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An inexpensive detector that works
There are many types of train detectors on market today
All with
varying degrees of dependability
Almost all
expensive if you need a lot
Here is a circuit
Reliable up
to one foot away in all lighting conditions
Relatively
low cost
Flexible
How it works
The circuit is
essentially an LED flashed about 10000 times a second
Next to this
is a light detector connected to a filter
In
simple terms, a few simple components that produce a constant signal
In
the presence of a intermittent signal
The weak
signal is amplified
By far, the
most reliable seems to be the IRDOT-1 circuit
Can be used
to detect trains from as far away as three feet and more
Only problem is expense
Simple detection
Ambient or
visible lighting
Simple
light-sensitive detector (cadmium cell)
Cheap,
simple effective
May not
work under certain lights or “night” lighting
Can be
triggered by shadows etc.
Infrared line-of-sight
Fairly
dependable
Simple,
inexpensive
May be hard
to conceal
Infrared – reflective
Most
dependable when set up right
But can be a
bit tricky
Tone decoder
Lm324
circuits
Considerations
Make sure
emitter/detector wavelengths match
Do not
have to be exact but should be close
The
two most popular emitter frequencies are 880nm and 940nm
Many
detectors will handle both theses ranges but check to make sure
Do not
underpower emitter
Devices
not liner
Little
output to minimum threshold value
Much
better output between threshold and max
To much
current will damage LED
Beams must
overlap
While
detector circuit may work at two to three inches
Quite
possibly not work at ˝ inch
Included
angle of emitter must illuminate
Area in viw of detector
Parallel beams
10 degree slant
20 degree slant
Greater angles may be difficult to drill with accuracy
I made a jig
to standardize positioning of these holes
For
standardized instalation
Circuitry to detect trains not that difficult to
assemble
The trickiest
part is the mechanics of assembly
To match
the sensitivity and dependability required
Working with infra-red can be tricky
A reflective
surface may detect a car at six
inches
A rough,
black surface may be invisible at a quarter inch!
Not to worry
Once you know
what to look for
Building a
reliable detector is a fairly simple matter
First step is to boost up the power to the emitter
To shine a
brighter light against the target
Second, most LEDs have a fairly narrow beam
Typically ten
to twenty degrees
Angling the
emitter and detector to about 20 degrees from centerline
To
converge about a half inch over railhead can do wonders
On my detectors I used a brass sleeve
to separate the beams.
I found
that blackening the inside of the sleeve
And
positioning the tip of the LEDS to just outside the sleeve
Yielded
maximum sensitivity but this can vary depending on
LEDs
and configuration used
I built a small test fixture to test the positioning of
the light guns
And a Shiny
side/rough black side target
To test
both extremes of reflective surfaces
The best circuit I’ve seen to date is the IRDOT circuit
that works
Surprisingly
well with no adjustments. The only problem
Is that each
circuit costs $40 so if you need 50 detectors
Well,
you do the math
The other extreme is the circuit built around a LM567
tone decoder
At around
three or four dollars per circuit
The circuit I built costs about six or seven dollars per
channel
Using (1)
LM734, (1) LM339 and (1) 7404 chip for each pair of channels
(The
7404 chip can be eliminated if you use a more expensive LED)
And
the LTV4 opto-isolators are a bit of overkill
But
absolutely guarantees safe, fool-proof interfacing with computer
All circuits work more or less the same way.
A pulse
circuit drives the emitter somewhere between 1khz to 10khz
The detector
circuit contains filters to reject higher or lower frequencies
The result is
passed through a diode to charge a capacitor
The last stage
amplifies the charge on the capacitor to drive some output device
(LED, relay, computer input)
The more sensitive you make the control circuits
The more
leeway you have in the adjustment of the LEDs
The better you allaign the LEDs, the less critical your
circuit
Adjustments
need to be
It s very easy to get some flickering output of most
circuits
But to get a
rock-solid signal it may take a bit of engineering
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