Section 6: Interfacing Analog to Digital Circuits (Comparators and A/D conversion): In the last section we discussed sensors and using the voltage divider to get a voltage that can be sensed out of the sensor for use in a robot. In this section we learn how to take that output voltage and use it to get a simple, digital output signal. We will examine two methods: the comparator circuit and the Analog to Digital (or A/D) converter. Comparator: As the name implies, a comparator circuit compares two values and sets an output depending on those values. A comparator is commonly available in IC form. The LM324 IC is a typical example. The LM324 is a general purpose operational amplifier circuit that can be used for many purposes. For now, we will only discuss its use as a comparator circuit. The comparator sets a reference voltage using a voltage divider. Then one sends the input signal to the comparator and it will compare the input voltage to the ref voltage. If the input voltage is equal to or greater than the ref voltage then the comparators output will be HIGH. A diagram of this setup is shown below.

The triangular symbol in the middle is the LM324. Note the numbers 1,2, etc. These are common pin numbers on an 8 pin IC for this device. But you should always check the pin out for the part you are using to be sure you are connecting it properly. Pin 4 is the supply voltage for the IC. Pin 1 is the comparator output. Pin 3 is the non-inverting input and pin 2 is the inverting input. The inverting and non-inverting name comes from the polarity of the input. The reference voltage is set by a voltage divider circuit to pin 2. Pin3 then goes to the output of the sensor. If we set a ref voltage of 3.75 V by choosing R1= 30K and R2 = 10K then the comparators output will be low while the sensors output voltage is between 0 and 3.74V. At 3.75V and above, the comparators output will be HIGH (1). We can graph this output response in terms of the input as below:

where Vref in this case is 3.75V. The opposite response can be had as well. That is, you can set up a comparator circuit so that the output is HIGH while the input is less than the ref voltage, and LOW when the input equals or exceeds the Vref. To do this you simply reverse the setup for the inverting/non-inverting inputs. In other words, put the Vref generating voltage divider on the non-inverting input (pin 3 of the sample circuit above) and the output of the sensor on the inverting input (pin2). Analog to Digital Conversion (A/D): With A/D, the goal is not to have an output that is simply ON or OFF depending on the input analog voltage, but rather to have an output that tells the digital device we want to interface with WHAT THE ANALOG VOLTAGE IS exactly. A/D converters are specified by the number of digital bits of precision that they have. The most typical is an 8 bit A/D. Recall that with binary numbers the total range of values is calculated with the formula

nrange 2= where n is the number of bits. So with 8 bits the range is 256 unique values. Remember that non-negative numbers start with 0 not 1, so this is a range of 0 to 255. A/D converters like a comparator are referenced to an input voltage that is normally equal to the full range value of output voltage. In our example, this would be 5V. This ref voltage is then divided equally among the range of the A/D. For this example, 256. Thus Increment scale = 5V / 256 = 0.02V Every increase of 0.02 VDC would result in the A/D converter output being increased by 1. By this scale then an input to the A/D converter from a sensor of 0Vwould result in an output of 0. If the sensor outputs 5V then the A/D output is 255. If the sensor outputs 3.75 V then the A/D converters output would be 187. 3.75 VDC / 0.02VDC = 187.5V Note that the .5 is dropped completely. An A/D converters output is an integer (whole number). Further note that it does not round the number. It simply truncates (cuts off) any remainder. If youre thinking ahead, youll be asking but how does the A/D converter communicate 187 decimal in terms of a digital signal. The answer is it drives 8 output lines (bits 7 down to 0), each one with a state of ON or OFF depending on the output. Binary numbers: Binary numbers use a number base of 2. The numbers you work with every day are base 10. Each place to the left of a decimal is a power of 10 you may recall. The number 187 in decimal is represented by 7 in the ones place (100), 8 in the tens place (101) and 1 in the hundreds place (102). Binary numbers work the same way except instead of powers of ten, we have powers of 2. An 8 bit binary number has 8 places. The right most is the least significant bit (LSB) and the leftmost is the most significant bit (MSB) ___ ___ ___ ___ ___ ___ ___ ___ Power 27 26 25 24 23 22 21 20

