This was one of my first designs: it is an ultrasonic
parking sonar.
Based on an article seen on a 1982 magazine, it was once installed on
the rear bumper of my Volvo Station Wagon, and it served very well for many
years. Connecting it to the reverse gear lights, it switches on automatically
and shows you the distance to the nearest obstacle (according to his beam) on a
led scale. When the last led lights, a buzzer is also activated telling you to
stop immediately.
It works on the sonar principle, sending an ultrasound burst and
listening for first echo. The burst generated by the oscillator built around
U4D (you must set the frequency using TR2 to have 40 kHz or the maximum
sensitivity), U4E buffers the output and U4F boost the signal doubling the
voltage span across the TX piezo transducer .
A new burst is generated each time the decade counter (4017 in the
circuit diagram) is in its reset state, that is output 0 is selected. The other
outputs (1 to 9) are scanned sequentially following burst generation, until an
echo strikes back the RX receiver. It is then amplified by the transistor input
stage, triggering the monostable built around U4A - U4B. The monostable stops
temporarly the scanning, and a led corresponding to the obstacle distance
appears as continuously lit. The buzzer bleeps when the first led (minimum
distance) il lit.
When the monostable expires, scanning is resumed and restarting the
send-and-listen sequence. If no echo is received, the scanning never stops and
all the leds are slightly lit.
To
set up:
Set TR2 for maximum sensitivity (usually 40 kHz for most commercially
available ultrasonic transducer pairs).
Set TR1 for your preferred range. Setting it to minimum resistance
shortens the distance for each led (minimum range). I suggest a range of 90 cm
(10 cm each led).
Hints:
First of all, be careful not to exchange the
ultrasonic transmitter with the receiver: they look very similar, and I suggest
you to mark them very clearly from the moment you buy them.
During setup, place the ultrasonic transducers over a soft surface, near
the border of a table, 10 cm apart of each other pointing outwards the table. Then
adjust TR1 and TR2 using a rigid surface (for example a metal sheet) placed in
front of sensors, at a variable distance.
I enjoyed the sonar for many years. It was installed below the car's
rear bumper in a plastic case. Do not choose an enclosure too small: always
separate the transducers by 7-10 cm and plenty of sound-absorbing material,
otherwise the receiver will reveal the direct sound instead of the reflected
one. The same applies if the sound travels through a rigid fixture, so it is a
good idea to fix them with separated supports.
And remember not to mount the tranducers exposed to direct sunlight nor
rain.