How sensitive are optical sensor?

[Omaha]

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.

[Jim Banner]

Cadmium sulphide photo cells are often used with incandescent lights for a simple system.  But if they are adjusted to work with normal room lighting, they will probably see a darkened room as a train obscuring the light and they will trip.

IR photo diodes or photo transistors are normal used with IR LEDs.  These LEDs can be modulated with an ac signal superimposed on the dc driving current.  The detector can be designed so that it detects the presence or absence of the modulation rather than the absolute light level.  That is how the IR transmitter in your TV remote control and the receiver inside your TV set work.  And like your TV remote, they can work from high light levels down to complete darkness.  As far as distance is concerned, how far away can you be and still change channels on your TV?  This leads to the idea of a modulated IR light source mounted on the ceiling and detector(s) mounted under the track, looking upwards.

The last time I was faced with this problem I got lazy and used a CdS photo cell under the track and a lighted tell-tale above it.  It helped that this was about a foot from a tunnel entrance.  Other possibilities are a street light or a lighted billboard or even a lighted building.  They do not have to be directly over the track like my lighted tell-tale.  They could instead be off to the side of the track and activate a CdS cell set at an angle under the track.