Multiplier 128 64 32 16 8 4 2 1 Some examples:0 in binary is 0 just like in decimal. 1 is also just like in decimal, after that though

2 in decimal is 10 in binary (pronounced one, zero, not ten). 3 decimal or 3d is 11b (b for binary). Thats 1 times 20 + 1 times 21 or 1 plus 2 which is 3 (decimal). So what is 187? With 8 bits what is the largest place multiplier that is just below 187? 128 or 27. 187 128 is 59. Now, what base 2 place multiplier is just under 59? 32 or 25. Notice 64 or 26 then gets a zero in its place. Now we have 128 + 32 = 160. That leaves another 27 that has to be accounted for in our 8 bit number. What is just under 27? 16 or 24. 128 + 32 + 16 = 176. 187 176 = 11, so 8 (23) is next which leaves 3d, so both 2 and 1 (21 and 20) get filled in. This gives us a binary number of 187d = binary _1_ _0_ _1_ _1_ _1_ _1_ _1_ _1_ Power 27 26 25 24 23 22 21 20

Multiplier 128 64 32 16 8 4 2 1 187 in decimal is binary 10111111. 9 in decimal is 8 bit binary 00001001. Zeros that are to the left of the most significant digit are meaningless just like in decimal notation, so binary 00001001 can also be written as 1001b, but if we are specifying 8 bit binary you should put those zeros there for clarity. Remember the idea of binary numbers and logic. You were born into a computer age world and I assure you, these concepts are not going anywhere anytime soon. Okay, back to our A/D converter output. On a standard A/D IC like the ADC0804 there are 8 output lines, DB0 to DB7 as the symbol in the schematic below shows. DB0 is the least significant digit and so is the 20 line. DB7 is the 27 line.

So now you have 8 ON/OFF circuits (a data BUS) that represent the analog value that comes out of your sensor. The output of this chip could be ran to LEDs if you wanted a visual representation of the analog voltage or inputted into a microprocessor that would take the data and perform some desired operation. The RD and CS with bars over them are equivalent to the nREAD and nCS signals you saw in the RAM access synchronous example.

Section 7: Mechanics Whew! Finally a break from electronics and physics. In this section well hit on the following major subjects: Mobile Robotic platform construction considerations Wheel size, speed, and distance traveled

Torque Gears and reduction Chains, sprockets, pulleys Walking robots Mobile Robot Platform construction: If you are building a robot that you want to move around on its own power and volition, you will be using a battery operated design. Batterys drive motors and motors and gear systems have to move the thing around. The more weight the motors have to move, the shorter the life of your batteries. Thus, weight is a big consideration when building a robot. There are several ways to approach design of the body or frame of a mobile robot. The easiest is to use an existing platform like a toy car or bulldozer. This way all the difficult mechanical problems are solved for you. Generally you will strip it down to just the motors and then build your own control system for them (see the next section on motor control). But you can also build your own design. Just remember that your robot has to have power to move that platform so generally you want to use light materials. Foam core board is a pretty good option. It is very light, easy to cut and shape and pretty cheap. IT is available at most hobby stores (Hobby Lobby, etc). But look around in your garage or abandoned toy box. There is probably all kinds of plastic shells and wheeled platforms that can be converted for robot use. For instance you might have a toy car that just has some wheels on it. No motors or that kind of thing. You could mount a servo motor (see the next section) on it with a wheel attached to move it around. The other wheels would keep it upright. As you learn your control systems you can add steering and sensors. Always keep an eye on your garbage or your neighbors garbage for big pieces of plastic or light metal. Such stuff can be converted for robotics use. Wheel size and speed: Everyone knows what a wheel looks like and does. It makes moving a load easier and faster. In robotics the things you will be most concerned with are size and speed. The basic parts of the wheel are the wheel itself which is a circular disk like object that is mounted on a rotating shaft or axle. In essence, the bigger the wheel, the faster the speed your wheeled platform will go. This is because the outer surface of the wheel has to go through as many rotations as the part that is near the axle since both of these parts are connected.

