Here is a low cost, wireless switch controller. It uses ultrasonic sound waves for remote control of a switch. As with any other remote control, the system basically comprises a transmitter and a receiver circuit. Frequencies up to 20kHz are audible. Frequencies above 20kHz are not audible. The transmitter circuit generates an ultrasonic frequency between 40-50kHz. The receiver senses the ultrasonic sound and switches on a relay. The transmitter uses a 555 astable multivibrator. It oscillates at a frequency of 40-50kHz. An
ultrasonic transducer is used to transmit the frequency. The transmitter runs on a 9v battery. The ultrasonic receiver uses a receiver transducer to sense ultrasonic signals. It uses a twostage amplifier, a rectifier stage and an operational amplifier in inverting mode. Output of the operational amplifier is connected to a relay through a driver stage. A 9v adapter can be used to power the receiver circuit. When switch S1 is pressed, it generates ultrasonic sound. The receiver amplifies the received signal via transistors Q3 and Q4. The amplified signal are then rectified and filtered. The filtered DC voltage is given to the inverting pin of operational amplifier 1C2. The non-inverting pin of 1C2 is connected to a DC voltage through VR2 that determines the threshold value of the signal received, for operation of relay RL1. The inverted output of 1C2
is used to bias transistor Q5. When transistor Q5 conducts, it supplies base bias to transistor Q6. When transistor Q6 conducts, it energises the relay RL1 . The relay can be used to control any electrical or electronic appliance. Frequency of the circuit can be varied by adjusting VR1. Adjust it for maximum performance. Ultrasonic sounds are highly directional. So when you are using the transmitter, the receiver should face towards the transmitter. The receiver is always kept on.
The transmitter circuit can be simplified to the following design as the driver transistors are not
needed. They do nothing.


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