automatic switch on on rear gear
audible bleep on last led
“good old” design style, no microcontrollers!
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.
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).
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
During setup, place the ultrasonic transducers over a
soft surface, near the border of a table, 10 cm apart of
each other pointing outw1ards 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.