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Two projects for model railways By JEFF MONEGAL Project #1 3-Aspect Signalling Many railway modellers strive to achieve the ultimate in realism yet the resulting layout usually has non-operating signals or signals that constantly show a red or green lamp. The project presented here will go a long way to increasing the realism of signals. The addition of a little of animation to any model railway layout can enhance train operation immensely. That is what this simple unit has been designed to do. It operates a three-aspect (red-amber-green) signal in a most realistic manner. As a train 34  Silicon Chip approaches the signal the green light will be displayed. As soon as the locomotive has passed the signal, it changes to red. A few seconds after the last car has passed the red signal, it changes to amber. After a further few seconds delay, the signal again shows green. If the signal is used on two-way traffic lines, then a constant red is displayed while ever the train runs against the flow of the signal. It is simple in its operation but it adds a lot of realism and interest to any layout. How it works A look at the diagram of Fig.1 will show that the circuit is quite simple in its operation. As the train passes the signal, it is detected by the sensor which is placed just nearby on the track; ie, past the signal. The sensor is an LDR (light dependent resistor), the resistance of which goes high as the passing train casts a shadow over it. Fig.1: this circuit provides three-aspect (green, amber, red) signalling for a model railway. The train is detected when the locomotive passes over the light dependent resistor (LDR1) which is mounted between the rails of the track. This causes the voltage at the junction of resistor R1 and zener diode ZD1 to rise. When this voltage goes above about +4.5V, the Darlington transistor pair comprising Q1 & Q2 will turn on and pull the cathode of diode D1 to ground. This lights LED1 which indicates that a train has been detected. Capacitor C1 will discharge quickly through resistor R5 and the forward biased diode D1. This process pulls pins 1 & 2 of IC1a low which causes pin 3 to go high. This turns transistor Q3 and the red signal, LED2, on. At the same time, pin 11 of IC1b will go low which discharges capacitor C2 quickly through R8. This causes pin 10 of IC1c to go high and pin 4 of IC1d to go low. This turns off Q5 and the green signal, LED4, goes out. This condition will remain as long as the resistance of the LDR is high. As the end of the train passes the sensor, its resistance will again go low. Q1 and Q2 will turn off and C1 will start to charge through R4 and R5. When its charge reaches about half supply (+4.5V), pin 3 of IC1 will go low. The red signal now turns off. Pin 11 will now go high, turning on the amber signal. C2 now charges through R9. When it reaches half supply pin 10 will go low. D4 is now forward biased which turns off the amber signal. Pin 4 now goes high and the green signal turns back on again. Q6 and its associated components, diode D5 and resistors R11 & R13, detect when the track polarity is reversed. When the rail connected to R11 is positive with respect to the rail con­ nected to D5, Q6 will turn on. When this happens the collector of Q6 pulls the junction of R4 and R5 to ground. This triggers the signal to the red condition and this is where it will stay as long as the polarity of the track voltage remains this way. This was done so that the signal will remain red when a train is moving against the flow of the signals. If this were not done the signals would indicate a green condition when a train was coming from behind – clearly un­ proto­typical. Q7 is connected as a simple regulator. Zener diode ZD2 holds the base at +12V so the emitter will be regulated to about +11.4V. Diode D6 provides reverse polarity protection with C3 and C5 providing supply filtering. VR1 is the sensitivity adjustment for the LDR. Construction The component layout for the PC board is shown in Fig.2. There is nothing difficult about assembly so go RESISTOR COLOUR CODES – PROJECT #1        No. 1 2 1 4 1 4 Value 470kΩ 120kΩ 47kΩ 4.7kΩ 1.8kΩ 1kΩ 4-Band Code (1%) yellow violet yellow brown brown red yellow brown yellow violet orange brown yellow violet red brown brown grey red brown brown black red brown 5-Band Code (1%) yellow violet black orange brown brown red black orange brown yellow violet black red brown yellow violet black brown brown brown grey black brown brown brown black black brown brown March 1997  35 PARTS LIST – #1 1 PC board, code 3ASIGNAL, 100 x 50mm 10 PC stakes Fig.2: the component layout for the PC board of the circuit shown in Fig.1. The PC board would normally be mounted under the lay­ out, quite close to the signal unit. Note that the coloured LEDs are not mounted on the board but are part of the signal itself. ahead and load all the passive components, watching the polarity of the diodes and electrolytic capacitors. If you want to use a socket for IC1 then solder it in now. Finish with the remaining components. Then go back over your work to ensure that you have done a good job and that all components are in the right places. Testing Connect a signal or three LEDs to the appropriate termi­nals. At this stage no LDR sensor is necessary. Switch on the power. The red lamp should come on as well as the detect LED. Using a clip lead short the two sensor terminals. The detect LED should go out. A few seconds later, the amber lamp should