Information on ordering a commercial kit or assembled and tested unit of these circuits is available from the TracTronics Price List.
This is the ninth in our series of articles describing a set of electronic building blocks we have designed to control our model railroad layouts: Rich Weyand's N&W Pocahontas Division, Bill Pistello's Union Pacific, and the Reid brothers' Cumberland Valley System. This month we are going to begin discussing signaling at a deeper level than we have before. We will start this month with a pair of signal driver circuits which will provide a standard connection to the many different types of signals we can use, and will show how these circuits can be used to implement Automatic Block Signaling. Those who are signaling club layouts pay special attention: we have repeatedly been asked for this type of circuit by the clubs we've met with, and we believe these circuits will give you the operation you want.
We are presenting these circuits both as circuit diagrams and as circuit layout patterns, allowing readers to breadboard or etch their own circuits. These two circuits are not yet available as commercial products, but TracTronics will be bringing them out as commercial products in the future. This series of articles has caught up with our product introductions, which has both good and bad repercussions. The good part is that the prototype board layouts are a little easier to etch, being single-sided and with wider traces than our commercial product. The bad part is that the boards are bigger than they need to be, and they have not been tested as thoroughly as our commercial products.
One problem with building a signal system for use on multiple model railroads is the multiplicity of signal types available. There are color light signals, which look like traffic lights turned upside-down, with the green on top and the red on the bottom. There are searchlight signals, with a single light which changes color from red to yellow to green to display the signal indication. There are position lights, which have lights arranged in a circle around a center light, with three vertical lights meaning the same as green, three diagonal lights meaning the same as yellow, and three horizontal lights meaning the same as red. These three basic types of signal heads are shown in Figure 1.
To further add to the problem, most real railroads have more than one signal type in operation. One signal type may be gradually replacing another as the old ones fail or need service, different divisions may use different signal types, or different predecessor roads used different signal systems which were never replaced. It is very difficult to model this variety accurately with signal systems that we've seen.
Additionally, some model signals use bulbs, some use LEDs, some even use both! We've seen this latter for yard dwarf signals which have red over white, where an LED is used for the red signal, and a bulb is used for the white because there is no such thing as a white LED.
What we wanted was a set of signal driver circuits which could drive all of the various signal types available, and which would all look the same to the electronics which runs the signal system. This way we could design signaling circuits and signal systems which were independent of the signal type used, and then use the proper signal and signal driver circuit for each signal installation, mixing and matching signals on our model railroads the way the prototype does. We got the whole variety of available signals down to only two signal driver circuits, AutoBlock for color lights and position lights using LEDs or bulbs, and AutoSearch for searchlight signals using bi-color LEDs.
The circuit we designed for color light signals, which we call AutoBlockTM, is shown in Figure 2, with the component values and part numbers given in Table 1.
Three op amps are used to drive the three LEDs. The op amp inputs are held to voltages which result in the YELLOW and RED outputs being high, or off, and the GREEN output being low, or on, whenever none of the inputs are pulled down. If either APPROACH input is pulled down to ground, but the STOP and DIRECTION inputs are not, the voltages on the inputs of the op amps are changed so that the YELLOW output goes low and the GREEN and RED outputs go high. If either the STOP or DIRECTION input is pulled down to ground, regardless of the state of the APPROACH inputs, the voltages on the inputs of the op amps are changed so that the RED output goes low and the YELLOW and GREEN outputs go high.
In other words, the circuit has two inputs, APPROACH1 and APPROACH2, which call for a yellow signal to be displayed, and two inputs, STOP and DIRECTION, which call for a red signal to be displayed. The circuit will drive the LEDs to display the most restrictive signal called for, displaying a green signal if no input is asserted. Note that this circuit does not have the fail-safes of a prototype signal driver, in that if all connection to the circuit is lost it will display a green signal. Building in the prototype fail-safes would have made the circuit and the entire system very expensive.
The circuit we designed for searchlight signals, which we call AutoSearchTM, is shown in Figure 3, with the component values and part numbers given in Table 2.
Two op amps are used to drive the bi-color LED. The op amp inputs are held to voltages which result in the GREEN output being low and the RED output being high, so current will flow through the bi-color LED from RED to GREEN, displaying a green signal, whenever none of the inputs are pulled down. If either the STOP or DIRECTION input is pulled down to ground, the voltages on the inputs of the op amps are changed so that the RED output goes low and the GREEN output goes high, so current will flow through the bi-color LED from GREEN to RED, displaying a red signal.
R6 limits the current through the bi-color LED in both the red and green directions. R9 and D7 allow the current in the green direction to be set to a larger value than in the red direction, as the green is usually not quite as bright. Start out by putting a wire jumper in for R6, and let the 20 mA internal current limiting of the op amps limit the current through the LED. If this works okay for your LED, then you don't need to experiment with R6 and R9 at all. If not, use R6 to set the brightness for red, and use R9 to set the balance between red and green. One way to do this is to use a jumper for R9, letting the green have the full current of the op amp, and then to select R6 to limit the red brightness to match. The values you need will depend on the type of LEDs and the voltage you run the circuit at. You should experiment with the values for R6 and R9 to get the brightness in each color that you want.
