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Further Notes On The Train Detector For Model Railways As soon as the June 1995 issue appeared on the streets, we received very favourable feedback to the Train Detector cir­cuit. But as often happens, one reader wanted to use it in a way we had not envisaged. In this short article, we feature his prob­lem and describe how to solve it. By LEO SIMPSON The reader’s problem is as follows and sets out an applica­tion where the block switching covered in the June 1995 article is not required. This application envisages the Train Detector being used in a one-off situation. Reader’s letter I am keen to build the Train Detector featured in the June 1995 issue and wish to use it with the Sound & Lights for Level Crossings, as featured in the April 1994 issue. My problem is that I don’t want to use the block switching system you describe for the Train Detector. I just want to use it to detect the train moving into the section which has the level crossing. I also don’t want to run wires all the way back to my train controller Fig.1: this diagram shows how to connect the Train Detector board to the Train Controller without using block switching. Note that the current for the isolated track section must flow through the two large detector diodes. ISOLATED RAIL COMMON RAIL RAIL 2 TRACK 1 +/- TRAIN CONTROLLER TRACK 2 0V AC SIGNAL RAIL 1 +12V 0V -12V OUTPUT GND 62  Silicon Chip (Infrared Remote Control, April & May 1992) because I have a large layout and I want to minimise the amount of extra wiring required. Therefore, instead of wiring back to the con­troller I just want to wire the Track Detector directly off the track where the level crossing is going to be. Now, if that doesn’t sound too confusing, how do I go about it? Finally, can I use the same power supply to run the Sound & Lights circuit as for the Train Detector? (K. A., Mona Vale, NSW). How it’s done It’s amazing, isn’t it? No matter how much thought we put into the presentation of these projects, someone always writes in to ask how to do something else that we hadn’t thought of doing. As it happens, this proposal does not present any real problems. First, you do need a section of the track to be isolated from the rest of the layout. One rail can be common to the rest of the layout and one section of rail must be isolated, as shown in the diagram of Fig.1. This shows one side of the controller wired to the common rail (rail 1) while the other side of the train controller goes to the 0V terminal on the Train Detector PC board. The rail 1 connection also connects to the track 1 termi­nal on the PC board (labelled “+/-” on the copper side of the board). The isolated rail (rail 2) goes to the track 2 terminal on the PC board, adjacent to the 0V terminal. Fig.1 shows how the train controller is connected to the isolated section of track but the reader wants to wire the Train Detector directly to the track, without wiring back to the con­troller. OK, Fig.2 shows how to do it. This shows a track with an isolated section. As before, the common rail is rail 1 while the isolated rail is rail 2. The corresponding rail connections to the COMMON RAIL SATELLITE SUPPLIES RAIL 1 TRACK 1 +/- AC SIGNAL RAIL 2 Aussat systems from under $850 ISOLATED RAIL SATELLITE RECEIVERS FROM .$280 +12V TRACK 2 0V -12V 0V OUTPUT GND LNB’s Ku FROM ..............................$229 LNB’s C FROM .................................$330 FEEDHORNS Ku BAND FROM ......$45 FEEDHORNS C.BAND FROM .........$95 DISHES 60m to 3.7m FROM ...........$130 Fig.2: this diagram shows how the Train Detector can be wired to an isolated section of track without the need for any connections to the Train Controller itself. Note that, as before, the current for the isolated track section must flow through the two detector diodes. Train Detector are the same as in Fig.1. Note that the DC and AC connections from the power supply to the Train Detector must still be as shown in Fig.3 on page 29 of the June 1995 issue. One small point to note with this proposal is that the voltage applied from the train controller to the isolated track section will be lower by 0.6V than the voltage supplied to the rest of the track. This is a consequence of the voltage drop across the current detector diodes, D1 & D2. This will lead to a small drop in speed as the loco enters the isolated track section and a slight pick-up in speed as the loco leaves the section but this is unlikely to lead to problems and may actually give an increase in realism. As far as the idea of using the Train Detector power supply for the Sound & Lights circuit is concerned, remember that the Train Detector has +12V, 0V & -12V supply lines while the Sound & Lights circuit needs only +12V and 0V lines. In that case, all you need do is to connect the +12V and 0V lines from the Train Detec­tor power supply to the Sound & Lights board. The output line from the Train Connector goes to the input of the Sound & Lights board. There is no need to connect the GND terminal on the Train Detector to the GND terminal on the Sound & Lights board since the circuit will be completed via the 0V connections. Do not make any connection from the -12V line to the Sound & Lights board. Especially For Model Railway Enthusiasts This book has 14 model railway projects for you to build, including pulse power throttle controllers, a level crossing detector with matching lights & sound effects, & diesel sound & steam sound simulators. If you are a model railway enthusiast, then this collection of projects from Silicon Chip is a must. Price: $7.95 (plus $3 p&p). Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or send a cheque, money order or credit card details to PO Box 139, Collaroy, NSW 2097. LOTS OF OTHER ITEMS FROM COAXIAL CABLE, DECODERS, ANGLE METERS, IN-LINE COAX AMPS, PAY-TV DECODER FOR JAPANESE, NTSC TO PAL TRANSCODERS, E-PAL DECODERS, PLUS MANY MORE For a free catalogue, fill in & mail or fax this coupon. ✍     Please send me a free catalog on your satellite systems. Name:____________________________ Street:____________________________ Suburb:_________________________ P/code________Phone_____________ L&M Satellite Supplies 33-35 Wickham Rd, Moorabin 3189 Ph (03) 9553 1763; Fax (03) 9532 2957 September 1995  63