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Light Following Robot,
By : Chiu Yun Fang
First off, if your new to BEAM and haven't had much experience with electronics, go to Solarbotics and buy the Photopopper 4.2 kit. It is an excellent kit with all the parts you'll need, a high output solarcell, new pager motors, omni-directional feeler switches, in-depth and informative documentation and a cute little PCB. It's well worth the money! Here's a couple pics to wet your appetite


The BEAM photovore is an autonomous robot that is entirely powered by solar energy. It is phototropic which means it searches out the brightest source of light which, in turn, gives it more power...pretty smart eh? The first step to building a photovore is to get the schematic.



And a parts list...
2 X Panasonic 1381J 2.6V-2.9V Voltage detectors (digikey part # = MN1381-J-ND)
These voltage detectors come in many different voltage ratings. If your motor is spinning too hard, losing traction with the ground, you may want to use a 1381G (2.4V-2.6V) or 1381E (2.2V-2.4V). If you want more power per step you can use the 1381L (3.0V-3.3V) although it will take longer to charge up.
2 X Photodiodes (digikey part # = LT1032-ND)
I'm not positive on this part number. I got a big box of photodiodes for free so I haven't ordered these specific ones.
2 X .22uF capacitors (digikey part # = P4958-ND)
2 X 2N3904 NPN transistors (digikey part # = 2N3904-ND)
2 x 2N3906 PNP transistors (digikey part # = 2N3906-ND)
2 x 2.2K ohm resistors (digikey part # = 2.2KQTR-ND)
1 X 4700uF eletrolytic capacitor (digikey part # = P5118-ND)
1 X 100K ohm potentiometer (digikey part # = 3386H-104-ND)
2 X Pager motors or small tape recorder motors. They have to be efficient. I suggest the Solarbotics pager motors. The Namiki 1701.
2 X Fuse clips. Or any thing you can use as motor mounts. (digikey part # = FO52-ND)
1 X Solarcell. Any solarcell that can output atleast 3.5V will do. Solarbotics has the best high-current solarcells around.
Next I suggest you breadboard the circuit just so you know all your parts are good and my schematic is right!
Test the circuit by approximately centering the potentiometer and throwing the circuit under a light. One of the motors should be spinning (popping). Cover one photodiode then the other with your finger. Do both motors spin? If alls good then you can build your photovore.
There are many ways you can build the photovore circuit. My favorite is free-forming.
First off, glue the two 1381's face to face. Then glue the two 3906's together and the two 3904's together and glue them all into one block. This block arrangement is ripped off from Paul Beckingham ;-) .



Make a ground loop. Solder together pin 3 on both 1381's and the Emitters on both 3904's. All four pins need to be connected together.

Now make a positive loop. Since the potentiometer wiper is connected to positive, now would be a good time to glue it to the photovore block. Solder the potentiometer wiper to the Emitters on the 3906's and leave a short length of lead hanging out the back. You'll need to solder other components to it later.

It's time to give out little creature some eyes. These photodiodes were given to me for free. They work very well for BEAM applications. Your photodiodes may look very different. Make sure to figure out the photodiodes polarity. They won't work if installed backwards. Anode goes to the potentiometer leads and cathode goes to pin 2 on the 1381's.


Install the capacitors from the photodiodes cathode leads to the ground loop you make in the first step.

The completed brain. The 2.2K ohm resistors are installed from the Base of the 3906 to the Collector of the 3904 on both sides. Leave a length of lead on the side of the resistors that connects to the Collector (3904) to hook to your motors. These 2.2K resistors are for tuning the solar engines to perform it's best with your particular motors. Experiment for best results. Check for visible shorts and bad solder connections. You may want to clean off all the flux too.

For those of you that couldn't wait...here's the final steps. Solder the capacitors positive lead to the solarcells positive lead then solder the capacitors negative lead to the solarcells negative lead. A capacitors negative lead is labeled on the can with a minus sign and arrows to the negative lead. Now solder the positive connection to the red "positive" lead and the negative connection to the white "ground" lead (ground loop). Now, choose with motor is going to be on the right (iny, miny, miney, moe works well for me) and figure out the lead polarity. Red is positive and the dark color or white is usually negative. The right motors positive wire goes to the photovores red "positive" lead and the negative motor wire is soldered to the green "left" lead (confused yet?). The left motor is installed the same but flipped. The left motors positive wire goes to the blue "right" lead and the negative wire is connected to the red "positive" lead. Put the photovore under the closest desk lamp and it should take it's first steps. Photovores are rather slow and boring by themselves but if you build 3 or more, you'll be quite surprised by the little quarks and attitudes of each one.





After leaving this progect hanging for a while I decided to finally make some motor mounts. The mounts are made from tubing found in my junk box that fit the pager motors perfectly.
The yellow stuff on the motor shafts is heatshrink tube. I find this stuff makes the best feet. One of the things I like about this photovore is that it's solarcell is tilted forward. Since it will be heading towards the brightest source of light, it just makes sense. I plan on changing the cap and solarcell to larger units to increase activity. With the 2200uF cap, it charges very quickly but it's steps are very small.

The finished photovore. Set it up facing towards a 100W light bulb and he should start popping towards it. Use the potentiometer to make fine adjustments until he walks dirctly to the light. One of these days I'll detail touch sensors...one of these days...

Introduction
The goal of our project was to build a robot which followed an active source of light. Our vehicle
uses optical sensors(photodiodes) to follow a moving light source which in our case is a torch held
by the user. The hardware is self constructed and thus was the major part of our work.
High-Level Design
Our vehicle uses photodiodes to locate its postition relative to the light source. The state of the
photodiodes is captured, and the car is controlled accordingly by an Atmel ATMEGA16
microcontroller. If the car detects a small perturbation(change in intensity of the oncoming signal),
it will correct its path by steering in the appropriate direction.
The original objective for this project was to optimize the car for the speed with which it navigated
the path. To do this, we would have increased the speed when little compensation was needed, and
decreased the speed as more was needed. This optimization, however, did not require much more
effort on our part than the heuristic outlined above. This is because the speed is mechanically
regulated by the gradual acceleration(increae/decrease) of the car in balance with the direction
reversal for large corrections.
Hardware
The hardware for this project was the bulk of the work. Because we were making a moving vehicle,
we decided that it would be best not to have any of the circuitry on breadboards. Breadboards are
also expensive, so we probably would have had to disassemble the car after its demonstration. We
therefore chose to wire-wrap all of the circuitry. This required a fair amount of labor, but it was well
worth it in the end.
The car we selected is fairly simple to operate. It has a simple two wheel drive, , which is powered
by a DC motor. The steering is achieved by applying a difference of voltages acroos the two
batteries. We used two H-bridges (L293D) to deliver power to the steering and the drive motor.
Because of the current requirements (the drive motor and steering solenoid could draw several
amps), we used 20-gauge wire for the high-current areas of the H-bridges rather than the 30-gauge
wire used for the rest of the circuit.


Download full report
http://me.iitb.ac~kaushikj/lightfollower.pdf

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