Remote control transmitter circuit

It is common for children and elderly people to be lost and searched everywhere, causing 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 remote control , wear a transmitter on the child or elderly person, and
wear a receiver on the guardian. When a child or elderly person is 3 to 4 meters away from their guardian (parent or nanny), the guardian will receive an alarm
sound. Since the distance is not far, they can be found quickly and are not easily lost. This device is especially suitable for crowded occasions such as holidays, shopping malls, and downtown areas.
    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 18-4. When VT3 is turned on,
forward bias is applied to VT1 and VT2, and
VTI and VT2 form a self-excited multivibrator to start
working, generating
a high-frequency constant-amplitude oscillation signal with a frequency of 1600 to 1800kHz. After
Li, Lz and cl resonant frequency selection loop, adjust
the size of cl, select the required transmission signal frequency
, and transmit it through the magnetic rod antenna.
    Among them, L1 and Lz are wound with 80 and 40 turns of yarn-covered wire respectively, and the trimming capacitor c] is 5-30pF.
    (2) Receiver The receiver circuit is shown in Figure 18-5. 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 cj tuning loops . Since the received signal is very weak, it performs multi-level amplification through vri, VT2, and VT3 to promote the four-sound
When the integrated music piece works, due to the small output power, after amplification by the final stage VT4, the speaker Y works to emit music sound to remind the guardian to
find nearby children or elderly people.