Infrared Train Detection System

In developing the software for automatic control of our layout, we needed a means to detect when a train passes a point on the layout.  We use two philosophies to feed the S88 modules which provide input to the computer: contact tracks and a home developed IR detector system.  The contact tracks are straightforward, so I include here a brief description of our IR system.

Sensors are positioned at the locations we want to monitor.  They consist of a IR LED and a Phototransistor sensitive to the IR spectrum.  Here are two pictures of sensor pairs:

This is a picture of one of the interface boxes.  Each box can handle eight sensor pairs and connects directly to the eight inputs of the S88.
One side (the sensor side) of the device has eight sets of three wires,orange for LED power, brown for ground, and blue for Phototransistor emitter.  All electronics except the LEDs and the transistors are contained in the interface box.

Here is the schematic for the power supply portion and one of eight detection cells per device.

This is a scan of the copper (trace) side and the component side of the interface board:

List of materials is as follows:

2 4001 CMOS
1 7805 VR  5 volt  voltage regulator
8 100k ohm 1/4w Resistors
8 560 ohm 1/2 w resistors
2 14 pin IC sockets
1 1N4001 diode
8 Fairchild QSC112 IR PhotoTransistors
8 Fairchild QED423 IR LED's
1 220uF 35v electrolytic
1 Flanged enclosure  (Hammond 1591GFLBK)

3x4 one side copper clad board

Update 1/28/07:

I have made a few changes to the design.

I have added a .047uF capacitor to the signal input to create a 30hz low pass filter to get rid of an annoying flutter due to flourescent lighting, and I have added screw blocks to make the hookup neater and more reliable.  Here is an updated picture of the interface box:

Updated BOM is:

4001 CMOS    2
7805 VR    1
100k 1/4w Resistors    8
560 ohm resistors 1/2 w res    8
14 pin IC sockets    2
1N4001 diode    1
IR PhotoTransistors    8
IR LED's    8
220uF 35v electrolytic    1
Flanged enclosure    1
3C 22AWG cable/100 ft    as needed
4x6 copper clad boards    0.5
4700pF Metal Film Caps    8
PCB Terminal Blocks (2 pos)    1
PCB Terminal Blocks (8 pos)    4
PCB (Radio Shack 276-168)   1

Final "production" model with local indication of train position:

The schematic for the above: