ultrasonic switch circuit diagram


Ultrasonic switch circuit
Click here for the circuit diagram

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
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
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
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

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