draw, so it only cuts power to the relay coils, which means there is a failure mode with a stuck relay that is not covered by the e-stop. The battery can be pulled up and out to kill the mower entirely (it is held in by gravity) if the e-stop does not work.

Mechanical Parts

The heart of the mower is the electric wheelchair chassis. I was able to get the Golden LiteRider on craigslist for $100, which was a great deal. I bought new batteries and it worked just fine. This is a small wheelchair but is a nice size for the small mower I combined it with.

Wheelchair chassis that I used next to gas mower that I did not use. Note that the chassis is mounted backwards on the actual mower frame

I found a Homelite 24V electric mower for very cheap, I think I paid $20, from a facebook ad. This mower was a nice find because the wheelchair is also 24V so I can run them both off the same battery bank. I had a gas mower in my original plans but switching to the electric was a great choice for ease-of-use and spare-the-air days here in Northern California.

Mechanical Fabrication

I am not a good welder! With an angle grinder and a cheap Harbor Freight welder I got it done anyway. I cut the wheelchair chassis in half and mounted the motors so backwards was now forwards. I used the wheelchair’s front axle with the caster mounts for the front wheels, it has a pivot which is handy for keeping the back wheels on the ground.

Here you can see the wheelchair frame (in red) and the new frame pieces

The mower is suspended from its original axles so that the stock height adjustment system can be used (see the Make Magazine article). It was tight to get the box frame over the axles and clear the height adjusting arm but with some careful mock-ups it all worked out.

Photo from early testing (before paint) shows the front axle mount and the mower hanging mounts

The battery box for the mower is empty, everything is powered from the removable wheelchair battery. Being able to slide the battery out and charge it means I can park the mower outside.

Electrical Parts

Wheelchair motors need a powerful controller as they can pull many amps. I did consider using the wheelchair’s original controller, it worked fine and had a joystick and everything. After some research into other people’s attempts to reverse-engineer the control so that it could be commanded by an arduino, I decided it was better to just start with a DC motor controller designed for robots. I was actually able to sell all the wheelchair controls on ebay for close to the cost of the whole chair, so it turned out to be cheaper in the end as well.

The speed controller I used is the Cytron SmartDriveDuo-30, which is different than many projects that feature a Sabertooth controller. It works great and has several options for control; I’m using the RC servo inputs. I did prove that you cannot connect the power with reverse polarity even for a second, so be careful when hooking up the batteries!

My remote control transmitter and receiver was built from scratch, but it works essentially the same as any hobby controller with 2 servo PWM outputs from the joystick, and one digital output from the motor on/off switch that drives a transistor that drives the mower blade motor relay. The RC project is based on this one:

/* DIY Arduino based RC Transmitter by Dejan Nedelkovski, www.HowToMechatronics.com Library: TMRh20/RF24, https://github.com/tmrh20/RF24/ */
Code language: JSON / JSON with Comments (json)

The mower blade motor is controlled with a relay as it is a cheap and easy way to control so much power. I’m using a 24V 80A relay right now. The original relay was rated for 40A and it eventually got so hot the contacts welded themselves together (fortunately it got stuck in the “off” position!). The relay is driven using a MOSFET, originally a 2N7000, but this seems to have a lifespan of just over a year, so I will try something with a higher rating next time. There is also a relay that can turn off power to the drive motors, this is hard-wired to the emergency stop switch.

Electrical Fabrication

I could have used an off-the-shelf RC transmitter and receiver, but built my own because it is fun. The transmitter uses a joystick from a very old Futaba Tx I had on a shelf, and was hand-wired with an arduino nano and nRF24. I originally powered it with a 9V battery but it is now powered from a lithium cell and a booster/charger module. I wasn’t sure if the booster would interfere with the RF module but the range seems ample for a lawnmower.

Messy hand wiring

The controller case is 3D printed.

Transmitter Case Design

The mower electronics are mounted into a cheap harbor freight toolbox. There are 2 relays, a battery voltage display, the motor speed controller, and the arduino. The emergency-stop is mounted to the handle of the toolbox.

Electronics Box

Notes

Traction is important on hills! With skid steering, the rear wheels are driving and steering, so if you are mowing across a hill, the downhill rear wheel is keeping the mower going straight and also moving forward. Separating the steering and driving wheels would have an advantage when traction is limited. My front lawn has a steep slope so I have glued and screwed knobby tires onto the stock wheelchair tires, this has helped a lot. I followed a youtube video on re-treading a push mower.

My front axle is angled slightly (caster pivots angled backwards) so that the wheels tend to steer straight. This is helpful except when mowing in reverse.

Electric power was a very good choice for the mower. I really like being able to turn the blade off when moving the mower, and I can mow when it’s too smoggy or smoky to run gas engines.

Conclusion

Ask questions if you have them and I will update with more information as the mower evolves.

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