Microwave induction control circuit

Microwave induction control circuit
Figure 1 is a schematic diagram of a microwave control circuit made with discrete components.

In Figure 1, the high-frequency transistor VT1 generates self-excited oscillation under the positive feedback of capacitor C1. The high-frequency electromagnetic waves generated by the oscillation are radiated by the antenna to the surrounding space, generating a three-dimensional microwave field around the antenna. When there is a moving object in this three-dimensional space, the electromagnetic waves reflected by the movement of the object are received by the antenna, causing the amplitude and frequency of the self-excited oscillation of VT1 to change. These changes are converted into a voltage that fluctuates with the movement of the object through the integration circuit composed of R2 and C3. After being amplified by VT2, this voltage can generate a voltage change of 2.5 to 6.7V on its collector (the voltage change is proportional to the speed of the object and the distance from the antenna). This changing voltage is sent to the dual-limit voltage comparator composed of IC1 and IC2. Whether the potential sent from the collector of VT2 to the pin ② of IC1 is lower than the pin ③, or the potential sent from the collector of VT2 to the pin ⑤ of IC2 is higher than the pin ⑥, the pin ① of IC and the pin ⑦ of IC2 will output high level. These two high levels are rectified by VD1 and VD2 respectively and added to the base of VT3 to make it conductive, and then the relay K is electrically attracted to control the controlled circuit.
In Figure 1, the inductor L1 is made of φ0.51mm high-strength enameled wire wound 5 turns on the φ5mm ballpoint pen core; the antenna uses a metal rod antenna about 15cm long used in shortwave radios. The parameters of other components are shown in Figure 1. C1 should be perpendicular to L1 during installation.

The microwave control circuit works according to the Doppler effect: a fixed high-frequency signal (usually 400-800 MHZ) is generated by the local oscillator circuit and radiated to the surrounding space through the antenna. When an object moves within a certain distance near the antenna, the high-frequency signal will be reflected back by the moving object and then received by the antenna, causing the oscillation frequency and signal amplitude of the original oscillation circuit to change. The integral circuit is then used to extract the change signal, and after amplification, comparison and other processing, a control signal is formed to start the control execution circuit (such as high-power thyristors, relays, etc.) to achieve the purpose of automatic control.