Problem with Light Tracking Robot

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Problem with Light Tracking Robot

Postby jaymanh1200 » Thu Apr 10, 2014 6:58 am

I am working on the Light Tracking Robot as extra credit project that is due on April 18, 2014. I have built the robot according to the diagram and it work for a few minutes, then stopped. I have switched out the batteries and went over the diagram again, but still cannot get the motors to work. I thought that maybe the board shorted and started over with the new one, but still have the same result, no response from the motors. My mom suggested bypassing the MOSFET and connecting the motors directly to the battery. I turned on the switch and the motors worked. When I switched back to the MOSFET and add the motor to the drain, nothing happens. But if I connect the motor to the S of the MOSFET the motor works. My mom and I can't figure out why the drain is not working. We have switched out new photoresistors and resistors, but not the MOSFETS.

[Moderator note: I have split your post into its own thread so that Experts can better assist you.]
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Project Question: Light tracking robot
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Re: Problem with Light Tracking Robot

Postby kgudger » Thu Apr 10, 2014 3:46 pm

Hello and welcome to the forums.

When you connect the motor to the "S" connection, you are connecting the battery directly across the motor, and you prove that the motor is good.

Have you measured (with a voltmeter) the voltages at the gate of the MOSFET? (vs. light) This will tell you if that leg of the circuit is OK.

If it is, then it sounds like the MOSFETs are dead. MOSFETs have protection circuitry to prevent shorting by static electricity, but sometimes they get overwhelmed and the MOSFETs die anyway. If your climate is particularly dry right now, you have to be really careful not to touch the MOSFET leads with your fingers, just with (preferably grounded) tools.

Let us know what you find out.
Best, Keith
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Re: Problem with Light Tracking Robot

Postby jaymanh1200 » Mon Apr 14, 2014 9:11 pm

Thanks for the response, but I am still having the same issue. I checked the MOSFET with the voltmeter and they looked liked they were dead. So I replaced them and made sure I did not touch the pins. I started with a fresh breadboard and double checked my work, but there is still no response from the motor when I connect it to the drain.
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Project Question: Light tracking robot
Project Due Date: April 18
Project Status: I am conducting my experiment

Re: Problem with Light Tracking Robot

Postby kgudger » Tue Apr 15, 2014 7:33 am

Hello:

I'm glad to hear you had replacement MOSFETs. Did you measure the voltage on the gate (G) pin while you varied the light? Could you please report those voltages versus light? That would be my next step in debugging the circuit. Thanks.

Keith
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Re: Problem with Light Tracking Robot

Postby jaymanh1200 » Wed Apr 16, 2014 8:01 am

The voltage from the gate said 1.5 without the light, around 2 to 2.5 with the light. I got some help from our neighbor last night who is a retired electrician just to make sure. He checked the breadboard to see if power was being distributed and it was. He thought that maybe I was not using a strong enough flashlight, so he used a fluorescent light and it still did not work. After that, he wanted me to connect the motor to the gate instead of the drain and test the voltage while shining the light. When he touched one of the photo resistors with the voltmeter the motor ran. So basically, the motor would run when connected to the gate while the voltmeter touches the photo resistors. The switch was on the whole time and used a fresh set of batteries. When we looked at the diagram, we were thinking that maybe the gate is not connecting when the light shines on the photo resistors. I don't know if the resistors are pushing enough power, because when our neighbor checked them, he said they were getting power. I have tested with Energizer and Duracell batteries. It seems like there could be some power loss or not enough being transmitted. I left my robot with my neighbor because he wanted to take a closer look and study it some more.
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Re: Problem with Light Tracking Robot

Postby kgudger » Wed Apr 16, 2014 9:30 am

Hi:

