Liittle Monster - Circuit Diagrams

The following wiring diagrams are to build the Little Monster Mower 


At various stages I have written code to test your components are correctly installed. Most of these test sketches are now integrated into the latest full code version or you can test components individually with cut down code from the GITHUB site.

Testing your mowers components page



Build Tips:

1. Nearly all of the Arduino sensors and modules operate (or are powered) on 5V.  The 5V for the sensors is generated by a step down module which steps down the 12V motor voltage to 5V for the sensors.

It makes sense to group all the 5V+ cables and GND cables from the modules together in parallel.  These can be terminated at a wire blocks.  Then have a single 5V+ lead going to the Arduino MEGA and a single GND cable going to the Arduino MEGA.  This results in a much tidier wiring inside the mower and saves on unnecessary wires 

2. It can become quite a mess of wire in the robot.  Try and keep wiring colors to

  • 5V red
  • GND Black
  • Communication wires to sensors Green/Yellow/Blue??



OK. so it can be a little intimidating with all the wiring diagrams, so where shall you start.  

Most of the electronics in the Little Monster are located on the inside of the mower Hood.  Print the hood first.




Initially you can just plug the Arduino cables into the MEGA pins and find the corresponding pins on the component using the diagrams below.

I recommend in the end to solder the wires to the pins, but for starters just get the wiring correct and working.



DC Step Down Module:

Before connecting it to the circuit, ensure that the DC Step down module is outputting 5V. Connect a 12V source to the input side of the module and turn the adjustment screw anti-clockwise until the output is 5V.  If you output more than 5V you are in danger of damaging your sensors.  This module is used to power all sensors and the Arduino boards





The mower is powered by 2x 12V 1:200 Geared DC motors.

12V from the 18650 Lithium Ion battery's (3S) is sent through a switching relay (to turn off main power when docked) to a motor driver.  The motor driver sends the pulse signals (PWM) to the drive motors to define the speed the drive motors shall turn at.  The Motor driver is connected to the ARDUINO MEGA with 6 wires  ENA/IN1/IN2/IN3/IN4/ENB as shown in the diagram.


In the same diagram the Step Down module is shown which is used to power the sensors and is connected to the 5V and GND pins on the ARDUINO Mega.




WIFI / NodeMCU Connections

The NodeMCU board gives the mower WIFI connectivity and can then be controlled by the smartphone APP.  The Smartphone APP details are in the All Mowers section.  A separate NODEMCU board is used due to the intermittent connection to the WIFI. If the WIFI signal is lost the board needs time to re-connect.  This can cause a disruption to the mowers operation until the WIFI is connected again.  For this reason a separate board is used which can handle the re-connection while the other boards take care of the safe mower operation.

The NODEMCU is added to the mower with a few simple connections




The Little Monster uses 1 sonar to detect objects around the mower.  If an object is detected the mower stops and turns. The sonar modules require 5V+ VCC / GND / Trig / Echo cables.  All the 5V+ wires can be ran in parallel and GND wires can be ran in parallel.

The Sonar module is wired to the Arduino MEGA as follows: 






The Voltage, Amp and Rain Sensor readings are read by a separate Arduino Nano board. As the perimeter wire detection requires some special software, the normal Analog-read function in the Arduino software is not available.  For this reason a separate Arduino Nano is used which then sends the data to the MEGA by serial communication. 

The Volt and Amp sensors are located in the Hood of the mower in the top right hand corner.  A Sepcial adapter plate must be printed to locate them together with the Arduino Nano


The 12V supply from the Lithium-Ion batteries is measured by the Volt and Amp sensors. (the sensors operate on 5V but they are measuring the 12V supply)

For the Volt sensor the 12V +ve and GND lead is measured. the volt sensor measures when the battery is empty and the mower needs to re-charge

For the Amp sensor only the charge GND wire is used.  The charge wire is required as we are only interested in the Amp reading when the mower is charging, or to detect when the mower has reached the charge station and should shut down (turn all motors off and turn the relay to off and cut the 12v supply to the motors).

The separate code for the Arduino nano needs to be loaded into the Nano.

The simple wiring for this setup is shown below:


Check communications here

If you are having problems with communications I recommend soldering the wires to the nano. I have found that any sort of loose connection can cause bad data to be sent from the Nano to the MEGA.





The Real Time Clock (RTC) is used when the mower is programmed with start and stop mowing times and dates.  The clock requires a small cell battery to power the clock when the mower is switched off.




Space is quite limited in the Little Monster so a bracket is used to hold the Mower Motor Controller on the side of the Motor Beacket.


The blade motor is also powered by the 12V Lithium Ion Cells.  the 12V supply passes through the relay (...which turns off all power to the blade motors when docked) to the 43A motor controller.  The motor controller is connected to the ARDUINO MEGA and to the blade motor as shown in the following diagram:







TFT Touchscreen & Compass:

I recommend to use the HMC5883L Compass.  There are many cheap QMC5883 modules, but they tend to freeze up causing issues.  Use the provded compass holder to secure the compass in place.(Y axis should go forwards and pins at the rear of the mower.

After the mow has been completed (due to batteries being empty) the mower needs to locate the boundary wire and follow it back to the charging station.  depending on the end orientation of the mower it could follow the maximum length of the boundary wire around the garden which can be problematic.  The compass angle (which can be set in the software for your garden) orientates the mower to the optimum angle to move off and find the wire.

The TFT Touchscreen needs a seperate Arduino MEGA as a shield controller.  The TFT MEGA then communicates via Serial 1 to the Mower MEGA Serial 3 ports.  The BMP files in the TFT folder need to be saved onto a Micro SD car and inserted into the TFT shield.

Pay attention that the SD card is on the right hand side of the touchscreen (oreientation).  Use the STL files to print the Shield bracket which will hold the MEGA and TFT in place. 

Ensure the latest library files are downloaded from the GITHUB site to support the TFT.







The perimeter wire is detected by the mower using a Amplifier and a 11P Ferrite 150mH Inductor.  A signal is passed through the boundary wire which is detected by this sensor.  If the wire detection is erratic try adjusting the amplifier by turning the adjustment screw.


This setup is from another Robomower project and is not my design



The bumper bar circuit is very simple and uses 2 small micro-switches.

These are connected to MEGA on Pins 47 and 48. The ground wires are tied together and connected to the GND pin.  When the microswitch is triggered it closes the circuit which is detected by the Arduino MEGA.  The test sketch can be used to ensure that the bumper bars are working correctly.







2 sensors are used to detect the angle of the mower and if the mower is upside down.  The sensors can be individually activated in the settings (requires Mower Software 8.0 or higher).  The 3D printed clamp must be used as this sets the angles for the sensor and ensures that the sensor "ball" is returned to the correct position after activation.




The 12V circuit is shown in this diagram. Use 1.5mm wire for the 12V circuit.

The main switch should cut power to the complete circuit.  The mower should however be able to charge when the main power is off or on.

The relay controls power to the motor drivers and blade motor drivers.  This relay is turned off when the mower is not in use to conserve power.








A basic overview of the 5V witing circuit.  Please refer to the detailed diagrams above for each wire pin connection.  Use the provided Arduino cables for this wiring.  I recommend to solder the wires to the pins pads rather than plugging the cables.  The plugged cables become loose with time leading to connection issues.

Try and keep all the 5V positive and 5V negative leads from the sensors together and group them into 2 bundles.  Then use 1 wire for the 5V positive and negative bundles to the MEGA, Nano and NodeMCU