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In November of 1980, Model Railroader magazine launched its "Electronics Symposium" column -- a regular feature that continued for almost 20 years. Earlier in the year the editor of the column, Don Hansen, had called for articles for this column. Here's the article I submitted, which appeared as the initial installment.
Symposium on Electronics / Train Situation Indicator
One of the contributions that interested me [editor Hansen] was Gerry Leone's applications of Eric A. Bracher's train situation indicator. I'll let Gerry tell you about it himself.
"Like most other model railroaders, what little I know about electronics came from the School of Blind Faith. I can usually tell a transistor from a transformer with a little prodding, and I know that the 'funny little colored things' are resistors. That's about it. Naturally, that hasn't stopped me from assembling my fair share of electronic projects over the years. (Ignorance is bliss + 10 percent.) Some worked; some I call 'modern art.'
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"One of the simplest, cheapest, and most useful electronic circuits I’ve ever come across is the ‘Train Situation Indicator’ by Eric A. Bracher, first published in the December 1973 issue of MR. See fig. 1. The train situation indicator works just fine for what it was designed to do -- show the passage of a train inside a tunnel, but with slight adaptation or modification, its uses are almost unlimited. I've found this little electronic marvel many a home on my Bona Vista RR.
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"One nice thing about the circuit is that it operates on 12 volts d.c. Also, the circuit can be built for about $3, excluding the beam and indicator lamps. Of course, because of component packaging, the more you build, the less expensive each is. And if you want to go all out, buy Radio Shack's package containing 15 2N3904 transistors (RS276-1603) to lower the price still more.
"Let's look at some of the ways I've used this circuit on my layout. On the Bona Vista RR. the village of Woodbury is the last stop at the south end of the run. In reality, it's merely a passing siding within a return loop. Since both are buried beneath scenery, I never knew if both ends of a long train were clear of the points of the turnouts. By adding a train situation indicator at each fouling point (fig. 2) and an indicator lamp for each on the control panel, I have yet to sideswipe a train.
"By modifying a train situation indicator, I solved a problem at my town of Thief River Falls. Here, one end of a passing siding is hidden in a tunnel. Since the siding is used quite frequently to switch Thief River Falls industries, I needed to know not only when a passing train had cleared the turnout's fouling point, but also when my way freight had cleared the points and could be shifted onto the main line. I installed a train situation indicator at the turnout, with the beam lamp and photocell at the fouling point. By adding a second photocell wired in series with the first, and a second beam lamp at the turnout point, the indicator lamp on the control panel would glow if either of the points were still occupied, or if a car was sitting beyond the fouling point. See fig. 3
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"When time came to add a crossing signal and bell outside of my main town, I searched through numerous articles, priced components, and decided on Peter J. Thorne's circuit published in the July 1977 MR. That circuit features four modules; a flasher, a bell clapper, a bell sound, and detection/timer. However, though I liked the flasher/bell aspects, I didn't care for using reed switches and magnets to activate the circuit. Train situation indicator to the rescue again.
"I built Thorne's crossing flasher, bell clapper, and bell sound modules just as he described. But I substituted the train situation indicator for his track detector and timer module. Here, instead of an indicator lamp, I added a dp.dt. relay (RS 275-206) to the circuit. I found that the relay would chatter without filtered power, so I added a 1000-mfd. capacitor and a 10,000-ohm resistor in parallel with the 12 volt d.c. input (see fig. 4). Existing room light and three photocells were used to trigger the flasher/ bell. The cells were wired in series and placed between the track ties -- one at the crossing and one on either side of the crossing, 2' 1/2 feet down the track.
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"Although this system doesn't have the built-in 'timer' effect of Thorne's original circuit (once activated, the flashers and bell remain on for a certain period of time), all but the shortest of trains set the lights flashing and the electronic bell ringing without interruption. And it wasn't necessary to go to the bother of adding magnets to any motive power or rolling stock.
"When time came to add an automatic indexing system to my turntable -- you guessed it, I used Bracher's circuit again. Each approach track has its own beam lamp mounted beneath the table top and activated with a multiposition rotary switch, on the panel. Two photocells -- this time wired in parallel -- are mounted on a disk attached to the turntable drive shaft (see fig. 5). This system is similar in principle to the one described by Wendell J. Stephens in the November 1966 MR.
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"The same filtered d.c./relay setup I'd used in the crossing flasher circuit was again wired into the train situation indicator, but this time a four pole, double throw relay (RS 275-214) was used. Attached to the contacts of the relay are the turntable rail power, the turntable motor power (an open) circuit when the relay is in 'normal' position, and a Kemtron switch machine which acts as a positive-locking brake. The contacts of the switch machine are wired for automatic coil cut off, as described on page 47 of Linn Westcott’s HOW TO WIRE YOUR MODEL RAILROAD.
"When light from an approach-track beam lamp is striking one of the photocells, the relay is in the 'normal' position (just as the panel indicator lamp is 'off' when the beam is unobstructed in the original train situation indicator circuit). In this position, the turntable drive is off, the switch machine is set to the 'brake' position, and power is applied to the turntable rails. When another approach track is selected, no light strikes the photocell, the relay is activated, the turntable drive starts, the switch machine brake disengages, and power to the turntable rails is cut off.
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"Other uses for the circuit? To use an old saw, they're as unlimited as your imagination. The management of the Bona Vista RR. already has plans to add one at a spur/ mainline double crossing. If a car is being switched on the spur, mainline power will be cut off to protect against a broadside collision. See fig. 6.
"The train situation indicator: A model railroader couldn't ask for so much, so easily, and for so little. And it proves to me that there is a place in electronics for us graduates of the School of Blind Faith.
"I have used Bracher's circuit in the many ways I have described, but I have a problem to be solved. Maybe some other reader can supply an answer. Is there a way to use an infrared LED and photocell in Bracher's circuit? With the one set available at Radio Shack that I tried, the photocell needed to be about one-fourth of an inch from the LED before I could get any reaction from it." - Gerry Leone