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Points Controller FOR MODEL RAILWAYS This Points Controller board uses a capacitor discharge circuit to energise the coils on a twin-solenoid switch machine. One Points Controller board can be used to operate all the points on a model railway layout. Most model railway enthusiasts operate their points with a twin solenoid connected to a 15V supply. However, if you keep your finger on the button for just a moment too long, you can easily burn out the solenoid coil. This points controller avoids that problem. Design by RICK WALTERS As any keen model railway enthusiast can confirm, even the simplest of model layouts include a few sets of points and most feature quite a few, for sidings, shunting yards and spur lines. While you can operate points by hand (the “big hand in the sky”) or by Bowden cables, that rapidly becomes unwieldy and unrealis­tic for all but the smallest layouts. Hence most enthusiasts operate their sets of points by twin solenoid assemblies which are usually referred to as switch machines. The most commonly available type is made by Peco and can be used for O, HO and N scale layouts. They are available from model railway retailers for about $7. As can be seen from one of the photos accompanying this article, these twin solenoid assemblies consist of two coils which drive a common solenoid shaft and a rightangle pin which protrudes from both sides of the assembly. The switch machine is mounted under the baseboard of the model railway layout and the solenoid operated pin fits into a hole in the sleeper of the move­able rail section of the points. To operate the points in one direction, one of the sole­noids is briefly energised, after which the points lock into their new position. To move the points back again, the other solenoid is briefly energised. In normal practice, the solenoid coils are energised from a 16V AC or DC power supply, with each coil connected via a push­button switch. The idea is that you briefly push the switch to operate the points for the new train direction. The operative word here is “briefly”. If you lean on the switch for more than a few seconds, the energised coil will burn out. The reason for burn out is pretty easy to understand. Each solenoid coil is wound with lots of turns of very fine enamelled copper wire and the total coil resistance is typically around 4.5Ω. With 15V across the coil, the internal dissipation will be V2/R = (16)2/4.5 = 50W. No wonder they can expire in a brief puff of smoke! The solution to this problem is to energise the solenoid coils with July 1997  29 14 Model Railway Projects Shop soiled but HALF PRICE! This book will not be reprinted Our stocks of this book are now limited. All we have left are newsagents’ returns which means that they may be slightly shop soiled or have minor cover blemishes. Otherwise, they're undamaged and in good condition. SPECIAL CLEARANCE PRICE: $3.95 + $3 P&P (Aust. & NZ) Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097.  Use this handy form Enclosed is my cheque/money order for $________ or please debit my    Bankcard   Visa   Mastercard Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ 30  Silicon Chip Fig.1: the 2200µF capacitors are charged via D1, Q1 and the 47Ω resistor. The capacitors’ charge can then be dumped into the solenoid coils via pushbutton switches S1 or S2. a capacitor discharge circuit. This charges up a capacitor to around 15V or so and then the capacitor’s charge is dumped via the respective pushbutton into the solenoid coil to be energised. This operates the points, discharges the capacitor and even if the push­ button remains depressed, no harm can be done to the solenoid coil since the capacitor cannot supply any more current. Fig.1 shows the circuit. The power supply can be any 12V to 15V DC or AC source, with a 12V plugpack being a safe and con­venient approach. This is fed via diode D1 and transistor Q1 to one or two 2200µF capacitors. From there, diode D2 couples the capacitors’ voltage to pushbutton switches S1 and S2. These switches then discharge the 2 x 2200µF capacitors via one or other of the twin solenoids in the switch machine. When power is first applied, the 470Ω resistor between collector and base of Q1 ensures that it is fully turned on and so it charges the 2200µF capacitors. The charge current is limit­ ed to a safe level (250mA maximum) for Q1 by the series 47Ω resistor at its emitter. The capacitors only take a few seconds to fully charge, by which time LED1 will be fully alight. The 1.2kΩ and 470Ω resistors form a voltage divider which prevents LED1 turning on until the voltage across the capacitor reaches 10V. This means that LED1 acts as a “ready” indicator. When either S1 or S2 is pressed, not only does it discharge the 2200µF capacitors, it also pulls the base of Q1 below its emitter, so it is completely turned off. Thus, once the capacitor is fully discharged, the only current which flows into the coil is from the 470Ω base pull-up resistor. As this current is around 20-30mA, depending on the supply voltage, there is no chance of damaging the solenoid coil. Q1 stays turned off, until the push­button is released, whereby the 2200µF capacitors begin to charge again. You might wonder about the functions of the three diodes in the circuit. Are they really necessary? Well, yes. Otherwise we would not have included them. Diode D1, provides reverse polar- PARTS LIST 1 PC board, code 09107971, 51 x 38mm 2 momentary contact pushbutton switches (S1, S2) 1 BC639 NPN transistor (Q1) 3 1N4001 or 1N4004 silicon diodes (D1-D3) 1 red LED (LED1) 1 or 2 2200µF 25VW PC electrolytic capacitors (see text) 1 1.2kΩ 0.25W, 1% resistor 2 470Ω 0.25W, 1% resistor 1 47Ω 0.25W, 1% resistor ity protection for the circuit if a DC supply is used and acts as a rectifier if AC is used. Diode D3 is include to prevent damage to the base of Q1 from voltage spikes which can be produced by the solenoids if there is contact bounce in the pushbutton switches (virtually all switches have some contact bounce). Finally D2 is included to allow Q1 to turn on and turn off correctly. Without D2, the base of Q1 would be connected directly to the 2200µF capacitors and so the transistor would be biased off. Assembling the board With such a small PC board, it will not take long to assem­ble all the components onto it. Make sure the diodes and transis­tor are installed correctly, otherwise the circuit won’t work. Our circuit and photos show the PC board fitted with two 2200µF capacitors but only one may be necessary. How do you know? Well, you could try the circuit with only one 2200µF capacitor fitted and see if it works satisfactorily. If so, then that’s all you need. However, if your input voltage to the circuit is 12V or less, you may need to fit two 2200µF capacitors to ensure that you have enough energy storage to fire the solenoids every time. We envisage that the points controller board will be in­stalled under the control panel for your layout. LED1 will be mounted on the control panel, adjacent to the pushbuttons S1 & S2. By the way, we suggest you try connecting the board to a switch motor and operating it before it is installed in your layout. Multiple points operation Note that while the circuit of Fig.1 and the PC overlay diagram of Fig.2 show provision for only two push­ This close-up view shows how the twin-solenoid switch machine fits under the points. A pin is fitted at rightangles to the solenoid shaft to drive the moveable rail section of the points. Points or Turnouts? If you are a model railway enthusiast you will find that American, Australian and European modelling magazines have dif­ ferent terminology for items such as points. Australian and English magazines refer to them as “points” while American maga­ zines refer to them as “turnouts” or “switches”. In fact, operations in railway marshalling yards are re­ferred to as “switching” in American parlance and “shunting” in Australian or English magazines. buttons, S1 & S2, you only need to build one of these point controller boards to drive all the points switch motors on your layout. All you need to run extra points is an extra pair of push­buttons for each set. So in theory, you could have 50 sets of points and 50 pairs of push­ buttons all run from the one points controller board. In practice though, it might be prudent to run no more than a dozen sets of points from each board. This would simplify the wiring and make troubleshooting easier if you ever have a short or an open circuit in your wiring. If you do decide to run multiple points controller boards, you can power them all from the same 12-15V SC source. Fig.2 (left): the component overlay for the PC board. Note that you can fit one or both of the 2200µF capacitors, depending on your input supply voltage (see text). Fig.3 at right shows the actual size artwork for the PC board. July 1997  31