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 and R2. Alarm Circuit Keeps Wandering Children The receiver comprises low-noise amplifier IC1, power detector IC2, comparator IC3, and a buzzer. When the transmitter is within range—for example, when a child or a pet is carrying it—the receiver detects the RF signal and provides a voltage greater than 400 mV at the inverting terminal of the comparator. Resistors R9 and R10 preset the reference voltage at the comparator’s noninverting terminal. The reference voltage is 3×R10/(R9+R10), and the comparator’s output remains low. Circuit keeps wandering children and pets nearby figure 2When the tra...