300 MHz RADAR or Motion Proximity Sensor

Transistor is a 2SC3355. Antenna output is connected to the 100pF capacitor on the collector. Output signal appears at the base and is bandpass filtered to remove the DC bias and the RF signal, only the detected signal passes to the 1st amplifer. Amplified signal is bandpass filtered again and amplified once more before being sent to an adjustable comparator. Output signal is a function of distance and velocity of the motion it detects. Can detect my body and large plates of sheet metal, actually works quite well at doing so. I assume it just detects changes in loading of the antenna. The coil L1 is 2 turns of 1.03mm enameled copper witre wound around a 4mm diameter air core and a width of roughly 2.06mm. Usually a capacitor between the emitter and ground is required for oscillation but for such a high frequency, parasitic capacitance can work in our favour. Objects moving towards the antenna generate positive voltages, and objects moving away generate negative voltages. Thus moving an object back and forth will cause the LED to blink on and off.

published 1 year ago