PIR circuit
This is a circuit I have used many times. It does not require a fresnel lens for the PIR, but you should arrange three or four slits in the enclosure directly in front of it. Make the slits about the same width as the enclosure is thick and space them apart by the same distance. With this arrangement, expect a detection range of 3-4 metres. The supply is not critical. I have used 9V and 6V in the past with good results. Be aware that relatively large switching loads may cause self-triggering as the PIR and op-amps respond to fluctuations in the supply. If this occurs, a large capacitor (470uF or 1000uF) may be required across the supply. The circuit needs a few seconds to stabilise after power-up.  The LED example load in the circuit diagram gives a good indication of operation. The output can be interfaced directly to CMOS logic, or a microcontroller (just watch that the output voltage level is compatible).
A pdf version of the circuit is available for download.