Circuit Keeps Wandering Children and Pets Nearby

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.

This 915 MHz receiver sounds an alarm when the comparator

This 915-MHz receiver sounds an alarm when the comparator’s inverting-input voltage drops below 400 mV.

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.

The transmitter comprises a voltage controlled oscillator, which R1 and R2 tune to approximately 915 MHz

The transmitter comprises a voltage-controlled oscillator, which R1 and R2 tune to approximately 915 MHz.

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.

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