OptoInterrupter

Exact location detection ...

This appeared in Modelspoormagazine 71 (june 2008). More text and pictures can be found there.

This page is a sequel to the uncoupler story. As an uncoupling point isoften out of sight, it is useful to have an indication if the car is at the correct position to uncouple it. A home-built photocell seemed a good solution.

To have our location detection precise and universal, we need a universal reference point. One spot that's always present on a car, is the space between the coupling and the buffer stamps. Light can pass here, and that's measurable.
To measure this light beam exactly, we need a light source and a sensor, nicely alligned.

The sensor shouldn't "see" any other light source, so we need to take measure for this. A tube around thesensor protects it from any incoming light from other angles.
Quite another problem: the sending light source should be camouflaged in the layout. An overhanging lantern post, aligned to send it's light towards thesensor, is a good solution.
These measures still aren't sufficient, however. Other light sources, e.g. on the ceiling, should be filtered, and that's done with some electronic synchronisation filtering.

The sender is build first. It is, in fact, a modulated power generator that will provide the leds in the lantern posts with short spikes of light. This schematic is therefore useable to control light in general. The small circuit is build with one IC and a few extra components.
The generator at the left hand side is made and amplified with schmitt-triggers. The exit "A" gives the synchronisation signal for the receiver. A power transistor gives the nescessary "juice" for lots of leds.

The schematic results in this PCB-layout design.An experienced electronics worker has enough information with these, but I'll expain it a bit more...

The circuit board is built, starting with the lowest components, the diodes and resistors. The polarity of the diodes should be looked after. Next is the "IC"é, the chip, which should be placed correctly, too. Some small and one larger condensor now, concluded wih the power transistor. A heat sink is nesccessary when connecting more dan 5 leds. A connector makes it easier to get the signals to the rest of our components.

The reciever's electronics is a bit more complicated. The light is picked up by a photosensitive transistor. Its signal is amplified and polished by the first chip. At its exit, we now have a signal that's synchrone with the rapid flickering of our lantern's light. This rithm is compared with our direct sender signal to filter any light pollution. Finally, we have two signals at the exit, capable of controling e.g. a led on a control panel. The PCB-design is at left, as you can see it contains more components, but there are in fact two receivers on one circuit board.

We start again with an etched and drilled circuit board and by mounting the lowest components. Next both IC's can be soldered to place, finishing with the condensors. Watch the polarity of the diodes, IC's and condensors.

As told before, our sensors should be shielded for stray light. A 3mm brass tube will do the job. As the sensor is 5mm wide, this tube is thickened with some shrinking tube. The sensor can now be alligned and fixed with another piece of heat shrinking tube. Don't heat it too long i nteh last step, as we don't want to "cook" our sensor.

As the light beam should pass between the rail and the service path, there's little room for a good position. We need to mill off a bit of the rail's inside, and drill a 3mm wide hole next. This hole should be drilled in a slightly inward angle - about 5 degrees - to align with the lantern's led.
If the hole is drilled right, the sensor can be clamped inside it. It's more likely that you need to correct the angle, and then we need some glue to fix the sensor.

The protecting tube shouldn't touch the wheels, so its top should be level with the rail ties. After placing the uncoupler's "pathway", it is hardly visible.
Connecting the sensor is fairly straightformard. We don't need the middle pin, so this is clipped. The small tab on the housing indicates the emittor, the signal output.

Connect the sensor to the circuit board. The arrows indicate the connection points. When the light sensor is not placed near the circuit board, it is recommended to use a shielded cabe type.

See: uncouplers