Let's Make Robots!

I want to try my hand at a robot vehicle.

Good day to you all,

Thank you for having such a great and informative page. I’m sure there are dozens of threads about this, rather then sifting through tons of pages…


I want to build a vehicle using a motor (?) a capacitor or two, a few switches and a power supply. I want the actuators/switches to change the polarity of the capacitors when it comes in contact with say…a wall, thus making it reverse.


As for the motor, can I use one and employ some sort of torque converter for wheels? Should I use separate motors for each wheel?


I’ll also need a battery pack; should I go with higher voltage and just step it down?


Lastly, what does anyone think of using PSPICE to put the circuit diagram together? It sure would make the calculations easier, you know ideal vs. real and all that jazz. Anything you can offer would be greatly appreciated, exception being snark and sarcasm. Thanks.

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Thank You

I would like to thank all you guys for doing a great job. I am an old retired guy who just build himself his first robot. I purchased the LMR Instant Robot Starter Kit. It’s wonderful for having all the help and all the trials on line from all the robot builders. The pictures and the instructions on line were great.

I did have some problems with loading USB Download cable drivers. After a number of tries with the self extracting  file and not finding the hardware I used the AXE027.zip. Extracting the contents it created DIR axe027. Running it loaded up the driver with no problem.

The kit came with a  gearbox motor that did not fit the wheel  but I did drill the wheel to accept the axle and it works fine. Now is the time to play with the program and all the possibilities.



Wow, thank you both for the input; it’s greatly appreciated. I suppose I should have used better terminology. I understand you can’t “switch polarity” per se. I just wanted to make a point simply as I could. As for the parts I mention, there is a great amount of flexibility I can accommodate. What you’ve both explained is pretty much what I want it to do. This is to be a simple and fun project, yet relatively inexpensive and I’m not above swiping parts from other devices. I just don’t want a kit in a box but rather something to build that is intelligent enough to change directions when it comes in contact with something.


Both of you gentlemen links are awesome, “an autonomous turd-combine that deposits its treasure in a standard disposable grocery bag”, that is the funniest thing I’ve read in a while.


Right, to clear it all up I’m thinking something like the Lego mindstorm, but again DIY. Can I use a PLC to tell it what to do?


Would a diode and relay be more costly? It sounds like a terrific idea. One last stupid question (for now) do you calculate voltage, current etc during your design phase? Assuming a top-down approach, or just by gosh and by golly as you go along? If not using pspice or what ever SW, what is the best way to go about determining power need?


Thank you again! Chuck.

Hi, Chuck,

In answer to you question about calculating voltage, current and designing, I personally calculate it all before putting the parts on the board, but that does not mean everyone does things the same.  I worked as an engineer for decades, so it is "second nature" to me.  The first and simplest rule in figuring such things is you MUST know/understand Ohm's Law, but then there will be other things to learn after that.

 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

Here is a (very) quick example of designing a circuit.

You look at what you need to run (DC motors, LEDs, servos etc.) and decide what batteries you need; —what voltage.

Four alkaline batteries give 6 volts or a bit more when new, but you decide you would rather use rechargeable batteries, so you don't have to keep buying new batteries all the time.  Ok, it happens that 4 rechargeable AA batteries do not charge to 1.5 volts, but rather to about 1.25 volts each.  Four of them will give almost exactly 5 volts when charged.  Will everything you want to run on the robot run on 5 volts?

You decide it will.  So now you know what voltage to use in your calculations.  Five volts.  If you want an LED to light on your robot at some point, you learn that the current through an LED must be limited.  (Actually, it depends on the one you used, but...) older style LEDs were normally figured for 10 milliamperes (or ma.) while the new "super bright" LEDs will accommodate up to 50 ma.  You decide to use a super-bright one, so you know you need to limit the current to 50 ma.  maximum so the LED does not burn out.  whether you turn it on with a switch, a relay, or a transistor, you know that to limit the current through the LED, you also need a resistor between the LED and the battery.  You say to yourself, by Ohm's law, if I have 5 volts and need 50 ma or .050 amps, then the resistor will be the volts divided by the current.  5.0 / .050 = 100 ohms.

