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 and R2.

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 transmitter moves outside the predetermined boundary, the detected RF produces less than 400 mV at the comparator. The comparator then generates an output of approximately 3V, which turns on the buzzer and sounds an alert that the transmitter has moved beyond the restricted perimeter. To increase the detection range, you can place additional low-noise amplifiers or VGAs (variable-gain amplifiers) in front of the power detector.