Imagine the wheels outer surface as a loop of paper. Then snip the paper and unroll it. You get a rectangular piece of paper that has a width equal to the width of the wheel. The length of that rectangle is the circumference of the wheel.

where the circumference of a circle is given by 2pr. p is a numeric constant that is approximately equal to 3.14, and r is the radius of the circle . Using these relationships you can calculate the speed that a wheeled robotic platform will travel given a wheel size and the rotational speed of the axle. You do this by using the rotational shaft speed which is given in rpm or rotations per minute. But in general is just rotations divided by some unit of time. Shaft speed = rpm = rotations / time A rotation of the axle at the middle causes a full rotation of the wheel at the outside which is the circumference of the wheel (2pr). You can substitute the formula giving the circumference into the shaft speed equation to get

Shaft speed = timeofunit

rrotations

__2

*p

=speed of wheeled platform

Example: Say that you have a pair of wheels both with a 1 inch radius mounted onto a motor output shaft that will rotate at 60 rpm. The speed of the wheeled platform will be

Platform speed = .min

8.376.min1

1*14.3*2*60__2* inchesinchtimeofunit

rrotations==

p

If you convert that to feet per second you get

ond

feetinchesfoot

ondsinches

sec52.

121

*sec60min1

*min

8.376 =

Torque: You probably basically understand the concept of a lever right now. If not mathematically, you surely understand the idea. If youve ever reeled in a fish and then held it up in the air at the end of your pole you have experienced torque.

Torque is the force that the fish exerts on you over the length of the pole. You must exert an equal force at the point where you grab the pole or the fish will fall back in the water. Mathematically, torque is the product (the product is the result of multplication) of the force times the length of the lever or moment arm upon which the force is exerted. If you pull in a 10 oz fish and you are holding a pole that is 24 inches long, the torque at the end of the pole that you must counteract is 240 ounce inches. If you wanted to build a robot arm, this same principle applies. The arms length is like the fishing pole and whatever you wanted to lift would be like the fish. You would need a motor that produced enough torque to lift that load at the end of the arm. The main thing to remember is that since torque is the product of the force and lever length, the longer you make the lever (in this case, the arm) the more torque you will need to lift a load at the end.

short robot arm long robot arm When a motor drives a wheel, it is also exerting torque. If you have a very heavy robot you will need a motor that has more torque. For instance the motor in your Scooter kit is most likely rated for about 2 oz inches of torque before it stalls (is unable to lift a load). This means that the motor could move a load of 2 ounces if the wheel mounted to it had a radius of 1 inches. This isnt much weight. Luckily we can use a gear system to get some help. Gears and reduction: If you have a bike with gears on it, you know the huge advantage that a gear reduction system gives you. On a bike YOU are the provider of torque. With the use of gears you can move greater loads with less force. When it is you doing the pedaling up hill this becomes a very important consideration. Pedaling up hill is a lot for work. On a bike with only one gear you can help yourself by standing up so that you get all of your weight on the pedals, but if the hill is really long youll wear yourself out. Your buddy Bob can get to the top of that same hill with a fraction of the effort because he has a bike with a system of gears.

Look at the diagram below. Here there are two gears actually gears must touch each other, so it is more accurate to say sprockets for a bike- attached by a chain. The largest sprocket has 20 teeth in it. The smaller one, 10.

