One refinement for occupancy detection is making it insensitive to momentary dropouts, as might be caused by dirty wheels or track. A simpleminded way to do this is to put a large electrolytic capacitor either across the rails or across the (DC) relay coil. While this can gain a second or two of protection, it creates a new problem when the train enters the block, that is the very sudden high current when the wheels discharge or charge the capacitor. Some further design, such as a diode shunted by a resistor placed in series with the capacitor can get around this.
Another approach uses two relays per block. The track operates the first relay; and the first relay operates the second, which handles the signals. Each relay has a latching contact; and there is one provision for interrupting the latching path for all the first relays, and another provision for doing the same for all the second relays. The first relay's latch is interrupted briefly first. If there is no occupancy, the first relay drops; but this doesn't affect the second relay if it is latched. After a delay of several seconds, the second relay's latch is interrupted briefly. If no occupancy has relatched the first relay in those several seconds, the second relay also drops. But, if there was occupancy however briefly during that time, it relatches the first relay, so that the second relay remains operated. Then the first relay's latch is interrupted again, and the process repeats.
There are various ways to wire this up. And I can imagine numerous ways to produce the interruptions, such as a motor-driven cam operating a pair of switches. At the club where I first encountered this concept, it was done with a relay pulse-divider that made a continuous "schlunk-schlunk" sound from deep under the layout; so it was given the name "the multischlunker". http://tmrc.mit.edu/dictionary.html
Bob Nelson
Perhaps the most straightforward logic implementation is with relays. Connect one end of a relay coil to your power supply and the other end to the isolated outside rail. Return the power supply to the layout common, that is, the other outside rails. DC relays are cheaper and quieter. They do not need to be modified with individual rectifiers for use with AC. Instead, rectify the AC supply and power the relays with the resulting DC. The supply voltage and the coil rating should match.
Virtually any signal scheme can be implemented with one relay per block. However, more complicated ones may need more contacts than one relay can provide. Your options are to wire multiple relay coils in parallel or to operate secondary relays from the contacts of one relay connected to the track.
For a simple 3-aspect setup on a 1-way track, each block's relay operates the red light guarding that block with a normally-open contact. Connect the corresponding normally-closed contact to the center contact of a relay in the following block, which will operate the green and yellow lights of the same signal. Thus each relay needs two SPDT (form-C) contacts, one for its own block's signal's red light and one for the green and yellow lights of the preceding block.
For bidirectional operation, double the number of contacts on each relay and implement the entire scheme facing in each direction.
There are some prototypical considerations that can make things more complicated. Railroads often use approach-lit signals, which remain dark until there is a train in the block facing the signal. This can be handled with yet another normally-open contact in that facing block feeding the red-light contact already described.
Another complication is including turnouts in the signal scheme. A turnout aligned against the train should produce a red signal, like an occupied block ahead. Then there is the sometimes used fourth aspect, usually a flashing yellow, which can be used, for example, to warn that a turnout at the next signal is aligned for a diverging path (like a passing siding). At the turnout, an upper signal head has red, yellow, and green lights and is for the non-diverging path; the lower signal head has only yellow and green and is for the diverging path. At least one of them is always red. The flashing yellow would be used at the preceding signal when the signal at the turnout is showing red over yellow, indicating that the train can proceed slowly onto the diverging path.
There are three parts to a signal system:
o Occupancy detection. This is easy in 3-rail. You just isolate one outside rail for the length of each block. That rail gets connected to the other rail, the layout common, through the train's wheels and axles.
o The logic to translate the occupancy information into signal aspects. This can be very complicated and can be done in a very wide variety of ways, including using a digital computer.
o The signals themselves. A three-aspect (red, yellow, green) searchlight signal can be made in at least two ways:
- A multi-color LED device that includes red and green LEDs with a common terminal (Radio Shack 276-028) can be operated from a DC supply. If, for example, the common terminal is the anodes of the LEDs, connect it to a positive DC voltage. Connect a separate ballast resistor to each of the red and green cathodes. Connect the free end of each of these resistors to the DC-supply common to light the red or green LED. Connect two more resistors to each of the red and green cathodes. Connect their free ends together. Connect this node to the DC-supply common to light both red and green LEDs together. Adjust these last two resistance values to get the shade of yellow desired.
