Current Sensor for RCX
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Discontinued   Current       $39.00                  .

This product is no longer being manufactured by Techno-stuff.

Measure how much electricity you are using..

A current sensor lets your Lego Mindstorms RCX measure how much current an electric motor or lamp is using.. Current is an indication of how hard the motor is working. Your program can use this to tell if it has run into an obstacle, or another robot as in Sumo wrestling. You can also use the sensor for physics experiments involving work and energy.

Relay brick for Mindstorms

Current Sensor for Lego Mindstorms RCX.

Connecting the sensor

One wire goes to a sensor port. One wire to a motor port. One wire to the motor. The sensor measures current flowing from the motor port to the motor. You do not have to power the motor from the RCX motor port. You can use a Lego battery box or any other low voltage power supply. You can use this sensor with any Lego motor, or any non-lego motor that runs on 12 volts DC or less. The lego train motor pulling a heavy load can draw more than 500 mA, which is beyond the capacity of this sensor.

You can connect two motors to the current sensor if their combined current does not exceed 250 mA. Plug the second motor into the empty connector on the bottom of the sensor, or on top of the other motor connector. The sensor will measure the combined total current.



The CurrentSensor is a powered sensor. If you are using Robolab, use the Light Sensor icons or the generic powered sensor icons. If you are using NQC or similar text languages, program the sensor as an active (powered) sensor in either Light Sensor or RAW mode. If you are using the standard Lego icon language (RIS), you can program it as a light sensor.

The Robolab program above reads the current sensor as a light sensor and displays the value on the RCX screen.

The RIS program above reads the current sensor as a light sensor and turns off the motor if the motor current exceeds 100 mA.

When programmed as a light sensor, the RCX value will change one unit for every 5 mA of current. Zero current gives a light sensor reading of 50. A reading of 100 corresponds to 250 mA in one direction. A reading of zero corresponds to 250 mA in the other direction.

Typical RCX values when read as a light sensor.

When read in RAW mode the output range is roughly 380 to 1020. One unit of change is equivalent to about 0.8 mA.

Sumo Wrestling

Scott Briscoe has used the current sensor for sumo wrestling competitions and offers some tips and techniques.

Classroom: Energy and Work

The current sensor can be used for experiments teaching the concepts of energy, work, energy conversion, and effieiency. In the setup shown below a simple winch lifts a 500 gram weight. Students calculate the electrical energy used and the mechanical work done. Comparing the two illustrates efficiency of energy conversion. Students may compare different winch models and motors for best efficiency. Calculations and a sample program are provided.

Connector for Mindstorms relay control



•  Brick Size: 2 bumps by 8 bumps.  Height is one and one thirds bricks.  (one brick plus one flat plate). Overall size: 16 x 64 x 15 mm.

•  Mindstorms electric plate on bottom of sensor.

•  Active (powered) sensor.

•  Measures +/- 250 mA DC. (Forward or reverse direction)


Technical Description

Current flows through a one Ohm resistor. A voltage develops across the resistor according to Ohm's law. The sensor amplifies this voltage and sends it as a retrun signal to the RCX. The amplifier contains a filter to smooth the current fluctuation that occurs when the RCX runs a motor at less than full power.