This activity shows how to build a circuit to measure rotation of a motor, which can be used to measure angular velocity, linear velocity, or linear distance.

How the motor rotation sensor works

A motor turns when current is run through it. Likewise, it creates current when it is turned: the motion of the coils generates electricity. By changing this current into voltage and measuring it, a motion sensor can be created. The following circuit does this. The voltage signal is proportional to the speed of rotation of the motor.

By adding a wheel to the motor and an integrator in the software, linear velocity and distance can also be measured.

• experiment board
• GoLink
• GoLink header
• small motor
• cardboard wheel
• hot-melt gluegun
• circuit parts:
  • 2 – 10K resistors (brown/black/orange)
  • 1 -1K resistor (brown/black/red)
  • 1 – 22 microfarad capacitor, OR a resistor-capacitor pair from the RC chart
  • wire

The Experiment Board

If you are not familiar with how the experiment board works, go to this activity for an introduction.

Plug the header into the experiment board so that each pin has its own row to connect wires to. If it helps, use colored markers to make it clear which is which. It is customary to use black for ground, red for +5V, and some other color for the signal.

The Circuit

Before beginning, disconnect the header from the GoLink.

Here is the circuit schematic.

The circuit consists of a half-bridge on the left to give a 2.5 V offset. This is made with a pair of 10 kΩ (brown-black-orange) resistors. The purpose of this is to put the zero point (motor not rotating) in the middle of the range so that the motor can go in either direction and a reading is still obtained.

The output of the motor is constantly fluctuating because of the brushes. If it weren’t smoothed out a bit, it would be noisy, erratic, and very hard to read. This smoothing can be done with the combined action of a resistor and a capacitor, called an RC filter. Such filters are very commmonly used to remove electronic noise.

An RC filter has a time constant equal to 2 pi RC, in this case about 0.1 second, corresponding to a frequency of 10 Hz. Fluctuations faster than that will be smoothed out.

Here is the schematic with the parts labeled.

If you have a 22 microFarad capacitor, use it with a 1Kohm resistor (brown/black/red).

Alternatively some ITSI kits have an assortment of capacitors. Read the capacitance value C of your capacitor and find a suitable R using the table below. These will be the R and C in the circuit.

The photograph shows the completed circuit. The yellow and green wires go to the motor. Make them at least 50 cm long so that the motor can be moved around independent of the circuit. If they are run along a meter stick to the ground, make them even longer.

This picture shows what is connected inside the experimenter board.

The motor

Now prepare the motor. It needs a wheel attached to the shaft so that you can attach things to it or roll it along the ground. The easiest way to make a good wheel is to cut a cardboard circle, punch a small hole in the center, fill the hole with hot-melt glue, and slide it onto the shaft. This holds quite well and can always be repaired. If you add a bead of hot-melt glue along the perimeter, it will not slip.

The diameter of the wheel changes the calibration. Use a radius of 3 cm for small cars and a radius of 6 cm for pushing it along with a meter stick.

This picture shows the completed motion sensor attached as a ‘fifth wheel’ to a cart, using a piece of cardboard as a spring arm. Of course your motor will have wires attached!

This picture shows the completed motion sensor attached to a meter stick. You can use this to measure your motion when you walk along.

Check your connections. When you are sure that you have it right, plug the header into the GoLink and the GoLink into a USB port on your computer.

Test the circuit using the raw voltage measurement. You should see a signal offset to about 2.5 V. Spinning the motor should generate a signal that rises or falls about one volt, depending on which direction you spin it.

If you attach a wheel or a pointer to the motor, you can calibrate it so that you know how much voltage corresponds to a given rotation rate. For instance, rotate the motor at 1 rotation per second and read the voltage. Then rotate it at 2 RPS and read the voltage. The relationship should be linear. You could make a graph or a table of your results.

Now try the motor and wheel with a calibration for velocity. The velocity will not be accurate. For one thing, it depends on the diameter of the wheel. This calibration is set approximately for a 3 cm wheel radius.

To adjust the calibration to be accurate for you, put on a wheel and roll it at 0.1m (10 cm) per second. Note the velocity reading. Find a multiplier that can be applied to your measurement to make it correct.

A custom calibration system will be available in the future.

Now try the motor and wheel with a calibration for distance. This calibration automatically integrates velocity (adds up little velocity X time steps) to graph the distance the wheel has traveled. The distance will not be accurate. For one thing, it depends on the diameter of the wheel.

To adjust the calibration to be accurate for you, put on a wheel and roll it at 1 m (100 cm). Note the distance reading. Find a multiplier that can be applied to your measurement to make it correct.

A custom calibration system will be available in the future.