Introduction: Weed Trimmer Motor Replaced With Brushless RC Truck Motor and Uses an Arduino Control Board

For years I have been using my trimmer with upgraded LiPo batteries, kind of like the ones used in an RC car. LiPo batteries have worked well and provided much more trimmer use time at quite a bit less weight. The problem was the motor finally gave out the other day. The trimmer overall performance was OK but it lacked power. I figured it is time to make use of more powerful RC truck brushless motors I had laying around. Using an RC motor means I can use RC LiPo batteries as well which are lighter and have plenty of capacity to far exceed trimming time of any battery operated electric trimmer on the market today.

This project is a little more advance because of the use of an Arduino board. This was because I needed some way to turn the trigger into a controller to tell the RC truck Electronic Speed Controller (ESC) to start. There may be other ways to do this and I would be curious to hear other ideas, meanwhile an Arduino will have to do. You will also have to have the ability to solder wires to the Arduino as well.

It is important that the weed trimmer is compatible to the brushless motor you plan on using. Turns out that the trimmer original motor had a 5mm shaft just like the larger 1/8 scale RC truck motors use. Note that if your trimmer has a 4mm shaft than look for a 1/10 RC truck motor for your swap.

Parts Overview
You will need the following:

  1. weed trimmer
  2. RC truck brushless motor
  3. ESC
  4. arduino
  5. misc wire

Lets get into more detail ...

Step 1: Install the Motor

Picking the right motor is important. Brushless motors have a KV rating to help pick the right motor for the job, the lower the rating the slower the top RPM but the more torque it has. I suggest using a 2100kv or less. In the end I set the Arduino code to about 80% throttle to replicate the original motor speed. In my opinion more RPMs is not good, you want more torque.

I purchase this Hobbyking 2100kv motor and ESC for $95 combo, though its not top quality for RC trucks it has plenty of power for this application. The plastic plate was a good fit for the new motor, I did have to drill two new holes as the original screw holes didn't line up, this was a minor modification.

Next I placed the spool on the end of the motor. Though it was pressed on the original motor but wasn't a snug enough fit on the new motor. I had to drill a small hole and insert a screw to securely connect it to the new motor ... you have to look closely at the picture for the little screw. This is common practice in the RC world as shafts on RC motors have a flat spot ending in a shape like a 'D'. A set screw is used to hold gears onto the shaft, it doesn't have to be fancy. My trimmer spool had a soft metal lead like base that was easy to drill. I let the screw do the tapping and added loctite. Turned out to be simpler than I expect.

Step 2: Connect the Esc and Battery

You will need a RC truck battery, really you need two to last for most larger yards,something in the 5000mAh 7.4v range.

In my case I had an existing battery and ESC setup so no wiring was needed. Of course you may have to buy connectors to finish your assembly if you are building from new parts. I have to assume you know how to wire and use a RC ESC and battery.

Step 3: Add an Arduino to the Mix

I needed some sort of way to recognize when I press the button on the weed trimmer and send a PWM of full throttle ... the trick is that it has to ramp up or the ESC may burn out. I did this using a Arduino Pro Mini but a nano would do the trick. You should be able to get one for less than $10 online.

Next I wired the ESC pulse to the Analog pin0 and routed power from the ESC to the Arduino board power pin ... and of course connect ground as well.

I think for the most part all weed trimmers have a switch at inside the trigger in series with the battery and motor at the bottom. I shorted the battery terminal at the top since I will no longer be using it. As a result the two wires that would originally plug into the motor function as my switch/trigger connection. This was soldered between the Arduino power and digital pin2. I wired a 10k resistor to ground then to the switch on digital pin2 like shown in this example instructions here.

The trick is the code, the code must ramp up the motor. Here it is in raw form:

#include <servo.h></servo.h><Servo.h>
int switchState = 0;
int switchPin = 2;
int servoPin = 14;
int ledPin =  13;<servo.h></servo.h>

int servoPwm = 1500;

Servo servo;

void setup() 
  pinMode(switchPin, INPUT);
  digitalWrite(ledPin, LOW); 
  delay (7000); // wait for ESC to startup
void loop() 
  switchState = digitalRead(switchPin);
  if (switchState == HIGH && servoPwm == 1500)
    digitalWrite(ledPin, HIGH);
    servoPwm = 1800;
  if (switchState == LOW)
    servoPwm = 1500;
    digitalWrite(ledPin, LOW); 
  delay(100); // waits for the servo to get there 

Step 4: Get Creative and Button Things Up

You will have to get creative to finish things up. I ran an extra long wire so I could carry the RC car battery in my pocket. As I mentioned earlier I use two. If you are going to use LiPo batteries make sure the ESC you are using is compatible. The ESC should cut the throttle if the battery gets too low which in turn prevents you from damaging the battery ... just like with a typical RC car.

In Review

Thought this is not a baby step type of instruction you have to admit it is a creative setup and a great way to revive that old weed trimmer. I always thought that this type of upgrade would be perfect for commercial use. This trimmer will keep up with any gas one out there in addition to reducing pollution, noise, and cost of gas. If you are interested in making a conversion but need me to build you an Arduino I would be happy to do so for a fee. Send me a message and I will give you a quote.