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title layout meta-description share author about cats simple-description
How to use a simple motor with the microbit.
text-width-sidebar
Using a transistor to drive a motor on the microbit.
true
jez
How to power a motor with a transistor and microbit.
external
Motor
 
too complex
Use a motor board.

A simple 6V DC motor takes takes electrical energy into mechanical energy. It has hundreds of uses in microbit projects.

{:.ui .dividing .header}

Electronics

Transistor

The motor requires a higher voltage and current than the microbit is capable of supplying by itself. The solution is to drive the motor with a transistor.

The micro:bit's pin0 is connected to the base of the transistor. The emitter is connected to ground. The motor sits between the positive of the battery and the collector of the transistor. The grounds of the battery and the microbit are connected together (a common ground).

Motor with Diode{:.ui .image}

When pin0 sends a signal to the base of the transistor, the transistor opens and completes the circuit between ground and the motor & battery.

Diodes & Back EMF

Electric motors are 'noisy' and can create electromagnetic interference. When the motor is stopped it can create a reverse voltage which can damage the circuit (or worse, the microbit!). A diode is added across the motor which stops this reverse voltage.

Speed Control

The speed of the motor can be controlled with python's write_analog() (or a similar block in PXT) and PWM. This stands for pulse width modulation and is where the signal is rapidly being turned on and off.

Whereas write_digital(1) or write_analog(1023) outputs a constant voltage of 3.3v, write_analog(511) the power is on exactly 50% of the time; or is at 50% duty. The total energy of the signal is 1.65v, or half of 3.3v. The transisor is able to receive the signal and turn on the motor for 50% of the time also.

There is a more detailed explaination of PWM on the micropython documentation.

Final Circuit

Motor with Diode{:.ui .image}

  • 1x 1N 4001 Diode
  • Small 6v DC Motor
  • 6v (4 x AA) Battery Box
  • 470Ω 1/4W Resistor
  • MPSA14 Darlington Transistor

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Code

Python

Power the motor at 100%, 50% & 25%.

{% highlight python %} from microbit import *

while True:

pin0.write_analog(1023)     # full power
sleep(2000)
pin0.write_digital(0)      # turn it off
sleep(2000)


pin0.write_analog(511)     # 50% duty
sleep(2000)
pin0.write_digital(0)      # turn it off
sleep(2000)

pin0.write_analog(255)     # 25% duty
sleep(2000)
pin0.write_digital(0)      # turn it off
sleep(2000)

{% endhighlight %}

Notes

  • This is only drive the motor one way. To go forwards and reverse you'll need an h bridge.
  • Using motors soon gets complex and unwieldy. Consider a motor driver board!

Further Reading