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A quickie project for your model railway By RICK WALTERS Traffic lights for a model layout Most model railway layouts have a few roads wending their way around and often a small town with an intersection is included. A good way to add life to such a scene is to have working traffic lights at the intersection. Any working light system on a model railway will add real­ism and having working traffic lights – cycling through the green, amber, red sequence – is a nice touch that can be easily and cheap­ly achieved. While you will probably have at 40  Silicon Chip least two sets of traffic lights visible and perhaps up to four sets for one intersection, you only need one PC board to drive the lot. These lights will change in the normal green, amber, red sequence which most of us, as motorists, are used to. We have chosen a timing cycle which seems realistic but it can be changed, as described later. Circuit description Fig.1 shows the circuit details. IC1 is a 555 timer which is wired as a free-running oscil­lator with a frequency of about 5.3 seconds, as determined by the 220kΩ resistor and 10µF capacitor connected to pins 6 & 2. IC1’s output at pin 3 is used to clock pin 14 of IC2, a 4017 counter with 10 outputs, each of which goes high in turn. Each time pin 14 of IC2 is clocked, the next output goes from 0V to +12V (low to high). Thus, each of the 10 outputs is high for about 5.3 seconds and low for about 48 seconds. We use a diode gating system from these 10 outputs to turn on the respective green, amber (orange) and red lights for dif­ferent times. Hence, the amber lights are only on for one clock cycle (ie, 5.3 seconds), while the red and green lights are each on for just over 26 seconds. By the way, if this overall cycle of 53 seconds seems too long or too short, it is a simple matter to change it by changing the value of the 220kΩ resistor at pins 6 & 2 of IC1; higher values give longer times and vice versa. The outputs are shown sequentially on IC2, going from output zero on pin 3 through to output 9 on pin 11. Only one output at a time can be high, as already noted. Let’s look at the outcome when pin 3 is high. Transistor Q1 will be turned on via the 10kΩ resistor connected to its base. This will turn on the two orange LEDs wired to its collector. In addition, transistor Q4 will be turned on via D1 and its 10kΩ base resistor, causing the red LEDs in its collector circuit to light up. By the way, we will use amber and orange interchangeably as we go through this article. Most people refer to the middle light as “amber” instead of orange but LEDs are available in orange, not amber. Typical intersection Before we go any further, we need to explain how all the light emitting diodes (LEDs) are wired up to control a typical intersection. Have a look at Fig.2 which shows a typical inter­ section with four sets of traffic lights to control the four directions of traffic. We have named the horizontal road “Cross Street” while the vertical road is named “Down Street”. (We hope readers appreciate how much of a mental strain it was for us to come up with these imaginative names.) As can be seen from the labelling of the four traffic lights, LEDs 1-6 Fig.1 (right): the circuit is based on a 4017 decade counter (IC2) which drives transistors and LEDs in a fixed sequence lasting around 53 sec­onds. IC1, a 555 timer, provides the clock signals for the counter. May 1997  41 Fig.2: this diagram will help in visualising the circuit opera­tion and also when the time comes to wire the lights (LEDs) at the intersection. control the traffic along Cross Street while LEDs 7-12 control the traffic along Down Street. Furthermore, the LEDs are paired up so that, LEDs 3 & 4 are the orange (amber) lights for Cross Street and so on. Traffic light cycle Fig.3: these waveforms are taken at three points in the circuit, with operation speeded up by 2.7 times. The upper trace shows the output at pin 3 of IC1, the clock cycle. The middle trace shows the signal at pin 3 of IC2. When this is high, Q1 and the orange LEDs 3 & 4 are on. The bottom trace is the signal at the junction of diodes D2-D6 and represents the signal driving Q2. When this is high, Q2 and the red LEDs 1 & 2 are on and so are the green LEDs 11 & 12. 42  Silicon Chip Thinking about how traffic lights work in practice, when the lights are green for traffic in Cross Street, they will be red for traffic in Down Street. When the lights change to amber (orange) in Cross Street, they remain red in Down Street. Final­ly, after the lights change from amber to red in Cross Street, there is a short delay before the lights in Down Street change to green. This short delay gives a slight margin of safety for those fools who run through red lights. In our modelling version of traffic lights, we have the same sequence except that when the lights change from amber to red in Cross Street, they simultaneously change from red to green in Down Street. This slight variation from reality can be toler­ated in a model railway scene, because the road vehicles in a typical model railway layout don’t actually move! And even if you were using wire-guided moving road models, you wouldn’t have to worry about the dangers of any vehicle running red lights. So now let’s resume our description of the circuit opera­tion. As we said, pin 3 of IC1 is high, Q1 is on so that LEDs 3 & 4 are lit, and Q4 is still on as well, so that red LEDs 7 & 8 are on. Traffic in Cross Street is coming to a stop while traffic in Down Street is stopped and ready to go. When IC2 is next