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FRED Photopopper
Some constructors have found very low-cost ways of producing a very effective Solar Powered wheeled vehicles. 
One excellent article (there are lots of excellent articles) has been produced by Ben Hitchcock. It is called
FRED. It is a Photopopper using low-cost components. It uses two red or green flashing LEDs to turn the circuit on when the voltage across the electrolytic has reached about 2.7v. 
The interesting part of the circuit is the two flashing LEDs. They change characteristics according to the level of the surrounding light and this turns the circuit into phototropic. 
Below is a photo of the Robot. Click to read the FRED article. 


The Schematic for FRED: 

The secret to the success of this circuit is the addition of a 220n and 3k3 around the flashing LED.  This is the cheapest and best circuit for a Photopopper. The only problem is the slight random activation due to the flashing LED. But this adds to the realism and creates unusual results. A circuit using a 1381 voltage-detecting chip is more uniform in operation.  Some flashing LEDs detect light and this creates a phototropic effect.  When the flashing LED detects light, the trigger voltage increases. You must make sure the solar panel is capable of delivering this extra voltage, otherwise the circuit will "lock-up."

Now you have the best circuit. You can produce lots of different designs for a light-seeking robot. 
Here are some photos from Andy's Page:

Note: Some bugs are named with their present owner

Black eyes better direction? looks like me?
Solar-bug V1 2 different solar panels looks like a bug?

Vibra-bot long legs, good vibration little pager motor with weight
Ground Explorer (Alex) with solar panel way up but bug close to the ground

40 More solar-bugs!!

Andy's links page will get you into hundreds of pages of Robots of all descriptions. 
This is only the start to an amazing world of "getting things to move."

There is only one more thing to help you produce a fantastic new design. It's a circuit to convert ANY voltage from a set of solar cells to a useable voltage. This means you can use as few as 2 or 3 cells and get almost any circuit to work . . . you don't have to buy an expensive solar panel!  It's a voltage step-up circuit. See it on the next page . . . 

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