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Pulsed Infrared Sensors
d. bodnar 4-09-05 3:59 pm
Over the last two articles on garden railway sensors we dealt with devices that can give good service when properly installed and utilized.  There are many other options including audible & ultrasonic sound sensors and devices that sense visible light such as photo transistors and CdS cells.  All of these are appropriate for use in somesituations but not necessarily in an outdoor environment where weather and mother nature play havoc with electronic devices.
The Best Sensor Yet?
In this article I would like to concentrate on the one sensor that I have found to be the most reliable non-contact sensor for outdoor garden railway use, the pulsed infrared sensor.  Although this device is similar to other units that use an infraredemitter and detector or visible light sensors it differs in that its two main components are designed to produce and react to pulses at a set frequency.  Pulsing the IR emissions, usually at 38,000 cycles per second or 38 kHz, makes this setup much less prone to interference from sunlight and other sources of infrared radiation. 
To give you an idea of how common and reliable this technology is,virtually every TV remote control made for the last 20+ years has used pulsed IR to help you select channels and adjust your volume!

Sensor Placement
The emitter and receiver module can be placed on either side of a piece of track and connected to a device that you want to start when the beam of pulsed IR is interrupted.  This is a great way to trigger trackside events whenever a train goes by. Note that the photo shows only the emitter (clear object on the right) and the receiver module (black object on the left) without any wiring or supporting circuitry.  Also, for simplicity's sake, the emitter and detector are placed directly across from one another.  A better placement may be to put them at a diagonal to the track so that the beam remains broken even when gaps between cars go by. 
A second method of placing these devices has more interesting possibilities.  You may have noted that your TV remote doesn't work very well when your dog or some other object blocks a line-of-site path for the IR beam.  That is a good illustration of what happens in our first example. You may have also noticed that you can face a remote control away from your TV and bounce the IR beam off ofa wall behind you or to the side and still control the TV.  Depending on the reflectivity of the surface you are utilizing you can get almost as good a result as with a direct path.  This is the key to the second method of use for our sensor.  Mount the IR emitter next to the IR detector, shielding the detector so that there is no direct path from the emitter to the detector.  The easiest way todo this is to put the emitter inside of a piece of tubing so that the beam can only go out the front.
Both devices, the emitter and receiver are placed as show here.  This setup, as noted above, will not work because enough IR is emitted from the sides of the IR LED to keep the receiver on at all times.

The addition of a shield, in this case a piece of opaque black tubing, removes the directpath between the IR LED and the sensor.  Infrared must be reflected from a passing train to reach the receiver.


38 kHz Pulse Generator with a 555 Timer
Now that we have an idea of how this sensor works let's get about the business of putting it all together.  First we need to build an oscillator to generate the 38 kHz pulses that will drive the IR LED.  There are a number of ways to dothis.  The traditional method is to use an integrated circuit called a 555 timer.  This device can generate pulses over a wide range of frequencies by adjusting the values of a resistor and a capacitor.  If a variable resistor, or potentiometer, is used you can easily adjust the frequency to a precise value.   The schematic below shows a 555 timer set up to generate pulses in the 38 kHz range. ...
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