How we get a yellow signal takes a little explaining. Components R1, R3, D1, D2, and D6 turn a 60 Hz AC signal from the power supply transformer into a 60 Hz square wave. When the APPROACH input is left high, the op amp output is high, and the rest of the circuit is unaffected. When the APPROACH input is pulled down to ground, however, the op amp output will be a square wave at 60 Hz, alternately forcing the circuit to a red signal and releasing the circuit to a green signal. The resultant alternating red and green output of the LED is integrated by our eyes to be a yellow signal. If the yellow you get appears too orange, decrease R9 or increase R6 to change the color balance toward the green. If the yellow you get appears too green, increase R9 or decrease R6 to change the color balance toward the red.
In summary, the circuit has one input, APPROACH, which calls for a yellow signal to be displayed, and two inputs, STOP and DIRECTION, which call for a red signal to be displayed. The circuit will drive the LED to display the most restrictive signal called for, displaying a green signal if no input is asserted. Note that this circuit also does not have the fail-safes of a prototype signal driver, in that if all connection to the circuit is lost it will display a green signal.
Note also that the operation of the AutoSearch circuit and the AutoBlock circuit are the same from the point of view of the signaling system. Each has APPROACH, STOP, and DIRECTION inputs which cause the circuit to display the proper signal. The AutoBlock circuit has an additional APPROACH input, which we will not use in the circuits we discuss this month. Leave this signal, A2, unconnected in all circuits.
The circuit etch patterns and component placement diagrams for the AutoBlock and AutoSearch circuits are given in Figure 4 and Figure 5 for those who want to etch boards, rather than perfboard the circuit. The component placement diagram includes the hole locations to aid in drilling your board. Note that the etch pattern is always printed as seen from the component side of the board, per electronic industry standards. The solder side image must be reversed on the board you build, so that the text and the image are correct.
The component listings are given in Tables 1 and 2. Note that no values are given for resistors R6 and R9 of the AutoSearch circuit, as these are determined by experimentation for your LEDs. Also, the AutoSearch circuit has one wire jumper, J1. The single-sided layout of this board was more difficult than for AutoBlock, and one jumper was necessary.
We haven't included any power supply caps on the boards, but if your supply lines are very long, you might want to add a 10 uF axial electrolytic cap (Radio Shack 272-1013) across pins 4 and 11 of the LM324 IC on the back of the boards. The plus lead of the cap should be connected to pin 4.
The circuits can be run on any filtered DC supply from 3 to 32 volts, depending on the application. We usually run ours on 5 volts, because that is the voltage we use for all of our logic circuits, and we have a lot of it available. You can go to a radio hamfest or flea market and buy an old PC or computer supply cheap. These supplies usually provide 10 to 20 or more amps of 5 volts, as this is the most needed voltage in a computer.
The AutoSearch board also requires an AC input. This input is used to alternate the red and green signals to produce yellow, and is not used for power. This AC voltage can be anything from 3 to 32 volts also. Take this signal off the secondary winding of the transformer of whatever power supply you use.
You can also use the power supply discussed for the MasterFlasher in the July 1994 Mainline Modeler. This power supply includes the AC line from the secondary of the transformer, which is called CLK in the figure.
Automatic Block Signaling, or ABS, is one of the simplest of the signaling methods used by the railroads. In modeling, ABS is the most often used of all signaling system types, and is especially popular on club layouts.
In ABS, signals indicate the occupancy of the blocks ahead. If an ABS signal displays a green signal, at least the next two blocks ahead are clear of trains and equipment. If an ABS signal displays a yellow signal, the next block is clear, but the second block ahead is occupied. If an ABS signal displays a red signal, the next block is occupied. The yellow signal gives the train crew one block advance warning that the next signal may display a red signal, giving the train crew time to slow the train and be prepared to stop.
Figure 6 shows the wiring for an ABS signaling installation on a single track main line with traffic in one direction, left to right in the diagram. The DetectTrain detectors were discussed in the February 1994 Mainline Modeler. The detector OCCUP outputs are used to provide the APPROACH and STOP inputs to the AutoBlock circuits which drive the trackside signals. The DIRECTION and APPROACH2 inputs are left unused in this application. The positive supply voltage provided to the detectors and the signal drive circuits need not be the same, or even from the same power supply, but the grounds of the supplies must be connected together. Of course one supply can also be used.
The limiting resistor RLED here is not strictly needed. The op amp outputs of the AutoBlock circuit have a current limiting feature which limits the current through the LED to the value we normally recommend, about 20 mA. Try this circuit first without RLED, with the LED anodes connected directly to the positive supply voltage. If the LEDs are too bright, insert RLED into the circuit to limit the brightness to the value desired. If one LED is too bright, for example the red LED is brighter than the green and yellow LEDs, you can instead insert a limiting resistor into the connection from the cathode of the red LED to the RED output of the AutoBlock circuit. Normally the red LEDs are a little brighter than the others, but that's the way we like it, so the operators can see those red signals!
