Science Buddies: "Ask an Expert"

Help! I have a project due on February 2, and I started my Bristlebot, but the motors won't start. Due to time, I couldn't order the kit, so I went out and bought all the supplies seperately. When I hook up the battery pack to the motors, the motors start, but when I test the robot itself, nothing happens. I've looked over the breadboard several times, and I can't find an issue with the wires. I can send pictures of what I have so far, but here's what I think the problem may be:

Potentiometers: I bought some that are also meant for 10k ohmn, but they are not single turn, so I have no idea how to adjust them

Transistors: When I got them at Fry's, a worker looked up what the same transistor would be for their brand, instead of spark fun

Please help, this project is due soon, if any pictures are needed, I can email them.

Hi Miarie-

Sorry to hear about your troubles. Your description doesn't make a lot of sense. You said the motors won't start but then said they start but you can't test the robot. It would help if we could pin down the description of what you did and what you're seeing happen. First though, if you could post a couple of clear pictures of the parts you have and the way you wired them that would be really helpful. You can upload them to one of the many free image sharing sites like http://postimage.org, http://www.freeimagehosting.net or http://s5.photobucket.com, then paste the URL in a reply. If you click on the 'Img' button above and put the URL in between the [ img] and [ /img] markers it will display nicely.

Hopefully the picture(s) will tel the tale and we'll get your robot up and running.

Howard

There are some pictures. Sorry for the unclear description. I meant when the motor wires are plugged into the breadboard, they don't start, even when I adjust the potentiometers and turn the switch on, but when I take out a battery and put the motor wires up to the battery, the motors start. The transistors I have aren't the same brand as the transistors in the procedure, but when I bought them, the person working at Fry's looked up what the same transistor would be, just in the brand Fry's carried. The potentiometers shown in the breadboard are single turn, (but they're not the ones in the procedure) and the blue one shown with the transistor are not single turn, so I don't know how to control the blue ones. I hope that makes more sense. The jumper wires got in the way of the breadboard, so if you want me to take some wires out to see certain areas of the breadboard, just let me know. Thank you so much.

Here's the blue potentiometers and transistor:
http://s9.postimg.org/j18ey6oxb/image.jpg

Hi again Miarie-

Thank you for posting the pictures. The separate one of the two components helped a lot. The transistor you got is indeed the same thing as the one pictured in the project. The blue potentiometer you got looks like a 10-turn pot. That means instead of turning it 270 degrees from end to end like the one on your circuit board, you have to turn it 10 full revolutions to do the same thing. For example, if you wanted to set the ones on your circuit board to 5K or halfway, you'd simply turn it to the left until it stopped, then turn it a little less than 1/2 turn to the right. With the blue one, you'd turn it multiple turns to the left *gently* until it stops, then turn it 5 turns to the right. Electrically it's the same thing but these 10-turn pots give you much finer control.

It's a little hard to tell whether the wiring is right or not. I haven't built one of these and I'm looking at the schematic to see if you followed it. I *think* so but here's how I'd debug it if i were you. Do you have a multimeter or can you borrow one from someone?

1) Set the meter to a 20V scale if it has a range setting. Connect the black lead of the meter to one of the breadboard connections where the negative (black) leads of the battery connect. You'll just leave that lead there.

2) Touch the red lead to where the battery connects to the switch. It should read 3V.

3) Touch the red lead to the switch connection which goes to the rest of the circuit. It will either read 0 or 3V, depending on the switch setting, Flip the switch the other way and you'll see it change from 0 to 3 and back to 0 as you turn the switch on and off.

4) Looking at the front of one of the transistors, the leftmost pin is called the Gate. That will be connected to one side of the light sensitive resistor and one side of the adjustable potentiometer. Set the potentiometer about half way. Put the red lead of the meter onto that leftmost pin. If the rom is kind of dark you'll see a very low voltage on the meter, close to 0. Shine a light on the sensor and the voltage will increase. With the light shining on it, keep the light steady and adjust the potentiometer. When you turn it one way the voltage will go up. Turn it the other way and it will go down.

5) Repeat step 4 for the other transistor, light sensor and potentiometer.

