The Speed Control Hardware

The schematics can be downloaded here.

The 5V supply

The 5V supply is only used if you want to power the motor and receiver/servo’s from the main battery pack. Skip this part if you want to use a separate battery pack for the motor and the servo’s.

The battery pack is connected to an LM2940 5v voltage regulator. This is a low dropout regulator and should not be substituted with a 7805 regulator. The reason is that as the battery gets depleted, the voltage will drop to not much more than 5V and with a normal regulator the output will be far below 5V – which might stop your receiver from working (this is not a good thing, especially if your plane is high in the sky!)

The switching circuit

We used an IRFZ44 power Mosfet as a switch for the motor. This mosfet can handle up to 50A and has an on-resistance of 0.028 Ohms. If you want to use a different mosfet, make sure it can handle the current (a starting motor draws quite a lot of amps!) and also a low on resitance (a high on-resistance mosfet will dissapate more energy as heat.)

The diode is very important to protect the circuit. The motor is an inductor and when the switch (mosfet) is open, it will try to force current to flow. The diode creates a path for the current to flow when the connection with ground is closed.

Current flow when mosfet is on

Current flow when mosfet is off

Any diode with a suitable current and voltage rating would work.

You will notice a transistor is used to drive the gate of the mosfet. This transistor converts the 0 to 5V output of the microcontroller to a 0 to Vbat voltage. If you try to drive the mosfet directly from the micro it won’t switch fully and behave more like a resistor than a switch (it will get really hot quickly and the motor will turn slower than it’s suppose to.)
You can use substitute Q1 with almost any general purpose NPN transistor.

The Microcontroller

We used the Pic12F675 micro because it’s small (8 pin DIP/SOIC package), cheap (around $2?) and easy to use. As a bonus it also has an analog to digital converter.Two LED’s show the status of the speed control.

Maximum Voltage Detection

An optional circuit is connected to the ADC of the micro. We added this because we wanted to use 3 lithium ion batteries (maximum of 4.2V x 3 = 12.6V) to a motor that could handle far less. The idea is that the micro monitors the battery voltage and never allow the output to the motor to be above a specified threshold.