Simple Alarm Circuit Keeps Wandering Children

The receiver circuit in Figure 1 sounds an audio alarm when the transmitter (Figure 2) moves beyond a designated perimeter. The transmitter, a voltage-controlled oscillator, operates at approximately 915 MHz in the unlicensed ISM (industrial/scientific/medical) band. It has a tuning voltage of 1.5V=3×R2/(R1+R2), which lets you easily adjust the frequency by varying the values of resistors R1