In this exercise, I will walk you through interfacing a potentiometer (variable resistor) to the NXT brick.
You will need:
- A stripped NXT cable
- A potentiometer with a maximum resistance no more than
- A small piece of wire
- An NXT Brick

This exercise is derived and expanded from a chapter in Extreme NXT by Gasperi, Hurbain and Hurbain.

THEORY

The NXT monitors the potential difference between the black and white wires with an Analog-to-Digital (A/D) converter. The A/D converter converts this potential difference to a RAW value between 0 and 1023 (10 bits accuracy). This RAW value is given by the ratio

(1)

where is the maximum RAW value of 1023, is the voltage used by the NXT A/D Converter, and is the voltage drop between the black and white wires.

The circuit diagram looks like this:

I have a little potentiometer that can turn over a range of about to . Below is a diagram. The resistance between the leftmost and rightmost pins is the maximum resistance of . We will focus on the resistance between the leftmost and center pins, which varies based on the angle through which the potentiometer has been rotated. To keep things safe, we wire the center pin and rightmost pin together. This doesn’t affect the potential difference between the leftmost and center pins.

I will assume that it is a linear potentiometer (a pretty good assumption), which means that the resistance at any given angle is given by

(2)

where is the maximum angle of the potentiometer and is the maximum resistance.

Equation (2) says that if the angle then the resistance of the potentiometer , and if the angle then the resistance of the potentiometer is maximum .

Looking at the circuit diagram for the A/D converter, the potential drop across our potentiometer (represented by resistor ) is given by the typical voltage divider relation

(3)

We can now substitute (2) into (3) so that the voltage between the black and white wires is determined by the angle of the potentiometer rather than its resistance. Then we can substitute the result into (1) to get an equation for the RAW value

(4)

with my particular values, this is

This formula will let us predict the NXT RAW value based on the angle of the potentiometer.

For my potentiometer, I find that a maximum angle of gives me a maximum value of 93. This is less than 7 bits of information, and each RAW value corresponds to . If you want a nice angle detector, you will probably need a potentiometer!

TRY IT

1. Before beginning, you need to cut and strip one of the NXT cables so that you can interface with the wires directly. I have placed a layer of solder on mine, so they can be inserted into a breadboard for easy connecting.

2. Next connect the center and right pins of the potentiometer together with a wire

3. Plug the other end of the NXT cable into the NXT brick.

I wrote a simple NXT-G program to read the sensor and display the RAW value. Notice that the Touch Sensor actually reads the resistance between the wires. So we are just replacing the Touch Sensor with a potentiometer. We will use the raw number output of the Touch Sensor Block, which is represented by the 1010 0101 symbol. We then need to convert it to text so it can be displayed on the NXT LCD panel.

You may download it here,
Potentio-01.rbt
or write your own.

When I try my potentiometer, I find that the RAW value goes from 0 to 95, pretty close to my predicted range of 0 to 93. So it works! Not bad considering I guessed that the potentiometer sweeps through and angle of .

Determining the Angle of the Potentiometer

Now, let’s convert this RAW value to an angle.
In Extreme NXT, the authors worry about the fact that the resulting relationship is nonlinear with respect to the RAW value. As far as I can see, this isn’t a problem. We simply solve (4) above for the angle in terms of RAW. We can output the angle if we wish, but here I’ll take it a step further and demonstrate the resulting equation by controlling a motor so that it maintains an angle equal to the angle through which I have rotated the potentiometer.

I will leave out the algebra. Try it yourself. Solve (4) for angle A:

(5)

for my potentiometer, this is simply

which is easy to code in NXT-G.
You can download my code here:
Potentio-03.rbt

The motor control is a bit crude, but it works well enough for the demonstration.
Check out the YouTube video to see it in action!

Enjoy!

MATLAB NXT Bluetooth Router

The system relies on the RWTH – MINDSTORMS NXT Toolbox, the NXT Fantom Library, and John Hansen’s enhanced firmware.  The brick software is written in Not eXactly C (NXC), which requires Brick CC 3.3.

Daniele Benedettelli also has a book published titled Creating Cool MINDSTORMS NXT Robots (Technology in Action)

NXT Reciever with Kalman Filter

Details on the project can be found at http://www.convict.lu/htm/rob/ir_us.htm#Kalman

The NXT cable has six wires. Below I list a table with the wires and their colors:

The WHITE and BLACK wires (Motor 1 and Motor 2) deliver power to the motor.
If standard batteries are used, the potential difference will be 9 volts, otherwise the NiMH rechargeable batteries provide 7.2 volts. If the white wire is positive and black is negative, the motor will turn one way. If you reverse the polarity, the motor will turn the other way.

The RED wire is connected to the ground (GND). Note that in the sensors, RED and BLACK are connected to one another. This is not the case in the motors.

The GREEN wire is connected to the +4.3 NXT power supply.

The YELLOW and BLUE wires are connected to the quadrature encoder, also called an incremental rotary encoder.

As shown in the figure from Wikipedia above, (http://en.wikipedia.org/wiki/Quadrature_encoder) the wires return square wave pulses that are 90 degrees out of phase. If the rising pulse on TACH00 leads the rising pulse of TACH01 by 90 degrees, then the motor is going forward. If it instead lags by 90 degrees, the motor is rotating backwards. One complete square wave cycle corresponds to 2 degrees of rotation. In the diagram above, if TACH00 refers to A and TACH01 refers to B, we can see that the motor is going backwards as TACH00 is lagging TACH 01.