Since both sprockets drive the same chain, the teeth must be the same size. Thus we can see that there is a 2 to 1 ratio between the number of teeth in the sprockets. If you turn the small gear at a speed of 100 rpm, the large gear will turn half that or 50 rpm but you get more power. Thats what happens on a bike when you are going uphill and shift down. You end up pedaling so that the chain travels over twice the distance than it would if you had no gear system, but you spend half the effort. Most electric motors run at very fast speeds, generally thousands of rpms. If you attached a wheel straight to the shaft of a typical motor the wheel would turn so fast your robot would not roll, it would fly probably into a wall and smash to pieces. By linking gears either by a chain like on a bike or with true gears that touch each other and mesh, you reduce the speed of the motor from 10,000 rpm to more like 100 rpm. Your Scooter assembly manual covers this subject in some additional detail. You will be using a gear reduction system on scooter to drive the wheels with a small DC motor. Remember these simple facts about gears/sprockets and pulleys: The speed always decreases when going from a small to a large gear The speed always increases when going from a large to a small gear Decreasing speed, increases torque (power) Walking robots: Making a robot move using legs instead of wheels adds complexity. But it is a cool idea. If you are truly interested in building a robot like this, start with a wheeled platform and learn the basics of motor control and electronic control signals, then move on to a walking robot. And start with a design that has 6 legs before tackling 4 or only 2. A bipedal (2 legged robot) is a very difficult problem that is being worked on very hard by scientists all over the world. The most success that has been made on this have been by Japanese scientists and engineers and their walking robots cost tens of thousands of dollars and took years just to get them able to take 3 steps. Its not impossible. Just very complex and not the best place to start. Think of like this. Insects took millions of years to evolve the walking pattern that you see commonly in your front yard. If you are interested in building this type of robot, I recommend John Iovines Robots, Androids, and Animatronics. There are also many web sites on this subject.

Section 8: Electric Motors and motor control Yeah, yeah, yeah, you say. All that electronic stuff is great. 1s and 0s and volts and ohms and blah, blah, blah. But when are we gonna get to how to move stuff around? Right now. Motors for robots are like muscles for you and I. It is how they do more than analyze and sample and calculate. It is how they move around. It is also if you are so inclined- how to get them to swing a hammer or turn a buzz saw like on battlebots. We will be covering DC motors in this section. There are 3 broad types: the standard DC motor, the servomotor, and the stepper motor. Standard DC Motor: A DC motor works thanks to the principle of magnetism. The running water analogy for electrical current is a good model for simple circuits. But it begins to fall apart very quickly if you study circuits in more depth. One big problem with this analogy is that running water does not produce a magnetic field. When current passes through a conductor it creates a magnetic field around the wire. It is how radio signals are transmitted but that is another topic. When you put a wire carrying current in the presence of a fixed magnetic field you get a very unexpected result. The wire is pushed out of the fixed magnetic field. The diagram below illustrates this idea.

The diagram shows two magnets oriented so that ones North pole faces the South of the other. This creates a magnetic field B between them that exerts a force in the direction of the arrows shown. A wire carries current I in the direction shown by the arrow. When it is inserted into the fixed magnetic field the forces combine in such a way that there is a RESULTANT force that is in a direction at right angles to the other 2 forces. For the example shown here, the force is downward. The force diagram at the bottom of this picture sums up what is known as the right hand rule. Using your right hand, hold your fingers straight and point them in the direction of the current. Then curl them in the direction of the magnetic field (B). The direction your thumb is facing now is the direction of the resultant force. But a circuit is a loop right? You cant just have a piece of wire like the simplified diagram above suggests. You have to have a loop of wire. Like in the next diagram.

Current first goes in one direction from the left and into the paper and comes out on the right and out of the paper. Each of these has a resultant force. Both of these forces act together creating TORQUE to rotate the loop of wire counterclockwise until their forces are now pointing directly away from one another and canceling each other. There is no torque at that point.

So how do you get a motor to keep turning and not stop there? Well before we answer that, how do we get current into the wires in the first place. They are turning, so we cant attach a wire to them. The wire would get stretched and broken. So, You use a flexible contact or brush to make contact with the two ends of the wire. The wire mounted on the shaft (actually it is many turns of wire, but the principle is the same) maintains contact with the brush as it rotates, so you have a circuit. This setup is called a commutator.