For situations where the main line branches, use the switch machine electrical contacts to select which detectors' OCCUP outputs will be passed back to the preceding signal driver circuit's APPROACH and STOP inputs. This requires a DPDT set of contacts. If you don't have enough contacts on your switch machine, using a set of contacts on the switch machine to run a 4PDT relay, such as Radio Shack 275-214, will give you plenty of additional contacts.
For multiple track main lines, in which each track is operated in one direction, the wiring of Figure 6 is repeated on each track. Make sure to orient the diagram properly for each track so that all your tracks don't run left to right! The OCCUP signals of the track block detectors should always be running back toward the approaching trains.
Note how clean the wiring in Figure 6 is. The detectors, the signal driver circuits, and the signal LEDs are the only components. Only one wire from each detector back to the two preceding signal driver circuits is required, together with the power supply connections which are bussed from circuit to circuit. This is the simplicity we set out to achieve with the signal driver circuits.
Figure 7 shows the wiring for ABS signaling on a single track which can be used in both directions. The left to right, or eastbound, signals and signal driver circuits are on the bottom, and the right to left, or westbound, signals and signal driver circuits are on the top. Only one set of detectors is required. Power supply connections are as before. Again, the RLED limiting resistors are probably not necessary, but are shown in case they are required for your application.
In this diagram, the OCCUP signals from the track block detectors are connected to the two westbound signal driver circuits located east of the track block, and to the two eastbound signal driver circuits located west of the track block. In this diagram we use the DIRECTION input for the first time. The DIRECTION inputs of all the eastbound signal driver circuits are all bussed together, as are the DIRECTION inputs of all the westbound signal driver circuits. These signals are connected to a SPDT switch in the lower right of the diagram which selects the direction of this section of track. If the switch is to the left, or west, the eastbound DIRECTION inputs are grounded, forcing all eastbound signals to red, and if the switch is to the right, or east, the westbound DIRECTION inputs are grounded, forcing all westbound signals to red. One switch can be used for each section of track for which independent control of the direction is desired. Do not use center-off switches here! If one gets left in the center position, traffic will be enabled in both directions, and bad things will happen.
Again, note how clean the wiring in Figure 7 is, with the only components being the detectors, the signal driver circuits, and the signal LEDs, yet it is a complete bi-directional ABS system. We were very pleased when the signal driver circuits worked out this well.
Figure 8 shows the modification to the ABS wiring diagram required to use the AutoBlock circuit to drive color light signals using bulbs instead of LEDs. The transistors here provide the higher current necessary for the bulbs. No resistors are required due to the 20 mA current limiting feature of the LM324 op amp used in the AutoBlock circuit. The transistors we use are the 2N3906, available as Radio Shack 276-1604.
No circuit for using bulbs with the AutoSearch circuit is given, as all model searchlight signals we have seen have been made with bi-color LEDs.
The Pennsylvania, Norfolk & Western, and Milwaukee Road used position light signals. Position light signals can be driven by the AutoBlock circuit using modifications to the ABS wiring diagrams as shown in Figure 9 for LEDs, and in Figure 10 for bulbs. These diagrams show the wiring for the center lamp, which some position lights have and some do not. These diagrams assume that LEDs in a position light signal will be connected in series, and bulbs in a position light signal will be connected in parallel. While this is the most common arrangement, if either is connected the other way, the circuit will still work.
Note in Figure 9 that, if you use a resistor to limit the current through the LEDs connected to the AutoBlock circuit, the values you will need for two LEDs in series, RLED2, and the resistor which is required for the single center LED by itself, RLED, will not be the same value. Experiment a little, or use a multimeter to get the same current through the LEDs. If you leave RLED2 out, and use the op amp's 20 mA internal current limiting, you should limit the current in the center LED with a resistor value RLED determined from the equation we presented before:
Resistor value in ohms = (LED supply voltage in volts - 2 volts) x 50 ohms/volt
Figure 11 shows the modification to the ABS wiring diagrams required to use the AutoSearch circuit to drive searchlight signals. Note the extra connection for the AC signal from the power supply transformer secondary winding. You will have to experiment to see which way to hook up the bi-color LED of the signal. Remember that, with no inputs connected, the circuit should drive the LED to produce a green signal. If you get a red signal with no inputs connected, reverse the two LED leads connected to the RED and GREEN outputs.
These two signal driver circuits provide a common set of connections for most of the signal types used in model railroading, allowing the signaling system to be designed independently of the signal type. This common interface also makes implementation of Automatic Block Signaling very easy when commonly available track block detectors are used. Finally, this common interface makes it easy to implement much more complicated signaling schemes. Next time we'll take on Absolute Permissive Block (APB) signaling, using these signal driver circuits and a new direction precedence circuit. We'll see you then.