clocked, pin 3 will go low and pin 2 will go high. So Q1 will turn off, Q2 will turn on showing red lights in Cross Street, and Q4 will turn off, allowing green LEDs 11 & 12, to light. So traffic in Down Street gets the green light. Note that both sets of green LEDs, 5 & 6 and 11 & 12, are not turned on by transistors. This is possible because both sets of six LEDs (red, orange, green) are each fed via a common 470Ω resistor. When transistors Q3 and Q4 are off, green LEDs 11 & 12 will be fed via diode D11 and the 470Ω resistor. Whenever Q3 or Q4 is turned on, the green LEDs will be extinguished as the voltage drop across the red or orange LEDs and their transistor will be less than that You can increase the realism at a road intersection on a model railway layout by having the lights working. This board is shown assembled with 12 LEDs to check its opera­tion. In normal use, the LEDs will be installed in the traffic lights at the intersection. Fig.4: the component layout for the PC board. Note that the LEDs are only installed on the board for checking its operation. across the green LEDs and diode D11. A similar situation exists with Q1, Q2 and the green LEDs 5 & 6. Thus we have red lights in Cross Street and green lights in Down Street. This condition is maintained for the next four clock cycles or 21.2 seconds (5.3 x 4) at which point pin 1 of IC2 goes high to turn on Q3 and orange LEDs 9 & 10 (for Down Street). This extinguishes the green lights and diode D6 keeps Q2 turned on to maintain the red lights (LEDs 1 & 2) for Cross Street. In the next clock cycle, pin 5 (output 6) goes high and pin 1 goes low. So Q4 turns on to light LEDs 7 & 8 and Q3 turns off. Q1 also turns off and so green May 1997  43 PARTS LIST 1 PC board, code 09205971, 95 x 80mm 1 555 timer (IC1) 1 4017 counter (IC2) 1 7812 12V regulator (REG1) 5 BC548 or BC338 NPN transistors (Q1-Q5) 4 green LEDs (see text) 4 orange LEDs (see text) 4 red LEDs (see text) 12 1N914 silicon diodes (D1D12) 1 1N4004 silicon diode (D13) Capacitors 1 100µF 25VW electrolytic 2 10µF 25VW electrolytic 1 1µF 25VW electrolytic 1 0.1µF monolithic ceramic 1 .01µF MKT Resistors (0.25W, 1%) 1 220kΩ 2 470Ω 0.5W 1 100kΩ 1 100Ω 8 10kΩ Where to buy parts Note: 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 is located at 66 Lorraine Street, Peakhurst, NSW 2210. Phone (02) 9584 3563; fax (02) 9584 3561. LEDs 5 & 6 are lit, via D12. If you keep stepping through the outputs of IC2 you will see that the traffic lights cycle in the correct sequence. Scope waveforms The oscilloscope waveforms of Fig.3 show the sequence speeded up by about 2.7 times. The upper trace shows the output at pin 3 of IC1, the clock cycle. In this case, the clock cycle is 1.35 seconds. The middle trace shows the signal at pin 3 of IC2. When this is high, Q1 and the orange LEDs 3 & 4 are on. The bottom trace is the signal at the junction of diodes D2-D6 and represents the signal driving Q2. When this is high, Q2 and the red LEDs 1 & 2 are 44  Silicon Chip Fig.5: the actual size artwork for the PC board. Check your board carefully against this pattern before installing any of the parts. on and so are the green LEDs 11 & 12. Traffic lights override One additional feature we have included is the ability to set IC2 (and thus the traffic lights) to a known state. This is done by grounding the 100Ω resistor in the base circuit of tran­ sistor Q5. This will reset IC2, so that pin 3 (output 0) is high. This is the initial condition which we described, whereby Q1 is on and the orange LEDs 3 & 4 are lit. If the lights for Cross Street were also used to control the traffic over a railway level crossing, an approaching train could ground the 100Ω resistor. This would immediately show an orange light to the traffic, followed by red on the next clock pulse. This gives the train an ‘all clear’ though the intersec­tion. The only remaining aspect of the circuit to talk about is the power supply arrangement. A 3-terminal regulator REG1 is used to obtain a stable 12V supply for the circuit and diode D13 provides protection against reversed polarity. Building it The PC board for this design measures 95 x 80mm and is coded 0910-5971. After checking the copper pattern for any defects against the artwork of Fig.4 you can start assembly by inserting the resistors and diodes. Note that all the diodes on the board face the same way; ie, with their cathode bands away from IC2. Next, insert the transistors and capacitors, ensuring that the electrolytic capacitors and transistors are correctly orient­ed. This done, insert the ICs and the 3-terminal regulator, REG1. Finally, you can insert the LEDs. While our prototype has been wired with the correct coloured LEDs on the board, this is not necessary for checking the circuit operation. You could initially use LEDs that all have the same colour. Testing To test the board, apply +15V to the input and check that the LEDs turn on and off in pairs. The red and green pairs should alternate with each other and the orange pairs should only turn on for just over five seconds each time. The total cycle time should be around 53 seconds but the exact value will depend on the tolerance of the 10µF capacitor connected to pins 2 & 6 of IC1. When wiring the traffic lights on your layout, 2mm LEDs are the closest to correct scale for HO layouts (1:87 scale) while 3mm would be good for O scale layouts (1:43). If you’re into N scale, the only way to produce a correct scale traffic light set would be to use optical fibres. Use the diagram of Fig.2 to aid in wiring the traffic lights for your SC intersection.