Science Buddies: "Ask an Expert"

I have built the Light Tracking Robot once, took it apart and rebuilt it twice and made a new one but they all are showing the same effect. Something is messed up with the robot photoaxis. The Robot shows negative (going away from light) instead of positive (going to light) AND the robot starts moving as soon as it is turned on even if it is turned on in the dark.

Please help me, my project is due on December 16 and I have not been able to test the robot as it is not operating the way I thought it should.

Hello and welcome to the forum:

For your first problem, did you make sure that the motor M2 is on the same side of the robot as Q1 (the photoresistor for M1) and that motor M1 is on the same side of the robot as Q2 (the other motor's photoresistor)? You must have these components "crossed" over to the opposite side, otherwise the robot will do as you say and go away from the light.

I need more information to know why the robot starts moving in the dark. Do you have a Voltmeter? Could you measure the voltages at the gates of the MOSFETs (labeled "G")? If we know those voltages in the dark, I might be able to help, Are all your components the same as in the project? If not, which ones are different?

And finally, did you notice that M1 does not seem to be hooked up to the output of the "ON" switch in the breadboard diagram? I will let Science Buddies staff know about this error.

Keith

Keith,

I'm a Dad helping my daughter with her light tracking robot ( https://www.sciencebuddies.org/science- ... ml#summary ). I don't want to hijack Jetifyred's thread, but we are having a similar problem. Even in the dark our robot turns on and vibrates both motors. We can make it turn by shining a bright light in either photo resistor, but it is on all the time. I have access to a multimeter, but I must be frank that I don't know what to measure. What setting on the multimeter do I use, and how should I measure the ground on the MOSFET?

Thanks,
John

Hi John:

I looked at the materials list and procedure (schematic) again, and noticed that the photoresistor is supposed to vary from 1K ohm (in the light) to 10 K ohm (in the dark). With a 3V battery pack, the gate input voltage (with reference to the source of the MOSFET) should be 2.5V in a very bright light to around 1V in the dark. I looked at the MOSFET characteristics in the data sheet linked to on the SparkFun web site, and this looks like it should turn the motor on and off. However, if you are having the motor turn on in the dark, you could halve the resistance from the gate to the source (instead of 1 4.7K ohm resistor, try 2 in parallel) and see what happens.

For the multimeter, use the V (volts) scale (make sure it's DC Volts) and the smallest scale that can measure 3 V (the maximum in the circuit.) (You can also look at Science Buddies Multimeter tutorial here: https://www.sciencebuddies.org/science- ... rial.shtml)

In all cases, if you could let us know the voltages across the gate to the source of the MOSFET that would help.

Best, Keith

Keith,

Thanks for your help and your very rapid reply! After I posted I kept tinkering with it, and found the problem! I could remove the photo resistor and the motor would still come on, so that made me think something was wired incorrectly. When my daughter hooked up the 4.7 k ohm resistor to the gate, I found she had accidentally hooked it up to the drain. I corrected the problem and everything works as expected.

Hi, this is Savar.

I am experiencing a problem in the project. It says in the instruction that you have to turn the potentiometers anti clock wise in order to stop the motor. My one of them is stopping but not the other one

CAN YOU PLEASE HELP HERE AS IT IS DUE IN ONE WEEK!!!!!!!!