This is my first attempt at an instructable . I have virtually no experience in electronics but at 80 years old I figure you are never too old to learn. Mowing the lawn was getting to be quite a chore so I decided to build an easy to use mower that I could either ride on or sit on my back porch and mow from there. I bought a used electric wheel chair on Craigs list for $100 to start the project off. After stripping wheels, motors, and battery box I mocked up the chassis with 2x2" lumber This also allowed me to fit the donor mower to the frame. I used the deck from an old Briggs 22" walk behind push mower. Once that was done I welded up the steel frame from 1 x 1/2" steel tubing. I tried building the electronics from a previous project in Make but I just didn't have the talent or experience. I decided to go with a Sabertooth 2x25. I already had the motor wiring and was set up for 24 volts. The folks at Dimension Engineering were a great help in advising me how to set up the DIP switch. I got a used 54 MHz. RC set up from a friend who fly's model airplanes. The Sabertooth is a no brainer to hook up. It even supplies the 5 volt power for the receiver. A switch, and fuse were all I needed to complete the job.
Step 1: Materials
Used Electric wheelchair $100.00
Steel Tubing $25.00
Donor 22" mower $ 25.00
Sabertooth 2x25 $125.00
Lawn mower batteries (2) $68.00
Misc. Nuts, Bolts, Switch , and fuse $15.00
Used RC TMX, AND RCVR $35.00
Step 2: DEMOLITION
Before taking the old wheel chair apart, look it over and see what you can use in the project. The motors came with a built in parking brake that puts the motors out of gear. This was handy for testing out the controls. The castoring rear wheels came with a nice bearing set up which saved a lot of work. The battery box was designed for U1 batteries and had a nice post mount for the removable seat. The chair came with a 24 volt charger, which I saved along with the charging plugs. I saved all the wiring from the motors to the batteries because they had very nice plugs and connectors with polarity indicators. I saved the joy stick and associated electronics for a possible future project that does not require a radio. The seat plugs into a pedestal built into the battery box. This makes it a riding mower complete with seat belts and arm rests.
Step 3: The chassis
Building a wooden mock up of the chassis saves a lot of mistakes and wasted metal. My best friend did the welding for me. My eyesight doesn't allow me to see well enough thru a darkened hood. He did a great job with his TIG welder. The wheel chair originally had the drive wheels in the front. Remounting them in the rear was an easy task using the same mounting points with motor pointing backwards instead of forward. The donor mower deck was then put in place to set up the overall length and position the front castors. This was also a good time to position the battery box for clearance with the deck. The final step was to fashion an electronics deck in the rear.
Step 4: Suspend the deck
A fresh coat of paint makes an old, rusty deck and motor look like new. Start by positioning the deck where it won't interfere with the front wheels and the battery box. Raise the deck on wooden shims to the desired lowest cutting height. Set the wheel adjusters to the lowest point.
Fabricate four straps to hang the deck. The top is bolted to the frame, and the bottom connects to the old wheel mounting axles. This allows the height to be adjusted just as it did on the old push mower. .The deck tends to sway fore and aft, but once the desired height is obtained simply tighten the top bolts and it will be stable. If you have planned your mock up properly there should be ample room between the deck and the front wheels. Shorten the pull starter rope and be sure to dis-connect the dead man switch. I installed a small push pull switch to kill the motor.
Step 5: The Sabertooth 2x25
The Sabertooth 2x25 by Dimension Engineering is really a giant in a small package. It provides everything and more than I need. The DIP switches allow it to be programmed to control the mower any way you choose. It has a built in fail safe that shuts down the system if you lose RC signal. The 5 vdc regulated power supply provides adequate current for most RC systems. The specs say that if you load it up with too many servos it may require additional power, but since I am not using any servos there is no problem. My Sabertooth is mounted directly to a steel plate and I have not noticed any heat build up. I am sure this unit is capable of much more than I am using. It comes with 4x40 mounting bolts for easy mounting.
Step 6: Installing the motor controller
Now comes the fun part! It is a good idea to put in a terminal strip between the power and the Sabertooth 2x25. This isolates the on/off switch and the fuse from the controller. Connect the positive lead from the charger to the hot side of the switch. This allows the batteries to be charged with the switch off. Hook up the power and motor leads to the controller. They are well marked . If the motors don't turn in the right direction, simply reverse the motor leads. My motors had an extra pair of leads that actuated an inner relay. I hooked these up to come on with the power switch. At this time you "MUST " set up the DIP switches . If you don't the unit goes nuts when you apply power. I set mine up for exponential steering. You can opt for tank steering . (Read the DIP switch instructions that come with the controller)
Step 7: Installing the RC receiver
Positioning the RC receiver is critical. It must be placed as far away from any electrical interference as possible. If you have limited range on your mower move the receiver to another location. I'm not an RC guy, but I have been told that 12 inches should be enough. The standard servo leads come with 12 inch leads I think you can also wrap the receiver in aluminum foil to shield it. I mounted it to the frame using two sided foam tape. Try to keep the antenna away from electrical noise. I taped mine to a 36 inch x 1/8 dowel.
Step 8: Final assembly
My chair didn't come with batteries. U1 wheel chair batteries are quite expensive. A trip to the local auto supply and I found heavy duty garden tractor batteries rated at 360 amp hours for $34 each. I have run for two hours or more and did not run out of power. The volt meter still showed 24 volts. I plugged the seat into the battery box and was ready to go. The blue led was not visible due to the cover plate . I hooked up an old 0-30 vdc meter and an old 24 volt light as a precaution to remember to shut off the main switch. I fashioned a 36"' dowel to attach my antenna behind the seat. I plan to add a pair of foot rests just for comfort.
Step 9: RoadTest
Now is the time that I wish I had spent more time playing video games. It takes a little practice to get a smooth turn and a steady straight run both forward and reverse. I have the DIP switches set up for exponential steering. That lets me control the speed and direction with one joystick. This is the way most wheel chairs are set up. The way my transmitter is set up, the left joy stick is used for the throttle and has a ratchet built in. This was confusing when I tried to use tank steering. I did try to use the left stick for forward and reverse and the right stick for direction. This seemed to work okay but I am more comfortable using just the one stick. The really nice thing about the Sabertooth is that you have the option to configure it any way you choose. My radio is very old and requires a long antenna to assure adequate range. My yard is 200 feet long and I had no problems once I moved the receiver away from the wheel motors. If I had a larger budget I would buy a
2.4 GHz system. These new systems only require a very short antenna, and are far more tolerant of noise. The new Spektrum 5XDE is available for $99.00.
This was a fun project and is attracting a lot of attention from my friends and neighbors. The model airplane club that donated the radio is having a ball running it up and down the grass runway. The surprising thing to me is the way young kids can zip it around with little effort. I don't however, let them ride on it for safety reasons. Except for the welding I would considerthis an easy project. The wooden mock up chassis would have worked just as well, but I don't think it would have held up over time. I will add a video as soon as I can borrow a movie camera. Feel free to contact me any time.