It was show and tell time in a big way over the weekend at the U.S. Department of Commerce Patent and Trademark office in Crystal City. About 15,000 visitors came to view, touch and try inventions ranging from homemade to ultra-slick professional.

It was the 7th Annual National Inventors Day Exposition to celebrate the birthday of Thomas A. Edison, the first Inventors Hall of Fame inductee.

On display by amateurs and professionals were "stopsmoking" ash trays, heat-conservation devices, luminescent writing paper, solar energy displays, another attempt to improve the mousetrap and a "Super-Dooper-Pooper-Scooper."

Rollin T. Williams of Allentown, Pa; was proud of his "SDPS," which for patent reasons is called, "Scavenging Device for Animal Waste." You press a button on the bigger-than-a-shoebox and a plate propels the waste into a plastic-lined compartment. "You remove the bag," said Williams, "tie it with a ribbon if you want to and dispose of it.

"Big cities definitely have a need for this item," said Williams, a slender intense man with only a wisp of humor. "I have spent every spare minute working on this invention. It took me three months to complete it. I am not divorced, but separated from my wife."

It was Williams' first attempt at inventing, and what made it tougher was not owning a dog. "But the neighbors have them," he said. "They're all over the place."

Presiding at a homemade booth where visitors could write on paper that glowed in the dark was 16-yearold Becky Jane Schroeder, a highschool senior from Toledo, Ohio, probably the youngest exhibitor.

"Most people say, I wish I thought of that," Schroeder said. She came up with her idea while sitting, at age 10, in a parking lot while her mother shopped.

"It was getting dark, and she was taking a long time," said Schroeder. "I thought, what if I had paper that glowed."

By age 12 she had a patent on her idea.

"It was simple," Schroeder said. "I coated a sketch pad with phosphorescent paint, drew dark guide lines, held the sheet in bright light for 5 to 10 seconds and it glowed in the dark."

One suggestion for its use was for reading menus in dark restaurants.

Anthony J. Souza of Lancaster, Pa; had U.S. Patent number 4,103,448 for improving the mousetrap.

It was the standard trap, with a stuffed mouse lying there looking miserable. The trap sat just outside a small paper bag attached to an elastic band. When the mouse went after the cheese, snap, the elastic band would yank the whole thing into the bag, mouse, trap and all.

Although no one came up with a solution for cooling off the hot and crowded exhibit hall, someone did come up with a "No-snow" summer sled. A two-piece mold is filled with water and left in the freezing compartment overnight. Presto! a superslick sled of ice with a plastic seat to ride on.

The inventors all stood proudly beside their efforts. And even if some might have thought their ideas a bit far out, they might have recalled that Edison himself once invented a talking doll. CAPTION: Picture, Becky Jane Schroeder, by John McDonnell (c) The Washington Post February 12, 1979