HUGO
–
Design and Development
Ultra sound detectors
The SRF04 Timing diagram is shown above.
You only need to supply a short 10uS pulse to the trigger
input to start the ranging. The SRF04 will send out
an 8 cycle burst of ultrasound at 40khz and raise its
echo line high. It then listens for an echo, and as
soon as it detects one it lowers the echo line again.
The echo line is therefore a pulse whose width is proportional
to the distance to the object. By timing the pulse it
is possible to calculate the range in inches/centimeters
or anything else. If nothing is detected then the SRF04
will lower its echo line anyway after about 36mS.
In operation, the processor waits for
an active low trigger pulse to come in. It then generates
just eight cycles of 40khz. The echo line is then raised
to signal the host processor to start timing. The raising
of the echo line also shuts of the MAX232. After a while
– no more than 10-12mS normally, the returning
echo will be detected and the PIC will lower the echo
line. The width of this pulse represents the flight
time of the sonic burst. If no echo is detected then
it will automatically time out after about 30mS.
Performance of this design is, I think,
quite good. It will reliably measure down to 3cm and
will continue detecting down to 1cm. Maximum range is
a little over 3m. As and example, it will detect a 1inch
thick plastic broom handle at 2.4m. Average current
consumption is reasonable at less than 50mA and typically
about 30mA.
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