Well the easiest way to look at it is part 101.1 -
FAR Part 101 Sec. 101.1 effective as of 05/26/1970
(a) This part prescribes rules governing the operation in the United States, of the following:
<SNIP>
(4) Except as provided for in Sec. 101.7, any unmanned free balloon that--
(i) Carries a payload package that weighs more than four pounds and has a weight/size ratio of more than three ounces per square inch on any surface of the package, determined by dividing the total weight in ounces of the payload package by the area in square inches of its smallest surface;
(ii) Carries a payload package that weighs more than six pounds;
(iii) Carries a payload, of two or more packages, that weighs more than 12 pounds; or
(iv) Uses a rope or other device for suspension of the payload that requires an impact force of more than 50 pounds to separate the suspended payload from the balloon.
Thus, if the payload is under 4lbs there are no rules except for part 101.7 -
FAR Part 101 Sec. 101.7 effective as of 06/29/1963
(a) No person may operate any moored balloon, kite, unmanned rocket, or unmanned free balloon in a manner that creates a hazard to other persons, or their property.
(b) No person operating any moored balloon, kite, unmanned rocket, or unmanned free balloon may allow an object to be dropped therefrom, if such action creates a hazard to other persons or their property.
So it appears that so long as the payload is under 4lbs and one uses good sense that all of the rules are followed.
Back to the original question - Any help with the optoisolator?
Thanks!