Outdoor Sensor

IR will work outside. This type of sensor is commonly used in parking lots/garages.

Here's one source...Banner

Dave's site has many projects such his Pulsed IR Outdoor Sensors.  His plans are on his site or he will sell them finished.  Dave is a great resource for train related electrical projects.  Many apply to O scale too.

http://www.trainelectronics.com/products.htm

Chris Sheldon

I would first consider converting the flagman. RC parts like a rechargeable battery and a servo to replace the coil would make the flagman standalone. The coil would need some serious current for a long train passing by.

Then, having DC power in it, you could power a detection sensor board, like something from Azatrax to trigger it with IR. The daylight may cause issues with inexpensive low power IR.

Current sensing boards (Azatrax, Dallee) would work, but require an isolation rail.

Interesting challenge. Check some outdoor rr boards for some ideas.

Do they make a simple mechanical track contactor for G gauge, like a 153C for O, that could be attached to the track easily?

In sunlight, IR sensors are likely to be problematic.  You'll need a pulsed IR setup that only responds to a specific signal from the emitter for daylight operations.

You should definitely consider pulsed-IR  If you are looking for an off-the-shelf plug-and-play that's one thing.  If you're willing to engage in some DIY I think you can do it for well under $10 total out-of-pocket.

I figure you need 3 things:

1) Voltage regulator to convert accessory voltage whether it be AC, DC, DCC track voltage, whatever to regulated DC (e.g., 5V).  Typical module shown on left - less than $3 (free shipping from Asia on eBay).

2) Pulsed-IR sender-receiver module. The two examples shown use the pulsed-IR method - less than $3 each (free shipping from Asia on eBay).  As shown the module is configured for "reflective" mode where a passing train reflects the pulsed IR and trips the sensor.  On each module you can see, facing the track, an IR LED which is pulsed and a pulsed-IR detector.  If you want to use the "transmissive" mode where the passing train blocks the beam, you can separate the transmitter LED off the board, run 2-wires, and position it on the other side of the track.

Note: if you look on eBay you may find IR sender-receiver modules which are NOT of the "pulsed" variety.  These are typically less expensive (less than $1) but are best left for indoor use.  BTW, you may find the term "modulated" used instead of "pulsed."  

3) Relay module.  Example shown on far right.  These run $1-2 (free shipping from Asia on eBay).  The pulsed-IR detector module would trigger the relay when a train is detected.  The relay would then switch power to the Accessory.

I like Carl's idea of looking into if/how the mechanism can be converted from a power-hungry solenoid to some kind DC gearmotor (e.g., servo).  Servos can be had for about $1 (free shipping from Asia on eBay).  There would be some DIY tinkering to pair-up the range-of-motion but would drastically reduce power consumption.

Note:

If you're familiar with "reflective" IR occupancy detectors for O-gauge (e.g., Lionel 153IR, MTH ITAD), they have a time-delay feature where intermittent drop-outs do not turn off the accessory.  So the accessory remains "on" for several seconds (adjustable) after a drop-out in reflectance such as between cars or perhaps against a dark colored car which doesn't reflect the IR beam as well.  You can duplicate this time-delay function in a DIY application by selecting a relay module with this delay capability or inserting some discrete components (e.g., capacitor) on the relay trigger.  Adds another $1-2.

Stan,

I think that the setup you described would work as the layout is in the shade.  Can anyone let me know how to wire it?  Im a dummy when it comes to electronics unless I can read a diagram.

Kelpieflyer posted:

...Can anyone let me know how to wire it?  Im a dummy when it comes to electronics unless I can read a diagram.

The roll-your-own approach can save a few bucks, but I do not recommend it to a (self-proclaimed) electronics "dummy".

I'd consider the Lionel 153IR which uses pulsed-IR.  Or, per website previously suggested by Fireone, you can buy finished pulsed-IR detectors.