So the wire loop experiences torque, up until the point that the wire loop has rotated to where it is no longer in contact with the brush. Then what? The shaft is in motion at that point and the force continues to rotate the shaft through until the wire makes contact again. The direction of current is the same so the process continues and you get continuous rotation. How do you speed up a motor? You increase the voltage, which increases the current. How do you reverse the motor? You reverse the polarity of the voltage source connected across the motor. That way current flows in a different direction, so the resultant force is in the opposite direction. If you have internet access have a look at http://www.geocities.com/CapeCanaveral/Hall/6645/electmag/electricmotor.htm This site has an excellent diagram of this process and its animated. It moves so you can see how the forces act on one another. And it has cool colors too, which beats this plain old black and white booklet any day. The main thing to think about with a DC motor is that it is a current driven device. The amount of torque that is developed by the magnetic field interacting with the current carrying wire is what gives it the umph to move your robot. Current and torque are DIRECTLY PROPORTIONAL, that is, as current increases, torque increases. This is a general plot and units are assumed to be proportional and are thus not included.

On the other hand, a motors torque and speed are INVERSELY PROPORTIONAL. If you increase the speed of a DC motor you lose torque and vice versa.

This tells us that if we decrease the speed of the motor we increase the amount of twisting power that will be available at the shaft. Thus running the motor at full bore speed is not the way to get the best power. DC motors rotate at very fast speeds, usually thousands of rpms. That is way too fast to attach to a wheel, so you must use gear reduction as discussed in the mechanics section. Setting up a gear box and attaching it to the shaft of a motor is a bit difficult. Thus if you are looking for DC motor to use, you want to find one that has a gear system already in place. This is called a gearhead motor. If you are converting a toy over to robotics use, they frequently have a gear box in place already. All you have to do is drive it electrically. Which brings us back to electronics. Sorry. By now you have seen that all you have to do to get a motor to spin is to attach a batterys terminals across the contacts of the motor. If you want to reverse it you just switch the connection. But how do you do it on your robot? Are you going to follow it around and when you want to switch directions reach down and switch the wires? No. You want such control to be in place on your robot. Imagine if you had a set of 4 switches set up like below.

If you closed switches 1 and 4 and left 2 and 3 open, you would have a path for current that ran from the voltage source, through switch 1, through the motor, through switch 4 down to ground. That would give you rotation in one direction. Then if you open switches 1 and 4, then close 2 and 3 you get a path for current going the other way through the motor, reversing its direction. You can wire this up physically with switches, but then you would need to flip the switches. AND YOUD HAVE TO TAKE CARE TO NOT LET SWITCHES 1 AND 2 BE CLOSED AT THE SAME TIME. Same for switches 3 and 4. THIS IS A SHORT CIRCUIT AND YOULL PROBABLY DAMAGE THE VOLTAGE SOURCE. But how about if you do this in such a way that you can control the switches electronically with digital signals? Its called an H bridge and it uses transistors as switches. It is commonly available as an IC for about $4 and will save you a lot of heartache. I include the pinout for two common versions of this type of IC in Appendix B of this booklet (see Pinout of L293D and SN754410 Ics) These ICs do this job nicely. Internally, they look very similar to the switch setup shown above. If you use a kit like the Lego Mindstorms set or the Parallax Boe Bot kit or any thing that uses motors that are reversible, this is probably the type of control system it uses. The Scooter robot kit uses the H bridge with discrete (non integrated) transistors. Your manual for the kit includes a schematic.

Servomotors: Hobby servomotors are a very elegant solution to the problem of adding a motor to your robot. They are mainly used in hobby RC airplanes, so they are very compact, powerful, light and power conservative. Since they have been in production for a long time they are also very cheap. You can buy a standard hobby servomotor for under $13. In a very small package you get a DC motor, gearbox, and feedback control system.

