This circuit generates inaudible ultrasound to make a barrier across the entry. At the moment the circuit detects a break in the sound barrier, it generates a loud alarm indicating the entry of a person. This can be used to protects the entry passages and helps to detect unauthorized entry of persons in protected areas.
The circuit has two sections. An Ultrasound transmitter and a receiver. Both the circuits use 40 kHz Ultrasound transmitter (TX) and ultrasound receiver (RX) pair to transmit and receive the ultrasound.
The Transmitter section uses the ultrasound transmitter TX in conjunction with an emitter coupled oscillator comprising T1 and T2, both BC 548. The values of resistors and capacitors determine the oscillation at 40 kHz frequency which exactly matches with that of the transmitter.
The receiver section has a 40 kHz ultrasound receiver RX which is connected to the base of T1. It is a common emitter amplifier which amplifies the signals received by the RX. The amplified signal available from the collector of T1 is coupled via C2 to the signal detector stage comprising T2, D1 and C3. When RX continuously receives the ultrasound from TX, T3 remains saturated and conducts. This keeps T4 output conduction since its base is at ground potential. When the ultrasound beam breaks, T3 turns off and T4 conducts. The piezo buzzer connected to the collector of T4 sounds the alarm. Red LED indicates the standby mode showing that the alarm system is active.
Range of the circuit
Range is a few meters .Properly align TX and RX on the opposite sides of the entry ,so that alarm will be off if the RX receives ultrasound from TX.