light. A further few seconds and the amber light should go out and the green should come on. Remove the shorting lead and the detect LED should come on as well as the red lamp. If this all happened, then your signal circuit is working correctly. If not, then go back over your work, looking for the fault. More than likely you will have inserted a component wrongly or a solder joint will not be done. Installation Installing the signal is simply a matter of choosing a place for the signal then drilling a 5mm hole down between the sleepers (ties) of the track. The sensor should be placed about 100mm past the signal. Connect power and then the two wires to the track. If the red signal is constantly shown when the train is travelling in Semiconductors 1 4011 quad NAND gate (IC1) 6 BC548 NPN transistors (Q1-Q6) 1 BD139 NPN transistor (Q7) 1 3.3V zener diode (ZD1) 1 12V zener diode (ZD2) 4 1N914 signal diodes (D1-D4) 2 G1G power diodes (D5,D6) 1 3mm red LED (LED1) 1 light dependent resistor (LDR1) Capacitors 1 220µF 16VW electrolytic 2 33µF 16VW electrolytic 1 10µF 16VW electrolytic 1 0.47µF monolithic Resistors (0.25W, 5%) 1 470kΩ 4 4.7kΩ 2 120kΩ 1 1.8kΩ 1 47kΩ 4 1kΩ Miscellaneous Solder, hook-up wire, etc. the normal direction then reverse the two wires to the track. If the signal will only ever see single direction traffic then these two wires need not be connected. Simply leave them unconnected. You need one of these PC boards for each railway signal on your layout. By using 2mm LEDs you can wire HO signals for realistic operation. 36  Silicon Chip Fig.3: Q1 is a phase shift oscillator running at 25kHz. Its signal is fed to power amplifier IC1 which drives the track via its 100µF output coupling capacitor. The two inductors provide isola­tion for the DC power controller which also feeds the track to drive the model locomotives. Project #2 Constant Brilliance Lighting Circuit Add constant brilliance lighting to your model locomo­tives and carriages with this high frequency drive circuit. This will add extra realism to your layout, especially if you model night-time scenes. Model railway rolling stock these days is very realistic. The detail in the plastic mouldings is quite astonishing and you need a magnifying glass to read the fine printing of rolling stock reporting marks. Where passenger rolling stock does fall down is with in­ terior lighting. Most carriages do not have interior lighting and if they do, it is not constant in brightness. So while the train is running the carriages may be lit but when the train comes to a stop, the lighting goes out, plunging the poor (imaginary) pas­sengers into darkness; not very considerate. Furthermore, if the train goes fast, the carriage and loco lighting is bright and as it slows down, it becomes dim. This is not how it happens in the real world. A model train layout where the lights in passenger coaches and locomotives remain on at a constant level of brightness regardless of wheth­ er trains are moving or stopped has greatly enhanced realism. That is what this unit does. Frustrated by the very unrealistic appearance of my own railway models, I decided to see what could be done. The princi­ple behind this system is not new and in fact, was proposed many years ago. The basic idea is a 25kHz sinewave oscillator which is fed into a power amplifier then applied to the tracks. March 1997  37 Fig.4: this is the parts layout diagram for the Constant Brilliance Lighting Circuit. Note that the TDA1520 power amplifier IC must be attached to a heatsink. Inside each carriage and locomotive is a small ca­pacitor connected in series from track collectors on the metal wheels to each lamp. The capacitor blocks the DC track voltage while allowing the high frequency signal through to light the lamp. The locomotive motor’s inductance will block the high fre­quency so that no damage will occur to the motor while it is standing still. The high frequency is combined with the DC train control voltage then connected to the track. Any lamp and series capaci­tor connected to the track, via the track contacts, will light at a brilliance level determined by the amplitude of the high fre­quency signal and not the level of DC motor control voltage. In other words, the lamps will burn at the same level of brilliance as long as the unit is switched on and will be unaf­fected by the train control voltage. This is much more prototypi­cal. In normal use the output of the controller is connected to the input terminals of this system. The output from the unit is then connected to the track. Any train can be controlled normally using the existing controller and the lights can be adjusted in brilliance PARTS LIST – #2 1 PC board, code CBLGEN, 127 x 50mm 1 heatsink (see text) 6 PC stakes 2 prewound inductor (L1,L2) 2 3mm bolts and nuts 1 20kΩ vertical trimpot (VR1) Semiconductors 1 BC548 NPN transistor (Q1) 1 TDA1530 power amplifier (IC1) 2 1N914 signal diodes (D1,D2) 4 G1G diodes (D3-D6) 1 12V zener diode (ZD1) Capacitors 1 1000µF electrolytic 16VW 3 100µF electrolytic 16VW 3 10µF electrolytic 16VW 2 0.47 monolithic 1 0.1µF monolithic 3 .0033µF ceramic 1 680pF ceramic Resistors (0.25W, 5%) 1 150kΩ 1 1kΩ 1 47kΩ 1 820Ω 3 10kΩ 1 270Ω 3 6.8kΩ 1 10Ω 1 2.2kΩ or even switched on and off regardless of what the train is doing. The unit presented here can drive up to about 20 3V grain-of-wheat lamps with an AC supply of 15V at 1A. 15VAC has been chosen because this is a commonly available voltage found on most power packs used for model railways. If you