From your description of the voltage at the gate of the MOSFET, I can tell you that your MOSFETs are not working. I don't know why, but if they are not conducting when you have 2.5 V on the gate, they are not working. However, I also wonder about your photoresistors. If you're only seeing 2.5V on the gate when brightly illuminated, then I think there might be a problem with the resistors, too. I know that this experiment works as designed, as other students have completed this project successfully.
Did you use these part numbers? GL5528 photoresistors and RFP30N06LE N-channel MOSFET?
Are you sure that you have the MOSFETs connected correctly? If you accidentally connected the MOSFET's G,D, and S terminals incorrectly, you might see the behavior you mentioned.
Also, what do you mean when you say: "When he touched one of the photo resistors with the voltmeter the motor ran." I don't know what this means.
Thanks.
Keith
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Re: Problem with Light Tracking Robot

Postby jaymanh1200 » Wed Apr 16, 2014 7:31 pm

When my neighbor moved the motor from the drain to the gate, and touched the photo resistors with the voltmeter, the motor vibrated. This happened while the light was shining on the photo resistors. When i removed the light, the motors stopped. Its as if the voltmeter was providing more power to the motors. It was really weird.

As far as the parts, I am using the IRF510 N-channel mosfet and CdS Photoresistors from Radio Shack. I made sure the volts for the MOSFET match exactly, but I am unaware there would be a difference in the parts. The resistors are exactly 4.7 Kohm 1/4 watt. Could there be conflict with the power between the parts? Could it be that I need to use a higher powered resistor?

I also made sure everything was set in the right place before I connected the motor and batteries.
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Project Question: Light tracking robot
Project Due Date: April 18
Project Status: I am conducting my experiment

Re: Problem with Light Tracking Robot

Postby kgudger » Wed Apr 16, 2014 10:00 pm

Hello:

I think I've found your problem! You say you are using the IRF510 MOSFET. I was going to upload the characteristic curves of this transistor compared to the specified one, but the bulletin board won't let me upload a picture. Here's a link to the data sheet for the IRF510: http://www.radiovilag.hu/images/IRF510.pdf and here's a link for the RFP30N06LE https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf. If you look at the Figure 5 in the first one versus the Figure 7 in the 2nd data sheet, you will see a huge difference for output current versus Gate voltage. Specifically, the IRF510 shows no output current for a gate voltage of less than 5V. This is why your circuit is not working. If you can please get the correct MOSFET, you should see your circuit work.

Best, Keith
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Re: Problem with Light Tracking Robot

Postby jaymanh1200 » Thu Apr 17, 2014 5:52 am

Okay, I will try to find the MOSFET locally since this is due tomorrow. What about the resistors and the photo resistors?

If not, is there a way to increase the gate voltage?
jaymanh1200
 
Posts: 6
Joined: Thu Apr 10, 2014 5:04 am
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Project Question: Light tracking robot
Project Due Date: April 18
Project Status: I am conducting my experiment

Re: Problem with Light Tracking Robot

Postby kgudger » Thu Apr 17, 2014 9:59 am

Hi:

I tried to find out the specs of the Radio Shack photoresistor, and they don't seem to be available. The best data I've been able to find for the specified photoresistor is that they should have between 8 & 20K ohm resistance at 10 Lux. Can you measure the ones you have? Ideally I would hope they are around 1 K ohm in bright light, assuming one is using the 4.7 K ohm resistors.

The only way to get a high enough gate voltage with the MOSFETs you have is to increase the supply voltage. I'm guessing you would need to at least double it. That might be a problem for your motors, if they are really 3V motors. If so, they could burn out with a higher supply voltage. You could try adding more AAA batteries, one at a time, to see if you can get it to work. I'm thinking these MOSFETs might require a supply voltage of 9V (6 AAA batteries in series) to woGL5528 photoresistorsrk.

Keith
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Re: Problem with Light Tracking Robot

Postby jaymanh1200 » Thu Apr 17, 2014 11:22 pm

Great news. We finally got the robot to work by adding the extra supply voltage. I was up to five batteries before the motors started to respond. I was thinking I would use this learning experience about the differences in MOSFETs as part of my report. Its a bit bulky for movement, so the steering is a little off, but none the less it works. Thanks again for the help.
jaymanh1200
 
Posts: 6
Joined: Thu Apr 10, 2014 5:04 am
Occupation: Student
Project Question: Light tracking robot
Project Due Date: April 18
Project Status: I am conducting my experiment


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