The code original from http://www.rcgroups.com/forums/showthread.php?t=2209667
It used cheap sonar module HC-SR04 with ATtiny85 microcontroller to convert signal to PWM.
I found an ATtiny85 SOC and revised the code to fit it.
If you want to get original code, it will be found in release tag.
FYI. ATtiny85 SOC http://goods.ruten.com.tw/item/show?21312271403983