Instructables
Picture of Simple Light Following Robot
MOSFET_Motor_driverNT.png
Build a simple light following robot with common low cost parts.  You will need a soldering iron, hot glue gun, and an ohmmeter if you do not have matched light sensor cells.  This robot will follow light from a flashlight.  These parts are available at many electronics suppliers, and are common in on-line surplus outlets.

Parts and Tools:

    1) Two small DC motors, with fairly long shafts.
    2) Three popsicle sticks
    4) Two potentiometers, around 10K
    5) Two (matched) CdS cells
    6) Insulated wire
    7) Two N-Channel MOSFETs
    8) Two diodes, preferably fast switching type
    9) Tube or shrink-tubing to use for wheels
   10) [optional] 1/2 ping pong ball, or metal thumb-tack, to use for rear skid.
   11) Hot glue, and hot glue gun
   12) Solder, soldering iron
   13) Multimeter (only needed if you need to select CdS cells)
   14) 9v battery plug
   15) 9v battery

On-line surplus electronics stores often sell large bags of Cadmium Sulfide (CdS) light sensor cells, which are Light Dependant Resistors (LDRs) for reasonable prices.  When using cells from such a grab-bag, you should be careful to select two matched cells with a similar resistance under both low and bright light.  The values of these cells can vary significantly.  Matching to within 10-20% is sufficient.

This robot is based off of two mirror-image motor driver circuits as shown here.  The photocell on the left side drives the motor on the right, and vica-versa.
 
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Step 1: Select Matching CdS cells

Picture of Select Matching CdS cells
If you are working from a grab-bag of cells, you will want to select two fairly similar photocells.  If you purchased matched photocells, you can skip this step.

Measure the resistance of cell in dim, ambient light. Repeat this measurement pointing toward a lamp. The second measurement should be significantly lower.

Repeat this measurement on several cells until you find two which have similar values both under dim light and when pointed toward a lamp.

Step 2: Mount Motor Wheels

Picture of Mount Motor Wheels
Mount wheels on motor axles
schel8 months ago

Hi Great Instructable! I swear I checked all the comments first before I ask this, I'm sure someone has before I, Is there any way to reverse this ciruit? I want the robot to STOP when the Photos' receive light. In other words, if the 'bot is in the dark...it should THEN move until it finds light or turns to face it, then stop. as if it wants to charge it's solar cells ya' know? since the motors are "triggered" only in light it would be running for the dark. Much Thanks!!

Mr.What (author)  schel2 months ago

Missed the earlier notification. If you swap the motor leads you can get a bot that runs away from a light, but stands still in the dark. You would "push" it around with a flashlight, instead of having it follow one. You will find that such a pusher is a little harder to control than a follower.

CRASH MA5TER2 months ago
To make*
Mr.What (author)  CRASH MA5TER2 months ago

Depends on how many spare parts you have around. Shipping will be an issue if you don't stock things. I can do it for about $5. $1 ea for motors. $0.10 each for transistors. $0.25 each for potentiometers. About $0.20 each for CdS cells, these are mostly only available as surplus. Cheap in bulk... bags of 100 or so. $0.25 for battery connector, $1 for battery. I have resistors, tape, wire, little tubes laying around. Solder. Hot glue.

CRASH MA5TER2 months ago
About how much is it toke one
Hey, this is a fantastic project. I loved the way you explained all the details of the project......
Mr.What (author) 1 year ago
Any small(ish)-signal n-channel mosfet will do. 200mA peak should be sufficient. I used several (whatever we had around the lab). 2N7000 will do http://www.goldmine-elec-products.com/prodinfo.asp?number=A20384
vn2222, for more power... bs170, for MASSIVE power (20A?) these are cheap (around $0.50 ea) http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
srao101 year ago
which N-Channel MOSFET???
killbox2 years ago
I believe that is cut up "screendoor/storm window Spline" being used for the wheels. but anything that grips well will work.
Mr.What (author)  killbox2 years ago
killbox: you ringer. you know it is screen holder... I got it from you! You could punch a hole in almost any rubber, and I bet there are grommets around that will work.