When you look at these schematics, I hope you will keep in mind that
this ain't popular electronics. These drawings are made after
the machine is working, not before. Good engineering practice is
recognized in the shop, but cut-and-fit tinkering is the process.
Art and design are not the same thing. Design is problem solving.
If you see a way to make it more simple, please let me know. It is
essentially a bistable switch which is set by an external pulse,
causing the send transducer to beep, and starting a counter that runs
until the beep echo returns and resets the switch - OR until the counter
goes around to zero, whichever comes first.
If this looks like a mystery, find out
about the 4093 CMOS quad Schmitt nand gate, possibly my personal
favorite.
Another possibility is to buy the cheapest ultrasonic 'tape measure'
you can find and pull the signals out of it. I can never get those
things to look right in my machines.
Passive Infrared Motion Detector
This circuit was originally reverse -engineered from a motion detecting
yard light that I ripped apart. That's still probably the best way to
get the parts at a reasonable price, especially the pyroelectric
sensor and the absolutely necessary Fresnel lens. The signal at pin 7
of the 324 is very interesting and fooling with the filtering around
the first amplification stage can make it even more so. The LM324 is a
wonderful little bug, and you will find many uses for the window
comparator if you look at it the same way you would learn a new really
useful knot. It all works on a single 5 volt supply. The sensor is
only sensitive to changes across its surface, so don't expect a signal
from a static object even if it is hot. Yard lights are turning up at
flea markets and yard sales as people find themselves heads up every
time the cat walks past. This circuit is in a machine that sees
people moving 40 feet away.
40KHz Ultrasonic Rangefinder
This is an echo detector which will reach up to about 25 feet. This
particular circuit resolves about 6-7ft. into six bits, so the
resolution is a little more than an inch. The digital output is
useful for interfacing with a processor, but if you get the send and
receive parts working, you can treat the signals any way you want.
The transducers are cheap and available in surplus these days. All
the ceramic ones I have tried seem to work, but watch out for those
requiring a high-voltage drive.
Digi-Key has a set that will do (Panasonic type U). Start by
building the send circuit (transistor 2n4401). The parallel resistors
are for trimming the frequency to 40KHz. You don't need to measure,
just connect pin1 to the positive supply, hook the receive xducer to a
scope and with the xducers close coupled, peak the signal by adjusting
the resistance.
The receiver is a little more touchy because the second stage is a
filter that's tuned by the two caps and feedback resistor. Work on it
live with the sender blasting away, and peak the signal at the output
of the second stage. This is not real selective filtering, but it
keeps most audible sounds from triggering the receiver. You don't
need some fancy scope to do this.
The start pulse should be short as possible at the gate input, and
almost any negative edge can be accomodated by adjusting the cap and
resistor.