Electronics Project

GpSpeed - Greenpower Speed Controller - Rotary Racer


An electronic speed controller for Greenpower Electric cars. We have been using a, team built, motor speed controller in the Rotary Racer cars for a number of years. 3 different designs have been built and used. This is a new design that tries to bring all of our ideas together to produce a relatively simple, pupil buildable, speed controller which is flexible enough for Greenpower teams to use and extend to provide additional features.

The idea is to provide a platform and a kit allowing teams to get pupils involved in electronics rather than provide a fully featured and built end system. The basic speed controller can be extended with software and/or other hardware modules to provide a highly featured electronics control system for a Greenpower car. As well as speed control it offers battery voltage, current and other parameter monitoring. We don't provide a full, turn-key system, as we are trying to encourage the teams to involve the pupils in electronics and software engineering rather than just provide a simple to use with little understanding system and also allow teams to differentiate themselves from each other.

We provide an electronic schematic (circuit diagram) and PCB layout in Kicad format. This allows modifications by teams if desired. We also can supply manufactured PCB's allowing teams to easily build the design given here and complete kits with all components. Not all of the components are needed depending on the usage desired. By installing extra components and changing the on-board software the design can be extended to perform many different functions. We provide basic software that will operate the system. It is for teams to extend this system as wanted and to get involved in the electronics and software design.

So what can it do ?

Some Features:

Info on our control/data logger project is at: http://www.greenpower.beamweb.co.uk/groups/electronics/gpControl/index.html

This is a simple, basic design plenty of scope for improvements :)

If you use this system all I ask is for you to involve the pupils as much as possible in the design, build, development and usage of the systems produced. Please get the pupils to build these, not provide a ready built one for them to use. The idea is to show, involve, educate and try to inspire young engineers.

Even better use this as a basis for ideas and develop your own system !

Note we do not provide any warranty that these systems will work or guarantee any support. The design and any kits are supplied on an “as is” basis.

We are, slowly, creating some Greenpower orientated electronics information at: http://www.greenpower.beamweb.co.uk/groups/electronics/GreenpowerElectronics/index.html



We can provide a basic kit of parts to assembly these units. These are sold at cost price plus a small amount of money to the CSS teams whose pupils package up the kits (put all parts into bags)). These are supplied "as is" with no support, just to help teams get going. We hope some enterprising older pupils from schools may be able to help other teams who have problems. We will set up a forum to provide support info and ideas from other teams somewhere ... Please contact: rotaryracer@greenpower.beamweb.co.uk

Pupil Involvement and Learning

There are various building and learning activities that pupils/students can do with the gpSpeed controller. Some of these include:

** TBD ** Ideas for bits of work.


Voltage 12 to 24V DC
Motor PWM Current Max 160 Amps, transient 640Amps
Motor Current Limit Hardware 70 Amps approx (can be changed). Software controlled limit normally about 60 Amps but programmable
Soft start Programmable.
Motor PWM Efficiency At about 90 - 100% PWM, about 98%
Motor PWM Frequency 20kHz (Programmable)
Fan PWM Current Max 1Amp 24V, filtered PWM at 20 kHz (Programmable)
External Power supply 12V at 500mA, 5V at 250 mA
Processor PIC18F2520:  Up to 32 MHz clock, 32 kBytes program FLASH, 1536 Bytes RAM, 256 Bytes EEPROM, 10 Bit 13 channel ADC, 2 CCP/PWM channels, SPI, I2C, UART, 4 timers

GpSpeed Block Diagram

This diagram shows the main operational blocks on the GpSpeed controller board.

GpSpeed Parts

Car Circuit Diagram Basic

This circuit diagram shows a complete circuit for a Greenpower Car using the gpSpeed controller. It is designed to be simple to use, no more difficult than using a relay, probably easier due to its use of screw terminals. The main batteries are on the left connected in series to provide 24 Volts. There are two main power switches, one SW2 in the -ve, ground connection accessible outside of the car and SW1 a current trip breaker accessible by the driver. Alternatively a 70 Amp fuse with switch could be installed in place of the breaker. It is ESSENTIAL to have a fuse or current trip breaker as near to the batteries as possible EVEN when testing on the bench. Batteries can generate a huge current that can be dangerous and damaging. It is also important to have a main power cut off switch close to the driver and easily used incase of speed controller or other electrical/electronic problems.

The GpSpeed controller is easily installed. It has screw terminals for the batteries B+ and B- lines and screw terminals for the Motors M+ and M- terminals. There are then three screw terminal connections for the, spring loaded, throttle potentiometer or hall effect throttle unit. Alternatively a simple push switch can be fitted to between two of the terminals.

This shows two possible throttle connections to the gpSpeed controller, a variable speed such as a twist grip and a simple on/off switch.


There is a version 1.0 and 1.1 of the GpSpeed. There are only minor differences.

