I just left my motion sensor in-circuit and soldered a wire to the appropriate terminal on the opamp output. It was easier for me to use their circuit rather than redesign the whole circuit. As long as you know the proper input voltage, you can use their circuit and repurpose the motion sensor for yourself. Once you've done some experimenting, you can buy one from radio shack or online for around $10 US. Sounds like you're on the right track. Have fun.