Ultrashort wave item left reminder alarm circuit

Ultra-short wave item left behind reminder alarm The item left behind reminder alarm introduced in this article is small in size, with dimensions of 17×35×100mm, like two matchboxes, with a control distance of 1-5m (can be adjusted to 15m), and a loud alarm sound. It is bright, has low power consumption, stable frequency, low cost, and is easy to make. It is very suitable for carrying around to prevent important items from leaving the owner. It can also be used to monitor children's movements. If you are interested, you may want to give it a try. working principle 　　　　　　　　　　　

The device is a system consisting of a transmitter and a receiver, operating in the frequency modulation (FM) band. The circuit working principle of the transmitter is shown in Figure 1. It is a multivibrator composed of transistors VT1, VT2 and their peripheral components. Its operating frequency is about 1000Hz. Its signal is taken out through capacitor C3 to modulate the FM oscillator composed of VT3 and its LC components. The modulated FM signal It is output from C10 and radiated to space through the micro-antenna inside the machine. The function of C6 is to broaden the frequency band and effectively prevent the influence of human body induction on the frequency. 　　　　　　　

The circuit working principle of the receiver is shown in Figure 2. The super regenerative receiving circuit is composed of transistor VT1 and its LC components, which is used to receive the FM signal from the transmitter. Gate circuits A, B, and C constitute a signal amplification circuit, which amplifies, compares, and inverts the received signal to control the 1Hz oscillation circuit composed of gate circuits D and E. The inverted output of gate circuit F is then used to control the transistors VT2 and VT3. Complementary audio oscillator. When the transmitter signal is received, the output of gate circuit C is positive, the diode VD is turned on, the low-frequency oscillator composed of gates D and E stops vibrating, the output of gate F is low level, the complementary oscillators VT2 and VT3 stop vibrating, and the speaker BL is silent. Once the signal from the transmitter cannot be received, that is, the target or child is 1-15m away from the owner, the gate circuit C outputs a low level, VD is cut off, the low-frequency oscillator composed of gate circuits D and E works, and the inverted output frequency of gate F is The 1Hz positive pulse controlled complementary oscillator circuit emits a rapid "beep...beep..." sound to remind the owner to pay attention. Until the target object returns to the control distance, the alarm sound will automatically stop. The component selection and production component list is shown in the table below. Transmitter part number name model quantity R1, 4 resistor 20K 2 R2, 3 resistor 100K 2 R5 trimmer resistor 91K 1 R6 trimmer resistor 100K 1 R7 resistor 51K 1 R8 resistor 470Ω 1 C1, 2 ceramic capacitor 0.01u 2 C4 ceramic capacitor 0.005u 1 C5, 6 ceramic capacitor 33P 2 C7 ceramic capacitor 0.1u 1 C8 ceramic capacitor 30P 1 C9 fine-tuning ceramic capacitor 10-3P 1 C10 ceramic capacitor 51P 1 L homemade inductor (see text after the table) 1 VT1 , 2 Transistor 9011 2 VT3 Transistor 9018 1 S switch micro toggle switch 1 E Battery pack 3V 1 Receiver part number name model quantity number number name model quantity R1 resistor 4.7K 1 C12 electrolytic capacitor 1u 1 R2 trimmer resistor 200K 1 C13 , 14 ceramic capacitor 0.1u 1 R3 resistor 470Ω 1 C15 electrolytic capacitor 10u 1 R4, 5, 7 resistor 1M 3 D diode IN4148 1 R6 trimmer resistor 100K 1 VT1 transistor 9018 1 R8 trimmer resistor 51K 1 VT2 transistor 9013 1 R9 Trimming resistor 510Ω 1 VT3 transistor 9012 1 R10 Resistor 100K 1 IC Six-NOT gate integrated circuit CD4069 1 R11 Resistor 750Ω 1 L1 Inductor is the same as L1 of the transmitter 1 C1 Ceramic capacitor 15P 1 L2 Inductor 6.5um color code inductor 1 C2, 3 Ceramic capacitor 33P 2 BL speaker small size 1 C4 ceramic capacitor 0.005u 1 S switch small toggle 1 C5 ceramic capacitor 0.5u 1 E battery pack 3V 1 C6-11 ceramic capacitor 0.01u 6 In order to reduce the size, all resistors are used 1/16W micro components, ceramic capacitors and electrolytic capacitors are all reduced type, and BL uses a Φ12mm micro electromagnetic sounder. The inductor coil L1 in the transmitter is formed by winding Φ1mm enameled wire 3 times on a Φ7.5mm pencil. The inductor L1 in the receiver is exactly the same as L1 in the transmitter. The transmitter and receiver each use two AA batteries. The quality of the selected components will directly affect the electrical performance of the device. After welding is completed, check and confirm that it is correct, and then turn on the power supply for debugging. Use an FM radio to adjust the transmitter so that the radio can receive the signal from the transmitter at the low-frequency end. Care should be taken to avoid local FM radio stations and adjust carefully.