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. You can also increase or decrease the desired perimeter
by adjusting R10 to change the comparator’s reference voltage. Link
