Ultrasonic switch
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.
