B.E.A.M.
Biology, Electronics, Aesthetics and Mechanics

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Frans Storms

            Mar. 6, 2002

 Universal BEAM Light Head
 1] Pick your gear motor or servo. Small gear motors up to 30mA can be driven by a 74HC(T)240. To be sure try to get a AC240 which can drive 3 times more as a HCT240.
 Cap C1 in the circuit delivers the current. In the first pic you see a small gear motor (10mA), for which 330uF was
 enough, andy's copal motor in the second pic needed 2200uF. With R1 you can adjust how big the steps are the motor takes. 33k is a good average.
 The less current the motor uses, the more active the light head is. The motor in the first pic runs continually in shallow dutch sunlight. Turning the stand kept it looking staight into the light, very funny.
 
2] Pick your sensors.
 Good wide angle sensors are sfh205, bpw34 and bpw41. Buy more, and pick a couple that has the same reverse voltage measured with a digital voltmeter. If they are not matched the head will always point at the brightest spot with a constant angle of deviation, so if you can live with that.... Line them up making an angel of 90 degrees with each other, somewhat like the eyes of a fish or bird. They have all some kind of marking to point to the kathode, look closely, or download specs from the web if you're not sure.

 3] Pick your solar cell.
 Solar cells deliver about 0.5 per stripe. So a 2433 (5 stripes) seems OK for a 1381J which triggers at 2.9 Volts. But uuuh, double the cells if your light head is indoor,
 double them once more if want it to move at night ...
 This is dependent ofcourse again upon how much current your motor needs. Solarcells can connected paralel or series.
 
4] Build the circuit, it has very little parts (10!)
 D1 and D2 are common diodes. Wiring the 74HC(T)240 or AC240 is easy as lots of pins are connected with each other ...
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