This is an excerpt from J. Seal, M. Suttlar, A. Landrith and A. Kay, " Key Chain Locator," ECE 4522: Design II, Department of Electrical and Computer Engineering, Mississippi State University, Spring 2003:

The phase-locked loop provides the Key Chain Locator with a dynamic digital filtering system. The center frequency is easily tuned while maintained a bandwidth within the constraints of design. The filter works remarkably well, with lab tests showing a 300 Hz bandwidth. The filtered signal is interpreted by a PIC, which is interfaced with a novel delay circuit using two switching transistors. Two switching transistors were controlled by the PIC and they alternate between the compared frequencies of the phase-locked loop. The delay circuit was designed using a PIC. The PIC also ensures the first tone received by the system is at least one-quarter of a second in duration and both tones two tones were received within a three-second interval. The device responded to a two-toned whistle sequence from a distance of twenty feet. The device actually responds to a user in excess of 30 feet away, regardless of orientation.