The user manual documenting the board for building, testing and usage. gpSpeedManual-1.1.pdf, (Older design gpSpeedManual-1.0.pdf)
A more detailed development manual describing all of the pins and giving information of software development gpSpeedDevelopment.pdf
The circuit schematic gpSpeedSchematic-1.1.svg, gpSpeedSchematic-1.0.svg
The component list gpSpeed-parts.ods, gpSpeed-parts.xls
Heatsink bracket design gpSpeedBracket.dxf, gpSpeedBracket.png
A diary of the GpSpeed development http://www.greenpower.beamweb.co.uk/diary?cmd=view&id=3
Some pictures of the boards during design and build http://www.greenpower.beamweb.co.uk/showcase?cmd=view&id=88
Some pictures from Crossley Heath schol. Note that they have produced some nice looking coil formers and large capacitor holder using a 3D printer. They intend to make the 3D designs for these available. extra
Forum for help and information on the GpSpeed controller. /forum/GpSpeed

Circuit Diagram

The circuit diagarm is at: gpSpeedScehmatic-1.1.svg, gpSpeedScehmatic-1.0.svg

Description: TBD

PCB Layout

The two letter initials at various places on the board are from the Rotary Racer teams year 7 to year 11 pupils and Gareth (17) who did most of the PCB tracking for the board.


See the documents list for the board components.

Other bits include:

Twist grip throttle (about £40) http://www.4qd.co.uk/accs/pots.html#mag
Twist grip throttle (about £9) http://www.petrolscooter.co.uk/twist-throttle-grip-electric-scooter-es-05.html?fee=1&fep=2650&gclid=CInpn4zVirwCFdShtAodd3QAtg


We can supply complete kits or parts or just the PCB available with some awkward to buy bits to Greenpower teams. These are sold at cost price plus a small amount of money to the CSS Greenpower teams whose pupils package up the kits (put all parts into bags)). The aim is to help get pupils involved in electronics, software and systems and control.

Part Item Price
gpspeed-pcb PCB with PicAxe processor, inductor wire, PCB thickening wire, aluminum heat sink piece and MOSFET mounting  pads, nuts and bolts £22.70 + P&P + VAT
gpspeed-kit PCB and complete pack of components to build the controller £77.00 + P&P + VAT

Note we do not provide any warranty that these systems will work or guarantee any support. The design and any kits are supplied on an “as is” basis.


See the user manual in the documents list.


The board requires the main Motor control MOSFET's to be bolted to a heat-sink using insulating pads. We use a short length of 19x19mm angle aluminum 3mm thick to transfer the heat from the MOSFET's to a small heat-sink or car body part. The actual amount of heat energy to dissipate is fairly low: (below 4 Watts at full power). The board has 4 x 3mm holes, one in each corner for mounting. It can be mounted "open frame" to sal an aluminium sheet or in some form of box as long as a degree of ventilation is provided to the electronics or the box is thermally conductive to get the heat out. There is a 2D CAD file for this angle heat-sink in the documents section although your own designs can be used. Do make sure they are well debured and sanded smooth where the MOSFET's fit to save short circuits.

Stephen Coughlan who helps at Crossley Heath school has provided some nice 3D printed parts they have designed and made to improve some aspects of the GpSpeed. Information on these are at:

Capacitor spacer http://www.thingiverse.com/thing:355480
Inductor Mount http://www.thingiverse.com/thing:360545


See the user manual in the documents list for basic systems test. A small board with 24V or 12V power supply, and 12V 5W car bulbs can be used to test and experiment with the unit.


The processor used is a PIC 28pin DIL device. A number of different PIC processors can be used. One that we recommend has 32 kbytes of program memory, 1 kbyte of RAM and various hardware peripherals. Two particular versions we would recommend include:


This comes preprogrammed to allow use in the PicAxe Basic programming environment. This system is commonly used by schools DT labs.


PIC 18F2520

This is a bare PIC processor. It can be programmed in 'C' using a PIC programming device. Alternatively a serial boot loader can be flashed into the device allowing programming from a PC via a serial programming lead. Various 'C' based software programming environment can be used. We use the sdcc compiler and sometimes the pikdev environment on Linux PC's.

We provide the following code"get started" code. This includes the bare speed controller software ans snippets of code showing use of various features.

Picaxe Basic simple speed controller. This allows speed control with switch or throttle and basic soft-start ability. softwarePicaxe
Sdcc 'C' simple speed controller. This allows speed control with switch or throttle and basic soft-start ability. softwareC


The PIC microcontroller dataSheets/PIC18F2420.pdf
The Hall effect current sensor dataSheets/hxs50-np.pdf
The MOSFET driver dataSheets/ir2125.pdf
The MOSFET's dataSheets/stp80nf10.pdf
The switch mode regulator dataSheets/lm2575.pdf

Extra Development

Some ideas for further development include:

Test and Play Rig

This is an example test rig. The GpSpeed is mounted to a small piece of MDF board (2.5mm holes drilled and plastic PCB pillars screwed in). A 10k ohm linear rotary potentiometer is connected to the throttle input and a TMP37 temperature sensor is connected to the secondary analogue input. A car 12V 20W stop light has been connected to the motor terminals and the power is supllied from a mains to 12V 48W "brick" type power supply. A fan can also be connected. A PicAxe AXE033 LCD display with real-time clock chip options is attached to the GpSpeed's display port.

With this arrangement quite a lot of experimentation and software development can be done in a safe and easy way. In the software area there is a suitable demo program available for this arrangement. It operates as a soft-start, current limited speed controller (the bulb will brighten and dim and fan go faster and slower with throttle position). It will also measure the temperature and read the real-time-clock. The throttle position, current being used, the time and temperature are displayed on the two line LCD.


The design is copyrighted to the Chipping Sodbury Rotary Racer Greenpower team. It is provided free for non commercial educational uses.


Note we do not provide any warranty that these systems will work or guarantee any support. The design and any kits are supplied on an “as is” basis.