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optical_block detection

As the name implies, the SOOD is an optical, rather than an electrical detector. That is, it senses the presence or absence of a loco or car at one specific point on the track; an electrical detector senses presence/absence of a resistive load (eg, loco, illuminated passenger car, resistive wheelsets) in an electrically-isolated block of variable size.

The SOOD works with both DC and DCC systems in all scales, but does require that you have some illumination on the layout (if you like to operate in the darkness, this one’s not for you). It senses “light” (no locos or cars on top of it) or “no light” (blocked by something on the rails). It has short built-in delays on both “Engage” and “Release” to prevent false triggering from passing operators and to allow for the inter-car gaps of a moving train; these delays may be adjusted if needed.

Speaking of inter-car gaps…IF you should happen to park the train with such a gap located just over the sensor, it’s going to interpret this as “no train.” If you think this is probable in your application, there is a solution: use two photocells connected in parallel(same type, please) located 1-2″ apart and change R1 to 10 Kohms (RS 271-1125), or use a 10 Kohm pot (271-1715) in series with a 4.7 Kohm fixed resistor (271-1123).

It’s great for staging yards (hidden and otherwise) where you’d like to know length and position as well as simple presence; since the cost is quite low per unit, you can put them every 3″ if you need such resolution.

The Photocell sensor is mounted under the track, between the rails and facing upward. In N- or Z-scale, you’ll want to remove the center portion of the tie where the sensor is to be located (optional in HO and larger — your call). Drill a hole the same diameter as the photocell (3/16″ for the preferred photocell from Jameco) thru the roadbed, subroadbed, sub-subroadbed and whatever else is there, and locate the sensor about 1/4″ below the surface of the roadbed. If you fix it in place with adhesive, be careful not to get any of the stuff on the face of the sensor….and remember not to ballast over the top.


Typically, you’ll be using the LED to indicate occupancy; if you like yellow or green LEDs, adjust R4 for the proper current. If you don’t want the LED, but DO want to connect the SOOD to other circuitry, just eliminate R4 and the LED. You’ll get a logic “1” when the sensor is blocked and a logic “0” when it’s clear.

Construction is simple; I suggest building SOODs in groups of three, so as to use all the inverters in U1 (which I suggest you socket to prevent soldering damage to its delicate innards). I made the connections to the photocell sensor using 2 runs of #24 stranded wire twisted together; be sure to insulate any bare wires to prevent shorting. Be particularly careful when soldering to the photocell — they won’t tolerate too much heat! Use either a clip-on heatsink, or solder to the ends of the photocell leads and insulate them with heat-shrink tubing (or something). The photocell can be located up to 18″ from the circuit board (maybe further, if your layout is electrically “quiet”).


The following circuit diagram depicts a 12V version:


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