Don't stop there, however.  There is another thing to consider.  The LED itself has a certain voltage drop across it.  Now this value changes with the brand of LED as well as the color.  For the sake of this example, the LED you are using has a junction voltage of 2 volts.  (Most are in the range of 1.2 to 3 volts)  Anyway, if the LED needs 2 volts and the batteries are supplying 5 volts, which leaves only 3 volts to drop across the resistor.  If you are with me so far, you say, "Ah, so to figure the correct resistor, I should calculate 3.0 volts divided by .050 amps.  That is 60 ohms instead of 100.  It happens there are "standard" values of resistors you can buy, and 60 is not one of them.  You can find 56, 62 and 68 fairly easily.  If you pick 62, reversing the ohm's law calculations, 3.0 (volts) divided by 62 (ohms) gives you a bit over 48 milliamps.  Great.  It is close to your maximum of 50 ma without going over, so to light that particular LED you need a 62 ohm resistor in series with it.

I said it would be a quick example.  That may have seemed long to read it, but it would take less than a second to think through the logic of it, and maybe a bit longer to do the maths, but hopefully you get the idea.  There are other things you will need to decide, but you do them one at a time.

 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

In you other questions you asked about turning things on and off with relays.  You can do that, if that is what you are most comfortable with.  Your motors will either be full on or full off.  If you want to be able to regulate the speed, then you add in the need for transistors in a simpler design or a microcontroller and a motor driver circuit in a more advanced one.

If you want something that is able to make decisions when it comes to seeing and identifying obstacles, then you should use a µC (a micro-controller chip) such as a Picaxe, a PIC, or an Arduino, or even a Basic Stamp (as well as several others available).

They have thousands of internal transistors to do all sorts of things from simply switching motors on and off, to computing angles and trajectories; deciding which way to turn; or putting on a show or dance routine.  (In a small car-type robot, the "robot dance" is simply having it spin round and round, first one way and then the other.)  Of course, anything you want the robot to do has to be set up in the programming which you will write (or get help with) to tell the µC what to do in a certain circumstance or how to "figure it out".

Microcontrollers such as the Arduino are more versatile as they accept programming in higher-level languages such as "C".  But if you have not done programming before, you may want to start with a Picaxe, which can do nearly all the same things, but the programs are written in a form of "Basic" language, which is easier to understand for the beginning programmer.

Finally, while you can hook up a microcontroller chip directly into your robot, most beginners find it easier to buy a board that is all wired up and ready to use on its own, and all you do is hook up your robot to it.  While small microcontroller chips are as low as $2 (US dollars), the finished boards will usually be in the $20 to $30 range.

Ok, that is enough to fuddle your mind for a bit.  ha ha.  The good thing about amateur robotics is that you can work at your own speed.  Nobody is setting a time frame.  You can take as long as you like to figure out how you want your robot to look and act and what parts you need to build it or how to program it.

Good luck.  —Dan


PICs and electromechanical systems are getting to be about the same in cost.  You'd have to program the PIC though and it might be overkill for what you're looking to do, but it would certainly have a lot more room for expansion.

Oh, also, while you can use a single drive wheel, as long as you use some method to steer it, using 2 or even 4 make things easier.


Well, here are a couple of the answers you wanted.

Batteries: If you use a higher voltage and then step it down, you are automatically wasting part of the charge in the step-down process. For example if you use a 12 volt battery, but only need 5 volts, the 7 volts your power regulator absorbs is being converted to heat which means it is wasted. You do not generally want your electronic circuits to over heat.

You can reverse the polarity on capacitors (using a double-pole, double-throw switch), but if you use caps large enough to drive a motor, you are probably looking at electrolytics which should *not* have their polarity reversed. I would suggest reversing power to the motors themselves. Following is a very simple little car, but I did a (schematic) drawing that might give you an idea of ONE method to do that... http://letsmakerobots.com/node/31986

It uses two standard DC motors like you find in many small toy cars, a couple capacitors and homemade bumper-switch, that makes the car back-up and turn while backing, so when it goes forward again, it will go at a different angle.

Lastly, on using SPICE or PSPICE, I personally never used them. Why? Because I have not seen them actually make a circuit that properly emulates what a real circuit will do. Perhaps they have gotten better lately, but if so, I have not seen it.


You can't change the polarity of capacitors with a switch, unless that switch triggers a motor that physically rotates an electrolytic capacitor, but it's kind of hard to see how that would make a motor reverse with the components you listed. The closest scenario I can think of is something like what I used in the scarab.. You need a relay and diode as well as a pair of resistors for each motor. (See the schematic at that node.) It requires two motors. The only way to do it with only one motor is if you use a slip wheel, but then it only backs up one way-left or right.