A Signaling Circuit For Your Model Railroad (218)

Electronics can be a problem for many model railroaders, especially if it requires actually building the circuits necessary to control accessories and other devices. Int this video I'll share with you a design for a fairly simple optical detection and signal control circuit that I sue on the Piedmont Southern layout to control traffic across a river crossing on a single width bridge shown in video 217. In the description below I'll include links to the website where I found this circuit and to my personal website where you can download closeup photos of the circuitboard and my hand drawn wiring diagram. So let's get started.

Here are the websites and parts info I mentioned in the video. I will try to have the plans and photos on my website by early this week so please check again later in the week. Redrawing the plans and fully annotating them and the photos just takes a while and I haven’t much time to do it all along with making and editing the videos.

My website https://larrypuckett.wordpress.com

Note: This website is no longer active but may be archived elsewhere. Rob Paisley's website: and http://www.circuitous.ca/PhotoDetecto...

Below is a parts list and sources. But first a note about substituting parts. Although I bought a lot at All Electronics, most of the electrical components can be purchased anywhere. However the relay and phototransistor are exceptions. Actually you could try any visible light sensitive phototransistor and some infrared ones will work well too—the one I used and listed is an IR device. Remember that IR devices are usually most sensitive around 940 nm whereas visible light runs from about 400-740nm. As for the relay pick a small 12VDC pc mount DPDT relay, with a coil resistance over 500 ohms. The one I used and listed is rated about 700 ohms. The perforated phenolic circuit board material is getting hard to find, especially the kind without copper cladding. But there is nothing special about it otherwise. You might be able to use the kind with copper dots around each hole but I have never tried it. In the parts list below I placed a * next to the ones I used. I used what ever solid wire I had on hand for the connections between components but smaller is better. In some cases I even used the leftovers after cutting the leads off the resistors and capacitors. All these companies sell small diameter solid wire—I would use 22ga or even smaller. The really small stuff in the photos is 30 ga wire wrapping wire.

Jameco.com
*Perf board 4.5x6” sheet #616690
Phototransistor visible light #1108144
*Relay 12VDC DS2Y-S-DC12V

Mouser.com
Phototransistor IR #604-WP3DP3BT (Mouser says it is the same as the DigiKey one).

Digikey.com
*Phototransistor IR #754-1468-ND

Allelectronics.com Values enclosed in (1) indicate the number required per board
1N4001 Diode 1N4001 (1)
2N3906 PNP transistor 2N3906 (1)
ICS-8 Eight pin IC socket (1)
LM393 Comparator (1)
291-10K 10K ohm resistor 1/4W (2)
291-1M 1 meg ohm resistor 1/4 W (1)
291-2.2K 2.2K ohm resistor 1/4 W (1)
291-470K 470K ohm resistor 1/4 W (1)
TER-405 5 pos screw terminal (1)
TER-403 3 pos screw terminal (1)
10R50-C 10uF 50v electrolytic capacitor (1) Note: these provide the delayed switching of the relay and give about 1 sec per 1uF. For less delay order a smaller value. For a longer delay use a larger value. A 10 uF device gives about 10 secs.