prefer, up to about 20VAC can be used with a corre­sponding increase in the number of lamps that can be driven. Be careful though, as lamps can be easily blown if the voltage is too high. How it works Understanding how it works is not difficult. Referring to the circuit diagram of Fig.3, Q1 is configured as a standard phase shift oscillator. R4, R5 and R6 together with C3, C4 and C5 cause a phase shift of the signal that is fed back to the base of Q1. This causes the circuit to oscillate at a frequency set by the values of these resistors and capacitors. The signal is tapped off from the emitter of Q1 and then fed to the brilliance control, VR1. From here the signal is fed to power amplifier IC1. It has its gain set at 11 as controlled by RESISTOR COLOUR CODES – PROJECT #2  No.    1    1    3    3    1    1    1    1    1 38  Silicon Chip Value 150kΩ 47kΩ 10kΩ 6.8kΩ 2.2kΩ 1kΩ 820Ω 270Ω 10Ω 4-Band Code (1%) brown green yellow brown yellow violet orange brown brown black orange brown blue grey red brown red red red brown brown black red brown grey red brown brown red violet brown brown brown black black brown 5-Band Code (1%) brown green black orange brown yellow violet black red brown brown black black red brown blue grey black brown brown red red black brown brown brown black black brown brown grey red black black brown red violet black black brown brown black black gold brown feedback resistors R10 and R11. The amplified signal is then fed to the track. Inductors L1 and L2 isolate the low impedance output of the controller from the 25kHz signal and this allows the DC train control voltage to operate the train but not block the high frequency signal coming from the amplifier. The output coupling capacitor C12 also prev­ents the DC voltage from the controller from upsetting operation of the amplifier and vice versa. Power for the system comes from a bridge rectifier, D3-D6, and a 220µF filter capacitor, C14. C13 provides more supply filtering at the power input pin of the chip. The supply voltage for the oscillator is regulated to +12V by resistor R7 and zener diode, ZD1. This has been included to prevent the oscillator from overdriving the power amplifier if a higher power supply is used. This board feeds a 25kHz sinewave at a level of up to 15VAC onto the track to drive grain-of-wheat lamps in locomotives and car­riages. Each lamp needs a 0.47µF capacitor in series to block the track DC. Construction The component layout for the PC board is shown in Fig.4. There is nothing critical about assembly of the unit. Start by giving the PC board a close inspection to make sure that no tracks are touching or have breaks in them. Load the resistors, capacitors and diodes, taking care with the polarity of the electrolytic capacitors and diodes. Next insert the transistor and PC stakes. Before inserting the power amplifier IC, prepare the heat­ sink. This is made from a piece of aluminium angle 50mm long, 40mm on one side and 25mm on the other, as shown in the photos. Using IC1 as a template, mark the two holes that have to be drilled. Ensure that the heatsink is aligned with the PC board and IC1. When assembled, the heatsink should be attached squarely to the PC board, with the two screws holding both the heatsink and power amplifier securely in position. Testing When the assembly is finished, it is time to test the unit. If you have an oscilloscope, you can look at the 25kHz sinewave signal which will be present at the emitter of Q1 and the output of IC1. Failing that, it is just a matter of hooking the unit up to the power and coupling a number of “grain of wheat” lamps, each via a 0.47µF monolithic capacitor, across the output of IC1. When power is applied, it should be possible to vary the brightness of the lamps up and down by adjusting trimpot VR1. When no lamps are connected to the circuit, the DC current drain should be less than 50mA. If everything works as it should, you can install the unit somewhere under your layout and install the lamps in SC your car­riages. Where To Buy Kits & Parts Kits for the 3-Aspect Signalling and Constant Bril­liance Lighting projects are available from CTOAN Electronics. Cost of the signalling kit is $14.00 plus $3 postage within Australia. The kit includes the PC board plus all onboard compon­ents including an LDR. Cost of the Constant Brilliance Lighting kit is $26.00 plus $4.00 postage within Australia. This includes the PC board, all compon­ents and heatsink, plus 10 0.47µF monolithic capacitors. Each 3V grain-of-wheat lamp requires one 0.47µF capacitor connected in series. CTOAN Electronics will be providing a repair service for both these kits. All kits sent in for repair should be accompanied with a repair fee of $14.00 which includes return postage within Australia. Fully assembled units are also available, priced at $25 for the signalling unit and $45.00 for the Constant Brilliance Lighting project. Add $4.00 for postage within Australia. Kits can be ordered by using Bankcard, Mastercard or Visacard or by sending a cheque or money order to CTOAN Electronics, PO Box 211, Jimboomba, Qld 4280. Phone (07) 3297 5421. Oatley Electronics can supply a pack of 2mm LEDs for installation in HO scale signals. Each pack contains 10 red, 10 orange and 10 green LEDs plus 30 1kΩ resistors. The cost is $10 plus $3 for postage and packing. Oatley Electronics are located at 66 Lorraine Street, Peakhurst, NSW 2210. Phone (02) 9584 3563; fax (02) 9584 3561. 3V grain-of-wheat lamps can be purchased from most hobby shops. March 1997  39