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C
ircuit of a new type of remote control switch is described here.
This circuit functions with inaudible (ultrasonic) sound. Sound of
frequency up to 20 kHz is audible to human beings. The sound of
frequency above 20 kHz is called ultrasonic sound. The circuit
described generates (transmits) ultrasonic sound of frequency
between 40 and 50 kHz. As with any other remote control system
this cirucit too comprises a mini transmitter and a receiver
circuit. Transmitter generates ultrasonic sound and the receiver
senses ultrasonic sound from the transmitter and switches on a
relay. The ultrasonic transmitter uses a 555 based astable
multivibrator. It oscillates at a frequency of 40-50 kHz. An
ultrasonic transmitter transducer is used here to transmit
ultrasonic sound very effectively. The transmitter is powered from
a 9-volt PP3 single cell. The ultrasonic receiver circuit uses an
ultrasonic receiver transducer to sense ultrasonic signals. It
also uses a two-stage amplifier, a rectifier stage, and an
operational amplifier in inverting mode. Output of op-amp is
connected to a relay through a complimentary relay driver stage. A
9-volt battery eliminator can be used for receiver circuit, if
required. When switch S1 of transmitter is pressed, it generates
ultrasonic sound. The sound is received by ultrasonic receiver
transducer. It converts it to electrical variations of the same
frequency. These signals are amplified by transistors T3 and T4.
The amplified signals are then rectified and filtered. The
filtered DC voltage is given to inverting pin of op-amp IC2. The
non- inverting pin of IC2 is connected to a variable DC voltage
via preset VR2 which determines the threshold value of ultrasonic
signal received by receiver for operation of relay RL1. The
inverted output of IC2 is used to bias transistor T5. When
transistor T5 conducts, it supplies base bias to transistor T6.
When transistor T6 conducts, it actuates the relay. The relay can
be used to control any electrical or electronic equipment.
Important hints:
1. Frequency of ultrasonic sound generated can be varied from 40
to 50 kHz range by adjusting VR1. Adjust it for maximum
performance.
2. Ultrasonic sounds are highly directional. So when you are
operating the switch the ultrasonic transmitter transducer of
transmitter should be placed towards ultrasonic receiver
transducer of receiver circuit for proper functioning.
3. Use a 9-volt PP3 battery for transmitter. The receiver can be
powered from a battery eliminator and is always kept in switched
on position.
4. For latch facility use a DPDT relay if you want to switch on
and switch off the load. A flip-flop can be inserted between IC2
and relay. If you want only an ‘ON-time delay’ use a 555 only at
output of IC2. The relay will be energised for the required period
determined by the timing components of 555 monostable
multivibrator.
5. Ultrasonic waves are emitted by many natural sources.
Therefore, sometimes, the circuit might get falsely triggered,
espically when a flip-flop is used with the circuit, and there is
no remedy for that
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