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automatic switch on on rear gear
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led-bargraph display
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audible bleep on last led
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"good old" design style, no
microcontrollers!
Click Here to View Circuit
Based on an ultrasonic amplifier from an
article seen on a 1982 magazine, it was once installed
on the rear bumper of my Volvo Station Wagon. 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 temporarily 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.
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).
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.
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