- A two-color LED device of red and green LEDs connected in anti-parallel (each anode to the other LEDs cathode) (Radio Shack 276-012) can be operated from an AC supply. Connect one of the two LED terminals to the AC voltage. Connect each of two ballast resistors to the other LED terminal. Connect a diode in series with each of these two resistors, with the diodes pointing in opposide directions. Connect the free end of each of these resistor-diode combinations to the AC-supply common to light the red or green LED on every other half-cycle of the AC voltage. Connect two more resistor-diode combinations to the same LED terminal. Connect their free ends together. Connect this node to the AC-supply common to light both red and green LEDs on alternate half-cycles of the AC voltage. Adjust these last two resistance values to get the shade of yellow desired.
I have made some scratch built signals, Green over Red color light and searchlight signals.
Using styrene tube for the mast and the LED housing and sheet styrene for the signal face, I could not find O Scale ladders so I scratch built the ladders with balsa wood and brass wire. The platform at the top of the ladder I found a plastic mesh in a craft store used for needlepoint. For the signal base I used a 1/2 PVC pipe fitting that I place sheet styrene on top and trim to match the PVC, drill a hole to insert the mast. My layout has 1/2 inch celotex over plywood. I cut out a 1/2 inch diameter hole into the celotex, stopping at the plywood to mount the PVC base into. Dill a hole through the plywood for the mast to extend through down below the layout.
Radio Shack has an assortment of LED packs. Green, Yellow, and Red and a 2 color LED, Red + Green that work well for a searchlight signal. Get an assortment of 150 ohm to 470 1/2 watt resistors. Spend some time playing around with what resistors work best for a given LED and the power supply. You will blow some LEDs before you get it right. The LEDs use only 1-2 volts DC. Use a small DC power pack or a bridge rectifier with an AC power supply.
For the signal circuit I have a 12 volt Relay that is tripped by the outside third rail. You will need a bridge rectifier to convert the AC to DC to trip the relay. Radio Shack has relays also. There are 3 sets of contacts, One pair of contacts connected to the 3rd rail closes the relay. The other 2 sets of contacts are DPDT switch, use one to control the LEDs - the other set can be used to control the track power to stop a train at a red signal. Telephone wire can be bought in 100 ft rolls, with red, green, yellow and black wire inside. Remove the outside insulation and the wire threads nicely inside the PCV tube. Mounting the signal head to the mast with the wires running down the inside of the PVC tube takes some work to get it right.
Good Luck
Here is some stuff about the red-green LED that I think is the one mentioned: http://www.play-hookey.com/digital/experiments/bicolor_led.html
You're right about the Hall signal's being the real "banjo" signal. I think I fell for another Lionel misnomer (like "crossover" for "crossing"). I don't seem to get any Google hits for "banjo signal" that don't involve Lionel. By the way, I came across the fact that it was Hall's company that also introduced the searchlight signal.
Hello all,
Bob - there was a "banjo" signal for train operations. The link below has a reference. Scroll way down on the link and there are several variations. The most well known is the Hall Enclosed Disk. The shape gave these signals their nickname.
http://mysite.du.edu/~etuttle/rail/sigs.htm
Not sure if this is what is being referenced by Crukshank. If not then the link can help provide a common reference point for what he is trying to build.
Regards, Roy
This turns out to be three LEDs in one package with a common-anode. http://www.radioshack.com/graphics/uc/rsk/Support/ProductManuals/2760028_DS_EN.pdf You should be able to get yellow by turning on the red and green in the correct proportions by selection of the ballast resistors that you connect to the red and green cathodes. Three-aspect block signals can get quite complicated; and there are a lot of different ways to do them. Some considerations are whether you want bidirectional signals and approach-lit signals. If you're expecting to stop a train automatically on red signal, that is a further problem.
This is a banjo signal: http://www.lionel.com/media/servicedocuments/71-2709-252.pdf I think what you want is a searchlight or target signal.
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