How sensitive are optical sensors?

[Omaha53]

I am considering using optical sensors to activate crossing signals on my HO layout. I want to embed them in the roadbed between the ties. I like to run my trains in a "night setting" so there are no ceiling lights on, just building and street lights. How much difference do optical sensors need between light and dark. Will a nearby street light on the layout provide enough light to work? Can most sensors be adjusted for sensistivity?

From what I understand IR sensors are usually placed beside the track and I do not have any place to hide them.

[Seamonster]

From my experience experimenting with optical sensors (photocells) of the general purpose variety, they require a fairly significant change in lighting to operate.  Basically, all a photocell is is a variable resistor whose resistance changes depending on how much light is falling on it.  They come in a variety of light and dark resistance values.  An electronic circuit senses the change in resistance and performs some action, like energizing a relay, when the resistance changes (i.e. a train passes over the photocell).  The electronics normally have adjustments ro make it more or less sensitive to the light changes.  Check out the commercial units available and see if any of them describe their sensitivity.  Circuitron sells some optical sensors, but their listing in the Walthers catalogue doesn't say anything about their sensitivity and their web site is under construction so I couldn't get any information there.  Hopefully someone who has used their detectors can advise you more.

 

[LogicRailTech]

Optical sensors (assuming you're referring to photocells here) are quite sensitive to the amount of ambient light. The photocell itself has no adjustment capability since it is literally just a light-sensitive resistor. The adjustment for ambient light level is then a function of the electronic circuit connected to the photocell. Some circuits (commercial or home-brewed) have such adjustments and others do not!

For example, all of our products that use photocells do have such an adjustment. In the case of products that use multiple photocells (e.g. Grade Crossing Pro) there is an adjustment capability for each photocell.

Having said all of this, a photocell will most certainly require SOME amount of ambient light. A well-placed street light MIGHT be sufficient but it's difficult to guarantee this without trial and error.

Finally, not all IR-based solutions require a "beam across the track" type of arrangement. For example, the IRDOT-1 product (http://www.heathcote-electronics.co.uk/DOT1.htm) uses a reflected IR arrangement with the IR emitter/detector pair installed between the rails. For what it's worth, we have application notes that illustrate how to use the IRDOT-1 in place of photocells with our products.

Sincerely,

Chuck Stancil

Logic Rail Technologies

http://www.logicrailtech.com

[larak]

For example, the IRDOT-1 product (http://www.heathcote-electronics.co.uk/DOT1.htm) uses a reflected IR arrangement with the IR emitter/detector pair installed between the rails.

Yes, this is probably a good solution. Install an IR LED AND phototransistor pair between the rails (not just a "detector"). Isolate the direct line between them. You can do this with heat shrink tubing. - wrap each and leave an opening on the top side only. Sensitivity depends on the output power of the LED, the sensitivity of the transistor (check spec sheet for the pair) and adjustment of the associated circuitry.

It also depends on the reflectivity of the underside of the train. You can increase it with aluminum foil or paint.

Disclaimer - This is theoretical. I have worked with optical controls but not this exact application (yet). It's something I will get to eventually. Right now it's across the track sensors on my layout.  Good luck with yours.

Karl