The Futaba S-148 standard servo. Available from towerhobbies.com or any RC hobby store (Colpar) Servomotors are designed to operate control surfaces on hobby RC planes. So they do not rotate continuously. Rather they are designed to rotate through 180 degrees with precise position control. If you want to use them as the main drive motor for a mobile robot you need to modify them so that they will rotate continuously. This is not a difficult thing to do. I will not cover it here but if you want to do it there are many sites on the web that cover this. An excellent one is http://www.seattlerobotics.org/guide/servohack.html They do not simply run on a DC voltage like a standard DC motor. They have 3 wires. Red is power (generally 3V 12V max), black is ground and then there is another wire, usually white or yellow that is the input signal wire. A servomotor is controlled by sending a pulse signal that is HIGH for a brief time, generally 1 2 ms. If you just connect a battery to power and ground, nothing will happen. You must have a timer circuit that generates this pulsed signal and by varying the pulse ON time (or the pulse width) the motor will move to a certain position over its range of motion and then stop as long as the input pulse width is the same. Depending on the pulse width, youll get a different position. You can build a fairly simple circuit to do the timing circuit. If you are interested in constructing this circuit, see Appendix B of this book where you will find a schematic for the circuit. A more advanced design approaches this problem by using a software driven microcontroller. This solution is way beyond the scope of this class, but I will demonstrate it. You can also modify the servo by completely removing the circuit board and other guts until you are just left with the motor and the gears and you get a gearhead motor out of it. On the next page is a diagram showing what position you will see approximately with a servo motor in response to different timed pulses.

Servo motor timing control diagram (typical for MOST hobby servo motors)

Stepper Motors: Another motor type is the stepper motor. It works similar to the servo motor in that this type of motor gives the user very precise position control. But it uses a different design. The stepper motor does not rotate and have a feedback system to know its position. It moves in exact steps every time you energize one of its pairs of wires. Stepper motors frequently have 4, 5, or even 6 wires. Their control system is more complicated so I wont cover it here. Suffice it to say that an advantage of a stepper motor is that you get a very precise position control out of it and when it stops, it STOPS hard. You can use a stepper motor as a very effective move to here and brake type of design. The disadvantage of the stepper is its complexity. If you have a printer for your computer at home, chances are it uses a stepper motor. Summary of motors: Motors are what make motion in robots possible. Whether it be to turn wheels, move legs, or power robot arms and grippers. They are current driven devices. The amount of current you put in will dictate how much torque (turning power) and speed that you get. The best way to implement a motor design into your robot is with an H bridge motor driver circuit, commonly available in an IC package.

Appendix B: Additional Motor Control Information A good sites for H bridge motor control http://www.ece.cmu.edu/~ece778/lecture-notes/Motor-drive-lecture/sld006.htm The following circuit is a simple one to construct using the L293D or SN754410 H Bridge Motor driver ICs to drive a small DC motor. Only two switches are necessary to turn the motor on and off and to control its direction. The switches could easily be replaced by digital signals such as the output of a comparator or similar circuit. The SN754410 and L293D ICs are available from Digikey (pn ) or from Acroname Robotics. They are about 5$ each. The 7404 NOT IC is commonly available from Radio Shack or Digikey.

EN1,2In1Out1GNDGNDOut2In2Vmtr

VccIn4

Out4GNDGNDOut3

In3EN3,4

1 162 153 144 135 126 117 108 9

7804NOTGate

+5VDC

10K

10K

L293D Or

SN754410

DC motor

MotorOn/Off

MotorDirection

555 timer IC control circuit for a servo motor. By varying the resistance on Potentiometer R1, you can control the position to which the servo rotates.

