So put a big electrolytic across the power inputs, and isolate the thing from other loads. When an echo is detected, ECHO will go high. INIT and ECHO can be connected directly to timer channels of the 68HC11 without harm, but the power supplies should be isolated. A separate regulator for the TI board is indicated, because it's a noisy load. Range is from 2 to 35 feet.
Shiny surfaces act like mirrors.