By measuring the frequency of the square wave oscillation, one can compute the rotational velocity. Since one cycle corresponds to 2 degrees of rotation, one cycle per second (1 Hz) corresponds to 2 degrees/sec. If you record a frequency of X Hz, then the rotation rate is 2X cycles/sec.

Note also that by tracking both square waves, you can identify quarter cycles, which gives you a resolution of 1/4 of 2 degrees, which is 0.5 degrees.

The motor speed is controlled by pulse-width modulation (pwm), which works by driving the motor with a variable duty cycle square wave. This effectively turns the motor on and off, fast. The longer it is on, the more torque it will generate and the faster it will go.

These details and more can be found in the excellent book: Extreme: NXT with a sneak peak here.

Additional details can be found in the excellent book Extreme NXT: Extending the LEGO MINDSTORMS NXT to the Next Level (Technology in Action) by Michael Gasperi, Philippe E. Hurbain, and Isabelle L. Hurbain.

Philo uploaded a comment, and reminded me that “Note that there are some internal photos of the NXT motor here: http://philohome.com/nxtmotor/nxtmotor.htm and schematics here: http://www.brickshelf.com/cgi-bin/gallery.cgi?i=1846577”

Happy Hacking!

So for those interested in some LEGO electronics hacking, here is a list of supplies that will get you up and running fast for about $275… just a but more than the cost of a single Mindstorms kit.  Plus you’ll now get to learn electronics!

First, check out the book:
Making Things Talk: Practical Methods for Connecting Physical Objects

This book explains how to wire, program and interconnect various microcontrollers, some of which are very closely related to those used by the NXT Brick.

Supply List

Item Number	Description	Quantity	Unit Price	Total
Amazon.com
	Making Things Talk	1	$19.79	$19.79
Jameco.com
19166	Desoldering Pump	1	$4.95	$4.95
159291	Wire Stripper	1	$10.15	$10.15
161411	Diagonal Cutter	1	$7.49	$7.49
35474	Needlenose Pliers	1	$5.49	$5.49
127271	Mini Screwdriver	1	$1.89	$1.89
681002	Helping Hands	1	$8.75	$8.75
159611	Power Connector	2	$1.79	$3.58
10444	Alligator Test Clip Leads	2	$4.39	$8.78
103377	Header Pins	10	$0.16	$1.60
119011	Push Button (PCB Type)	10	$0.27	$2.70
29082	Potentiometer	2	$1.05	$2.10
242115	LM1117T-3.3 Voltage Regulator	3	$1.39	$4.17
51262	7805T 5v Voltage regulator	3	$0.32	$0.96
38236	2N2222A Transistor NPN	5	$0.41	$2.05
32993	TIP120 Power Transistor	5	$0.45	$2.25
643488	3.3V Zener Diode	5	$0.03	$0.16
35991	1N4004 Diode	5	$0.04	$0.20
152792	LED Yellow	10	$0.17	$1.70
152805	LED Red	10	$0.21	$2.10
153139	LED Orange	10	$0.35	$3.50
156962	LED Green (567 nm)	10	$0.20	$2.00
334529	LED Bargraph Red	1	$1.31	$1.31
334537	LED Bargraph Yellow	1	$1.23	$1.23
334511	LED Bargraph Green	1	$1.28	$1.28
17187	7-segment LED Display	3	$0.88	$2.64
38818	4-switch DIP	4	$0.48	$1.92
38842	8-switch DIP	2	$0.89	$1.78
103166	Resistor Refill	1	$12.95	$12.95
15270	0.1 uF	10	$0.15	$1.53
94161	1 uF	10	$0.12	$1.20
29891	10 uF	10	$0.06	$0.60
158394	100 uF	10	$0.11	$1.08
MPJA
4443 TE	Solderless Breadboard	1	$4.95	$4.95
4447 TE	Large Solderless Breadboard	1	$22.95	$22.95
7027 TE	Jumpers	2	$3.95	$7.90
14213 TE	Digital Multimeter	1	$14.95	$14.95
15860 TL	Mini Soldering Station	1	$14.95	$14.95
Sparkfun
Wiring Platform	DEV-00744	1	$84.95	$84.95
Radio Shack
64-025	Lead Free Solder	1	$3.89	$3.89

Note that the light gray items are optional, and not necessary.

Also, this list does not include some sort of power supply. Pulling one out of an old computer is an easy option. Or rechargeable batteries work well too (in which case you will need battery holders).

Last, there are special items in the book Making Things Talk that you may decide to purchase separately, such as flex sensors, or bluetooth boards, etc.

You can store your electronics in much the same way you store your small LEGO parts. Check out the article on Storage.

Enjoy Hacking!

See also quadrature encoder or rotary encoder at wikipedia and the LEGO NXT Hardware Developers Kit (LEGO NXT data sheets). Philo also has the pinout diagram on his page on deriving power from the Motor A port.

Additional details can be found in the excellent book Extreme NXT: Extending the LEGO MINDSTORMS NXT to the Next Level (Technology in Action) by Michael Gasperi, Philippe E. Hurbain, and Isabelle L. Hurbain.

Additional details can be found in the excellent book Extreme NXT: Extending the LEGO MINDSTORMS NXT to the Next Level (Technology in Action) by Michael Gasperi, Philippe E. Hurbain, and Isabelle L. Hurbain.