The model I chose for this conversion was SIKU no. 3258, a 1:32 scale diecast model of a Fendt 936. The model has quite good detail however the main reason I chose this was because SIKU models are generally made from various diecast parts screwed together which suits modification to radio control perfectly.
I learned from the Massey and decided to use lower voltage battery which removed the need a voltage regulator and all the problems associated with that. That meant I had to squeeze less components into the small tractor and could power the tractor with a battery from 3.3 to 5 volts. This worked well with my original design but I have since moved away from the XBee radio modules and now need to upgrade the model to use the NRF24L01 radios.
Final Design Costs
These are the costs of the components used in the final design.
|SIKU Fendt 936||1||€26.00||SIKU No. 3258|
|Hobby King Servo HK15178||1||€1.50|
|SIKU Fendt Gearbox||1||€16.80|
These are l parts I bought the original design before I updated the model.
|XBee Radio Transciever||1||€20.00|
The steering is controlled with a hobby king servo, you can find the details here. First I cut the steering wheel shaft and glued the steering wheel in place as the shaft was in the way. I then removed all the shafts and gearing associated with the steering knob on the cab as this was also in the way. I then simply drilled a hole in the steering rack, connected a rod to the servo arm (a stiff paper clip will do) and mounted the servo inside the body of the tractor. This seemed to be the best approach because you can adjust the servo position slightly if you have to, whereas a gear to the steering rack requires the servo to be fixed in the position which best suits the gear and can be difficult to achieve.
I used a SIKU motor and gearbox assembly which can be bought from a number of small model websites or on ebay. The one I chose was to suit a SIKU 6752 or SIKU 6754 which are Fendt 930 models. This gearbox comes with wheels which have a socket to match a nut on the axles so that the wheels don’t just spin on the axles. Try to choose a gearbox for your tractor manufacturer as the wheel rims will be the colour to suit, this is not possible for all brands unfortunately. The install for this gearbox was quite simple there was a small bit of filing required to fit the gearbox into the model but apart from that there was no major issues.
This tractor originally relied on an Atmega 328P microcontroller loaded with the arduino bootloader. The Atmega chip ran at 16MHz which required a 16MHz crystal and two 22pF capacitors to achieve. I also added a 100nF capacitor to filter out high frequency noise in the power supply. These components all come install on the Arduino Pro Mini which I use now.
The LEDs need to be wired in such a way that they don’t damage the output pins of the microcontroller. You need to calculate the resistor values for your own LEDs to make sure you are doing it right, you can find a resistor value calculator here. There was no voltage regulator in the model so I had to spec the LEDs to suit the higest possible voltage of 5V.
I placed a power sockets under the front linkage and another above the rear linkage. There is a socket above the rear hitch which can control a servo on an accessory or trailer and a final socket which can communicate with a microcontroller on a trailer to control LED indicators or brake lights on the trailer.
I will be updating the model to use the NRF24L01 radio module soon and the wiring will basically be identical to the Massey 8680. Hopefully it has better steering though.
RC Fendt 936 Circuit Diagram
Fendt 936 YouTube Playlist
There is only one video about the Fendt 936 on the RC Tractor Guy channel so far but I will soon upgrade it to match the other tractors and that will all be in upcoming videos. If you want to view the videos related to the RC Fendt 936, follow the link below to visit the playlist on the RC Tractor Guy YouTube channel.
RC Fendt 936 Code
The most up to date code for the Fendt 936 can be found in the forum. I post the code here so that I can update it easily and discuss any problems people are running into when trying to use or modify the code.