Lost child alarm circuit b

The phenomenon of missing children often occurs, and they are searched everywhere, which brings undue pain and economic losses to families; in addition, it also gives
opportunities for criminals and human traffickers to take advantage of them. To this end, design a radio alarm, wear a transmitter on the child or elderly person , and
wear a receiver on the guardian. When a child or elderly person is 3-4 meters away from the guardian (parent or nanny), the guardian will receive an alarm sound.
Since the distance is not far, it can be found quickly and is not easy to lose. This device is especially suitable for crowded occasions such as shopping malls and supermarkets during holidays.
    The device consists of a transmitter and a receiver. Its circuit composition and principle analysis are as follows:
    (1) Transmitter circuit The transmitter circuit is shown in Figure 13-3 2a. When VT3 is turned on, forward bias is applied to VT1 and VTZ. VT1 and
VT2 form a self-excited multivibrator and start working. A high-frequency constant-amplitude oscillation signal with a frequency of 1600 to 1800kHz is generated. Through the Li, L2 and Ct resonant
frequency selection circuit, adjust the size of c], select the transmission signal frequency of the pile spring, and transmit it through the magnetic rod antenna.
    Among them, Lj and Si are wrapped with yarn wrapped thread respectively. For 80 turns and 40 turns, the trimming capacitor c1 is 5-30pF. Others are as shown in the picture.
    (2) Receiver circuit The receiver circuit is shown in Figure 13-32b. After the receiver antenna Li receives the signal from the transmitter, it selects the remote control signal with the same frequency as the transmitter
through the L2 and G tuning loops . Since the received signal is very weak, it performs multi-level amplification through VT1, VT2, and VT3 to promote four The sound integrated musical film works. Since the output power is small, after being amplified by the final stage VT4, the speaker island L works and emits musical sound to remind the guardian to find the lost child.