A typical connector for a servo motor will have 3 wires which look like

Good resources for servo motor control in general. http://www.chipcenter.com/circuitcellar/october01/c1001rr1.htm

Appendix C: Suggested Reading

Books: Robotics: Robot Building for Beginners, by David Cook Building Robots with Lego Mindstorms: The Ultimate Tool for Mindstorms Maniacs, by Mario Ferrari The Robot Builders Bonanza, by Gordon McComb, 2nd edition How to Build Your Own Underwater Robot, Harry Bohm & Vickie Jensen Mobile Robots, Joseph Jones &Anita Flynn &Bruce Seiger, 2nd edition Applied Robotics, Edwin Wise Electronics: Get Started in Electronics, by Forrest Mims Radio Shack part 276-5003A Basic Electrical Theory With Projects, by Delton T. Horn

Websites: www.robotstore.com - Good supplier of basic robot kits and parts www.acroname.com - Supplier of robot kits, parts. Located in Boulder, CO. They also sponsor an annual robotics convention, usually in April. Many good links as well www.imagesco.com - Good supplier, generally a bit more advanced type stuff though www.seattlerobotics.org - The best club robotics club site out there. VOLUMES of fantastic member projects, information, circuits, schematics, source code, pictures, movies www.robotroom.com - Chicago based robot club site. Also very good project site www.ranchbots.com Magazines: These are typically a bit advanced Nuts and Volts, their site is www.nutsvolts.com. Poptronics, www.poptronics.com

WHERE DO I GO FROM HERE If you are really interested in robotics and want to keep going with it, here are some suggestions. Electronics kits: A cheaper way to start out is to get more experienced in electronics with some kits that are available. Radio Shack has several 50 in 1 type of kits for electronic experimentation. These things are a blast and are considerably less expensive than the kits above. They are a great way to get your feet wet and to learn the skills that you are going to need later anyway. These kits run from $30-100. Some examples are the Electronics Learning Lab Radio shack part number 28-280- $59.99 Electronic Sensor Lab Radio shack part number 28-278, $49.99 Timer Kit, $6.97 part number 990-0202. FM microphone kit. Make an FM radio transmitter. Turn an FM radio into a PA system, part number 990-0191, $7.95 Sumo Robot kit, $29.95 on sale, normally 49.95, part number 990-0409 Lego Mindstorms set. This is a kit made by the famous Lego company. But it is more than just the familiar toy construction set. It includes motors, sensors and a programmable microprocessor brain. You can build a robot, program it with your own software to control its behavior and let it go. But beware the $200 price tag. The kit keeps the simplicity of Legos at its heart, but it gives one the power to remarkable things with robotics projects. The software programming interface even has a Lego look and feel to it. The programmer assembles logic blocks that look like Legos in a windows interface. The user can also download another language compiler into the micro such as NQC (Not Quite C) or recently a Java Compiler for more advanced programming. NOTE: I FOUND ONE ON EBAY FOR $90. Look around if you are interested in this. You can get it at Toys R Us, www.amazon.com, www.robotstore.com, or www.acroname.com (they also have a store in Boulder) Parallax Boe Bot: A microcontroller (programmable computer like chip) based robot kit. Their most versatile kit is the Boe Bot which is a wheeled platform robot that comes with the computer chip (called the BASIC STAMP), prewritten software routines to control the motors, etc. This kit is very well documented and there are hundreds of website resources that are free. This is a great kit for those that like the simplicity of the Lego Mindstorms kit, but want to expand and do more. Price: About $230. Available from The Robot Store (www.robotstore.com) or Acroaname Robotics (in Boulder, CO) or directly from the Parralax web site. You generally will not see these on ebay. People that get them, keep them. Do it yourself: Price? Nothing to several hundred dollars. The downside to this is that it either takes experience, a patient mentor, or incredible drive and enthusiasm to start out this way. But if you dont have $200 to drop on the kits mentioned above, you can start out by applying some of the circtuits I presented in the class and hacking them into a toy you have around the house. I highly recommend starting out with one of the electronics learning kits listed above. Learn the pieces and then go and put them together into a working system.