6) That particular transistor will turn *on* when the voltage on that pin you looked at reaches 1.5V. The setting you're looking for on the potentiometer is the one that makes the voltage lower than 1.5V when the sensor only sees room light and greater than 1.5V when the light from your flashlight shines on the sensor.

7) Assuming you got this far, we'll look at one motor. One lead from the motor connects to the switched voltage from the switch and the end of the light sensor that isn't connected to the transistor. The other lead of the motor connects to the center lead of the transistor, the Drain. The rightmost lead of the transistor connects to the negative (black) side of the battery. If the voltage on the leftmost pin is varying from above and below 1.5V dependng on the light, the motor should be turning on and off.

8 ) Repeat step 7 for the other motor and transistor.

One rather important thing about MOSFET transistors is that they are sensitive to static electricity. If the room you're working in is dry, it's very easy to damage the transistor simply by touching it. It came in a gray or pink plastic bag that protects it - as soon as you remove it you have to handle it in a static-safe manner.

If you've made these checks and can't get things to work, maybe it's worth taking the parts off the board and rebuild one side at a time, checking the voltages as you go. Start with the battery and switch, then the light sensor and potentiometer. Then add the transistor and put the motor in last.

I followed what I could of the pictures and it does look like you wired things correctly. I was hoping it as something as simple as an adjustment of the potentiometer. The board you photographed has simple 3/4 turn pots and those are easy enough to adjust. If you substitute in the blue ones, it's the same as long as you remember to turn multiple times to go from end to end.

There are others here who have built one of these and the designer reads the forum too. I hope this helps a little and if not, maybe someone else can give you more specific advice.

Howard

Hi Miarie -

Howard wrote out a very thorough troubleshooting procedure so I won't add to that. I only have one more suggestion. I can see in your pictures that you already have a pair of wire strippers. If possible, I would recommend buying some "22 AWG solid-core hookup wire" at Fry's. It comes in spools like this in different colors:

http://www.frys.com/product/3018459?sit ... IN_RSLT_PG

That will allow you to cut shorter jumper wires to length and use them on your breadboard. Make sure you get "solid core" wire and not "stranded," which is too flexible to push into the breadboard.

With all those long, loopy wires in the way, it can be hard to debug your circuit. Using shorter wires makes the circuit less messy and easier to debug. Usually when students have trouble with this project, it is because just one wire is misplaced somewhere, and that prevents the whole circuit from working.

In case it helps, we have more information about the different types of wire here:

https://www.sciencebuddies.org/science- ... hookupwire

and here:

https://www.sciencebuddies.org/science- ... mper-wires

-Ben

About the potentiometers, the blue ones never stop turning whichever way I turn them, so how do I know when to stop turning them left, and when to turn them right again? I looked them up and found the exact model, so I don't know if this helps: http://www.kingtronics.com/pdf/rkt-3296.pdf . I've been testing the two on the breadboard, the 270° turn ones, and haven't had much luck. I'll try to find a multimeter for the rest of my breadboard, and hopefully I'll be able to get it to work. Once again, thanks.

Hi again Miarie-

If you're local to Fry's, any meter they sell will do. For your measurements you don't need any exotic ranges or functions. The blue potentiometer (thanks for the link to the data sheet) is a 28-turn pot. If you knew the part number that might have been on the packaging we could tell what the maximum resistance is. Anything over 10K should work. To go from the minimum to the maximum you have to turn the little screw 28 times. It's possible that it won't physical stop turning but that's unusual. With a multimeter you can measure the resistance from the center pin to one of the others and you'll see the resistance increase or decrease as you turn.

If you redo the wires with shorter ones as Ben suggested and follow the circuit along with the voltmeter, I'm pretty confident you'll find the problem. You might consider picking up a spare transistor or two in case your problem is static damage to those. The pros use a 'grounding strap' which is an elastic wristband made of special material that has a cord that you attach to a water pipe, the screw on an outlet cover or plug in. That keeps any static charge off your hands. It also can help to work on a piece of aluminum foil or on a cookie sheet which is attached to a water pipe.

Howard