I have a Fairchild QRB1114 IR emitter/detector PhotoTransistor. I want to use it to light a panel LED to show train position on "hidden" track on a DC layout. I need help in creating the circuitry to accomplish this goal. I am totally ignorant about determining values in an electronic circuit BUT I do know terminology and how to read and build from a schematic. Any direct assistance - or a finger pointing to a reference source - will be greatly appreciated.:tb-wacky:
Start with the data sheet for the device: http://www.fairchildsemi.com/ds/QR/QRB1114.pdf With a 12VDC supply, put a current limiting resistor of between 560 and 1000 ohms between +12V and Pin 1 (the anode of the emitter LED). Ground the cathode (pin 2). This will light the IR emitter diode. Connect a 1K resistor in series with your indicator LED and connect these between + 12V and the collector of the IR phototransistor (pin 4). Ground the emitter of the transistor (Pin 3). Then all you need to do (per the data sheet) is have a reflective surface within 4 mm of the device to reflect the emitter LED's IR back to the sensor. That will likely be difficult/impractical unless you can do it from below and the IR reflectivity of the undercarriage of your rolling sock is high enough. Good idea though.
If you have a choice, I would strongly suggest using the IS471 chip from Sharp. Take a look at an article I wrote on my web site about inexpensive IR detectors for train layouts using this chip: Rangachari Anand's site The Sharp chip has many advantages in its ability to reject false signals.
Yes I had planned on doing it from below. In fact I have already fabricated the bracket that would hold the unit and related resistors in position. [Based upon what follows that is probably for naught.] The undercarriage could have a bit of reflective material glued to the bolster pin as BrownBear45's linked page suggests. That appears to be within the 4mm(a shade more than 5/32"). But, that presents 2 limitations that my original thinking overlooked: 1. Every bit of rolling stock would need this tape modification, on both ends. 2. Once the bit of reflector tape moved on, the LED on the panel would go out. (Taping the entire length of the undercarriage would fix this. UGH! No, thanks) Thus, it seems like breaking a sensor beam across the tracks seems more logical, using a NO device that would close when the light source is blocked. The gaps between cars would cause the LED on the panel to go out - but that could be solved by a pair of light sources and detectors, wired in parallel to the LED, about 1" apart. Does that make sense? And, if so, can anyone tell me what I need to accomplish this?
RadioShack sells an emitter/detector pair, just arrange them at an angle across the track so that the gaps do not cause a break. Works for me. Al
Thanks, Al. The local RS has them in stock. I'm on my way to get them now. Thanks, also for the tip about mounting them on a bias. Funny, how the obvious hides from us, at times!
Rereading your initial posting, I realize there is a significant difference in the way we use sensors: I use a sensor to tell me when a train reaches a particular point whereas you wish to use the sensor to tell you whether a particular track is occupied. In my case, I need to add a reflector only to my locomotives since they will cross the detector first. It occurs to me that occupancy is better detected by track current detection - you would need to use resistor wheelsets in your cars though.
I probably should have defined my scenario better. I have a hidden reverse loop with "stop-and-reverse-it" blocks leading into the feed turnout. I want the occupancy LED at the control panel. Part of the purpose of the hidden reverse loop is the operational staging effect of delaying the reappearance of the train. While it is stopped there will be no current to be detected, so photo detection fits my needs.
