Grade Crossing Signals

Chapter-5d-1 Page 33

Crossing Lights

Here is a simple design to provide flashing crossing lights on a single track for trains going in either direction.

The circuit is similar to that of "An Intermediate Signal" and basically detects the train entering and exiting the area between the two spot detectors. When a train is detected the circuit activates a set of alternately flashing LEDs on the crossing signs. The flash rate is controlled by a Flashing LED. When ON it pulls power through two of the crossing LEDs and when it turns off a transistor switches on and pulls power through the other two LEDs.

Installing the spot detectors, called Entry and Exit, will need some thought. The spot detectors should be located an equal distance either side of the crossing. The distance being far enough away so that the last wagon of the longest consist just clears the crossing when the train has passed the exit detector. Use your longest consist running at its normal speed. Single locos or shorter consists can't be controlled so closely but most observer attention will be as the train approaches and the lights start up so any extended flashing after the train passes may not be significant. This is only a simple logic and the train is expected to pass the entry detector point then exit past the other detect point. If it stops and reverses over the entry point the sequence is not completed and the lights will continue going until the exit detect point is crossed. The circuit will work on a double track but if two trains are in the detected area together the sequence will probably not be correct.

How it works:

The two timer circuits work interactively. Either spot detector can start the sequence and the opposite one will finish it. After the exit detector is crossed there is a delay, about 5 seconds to allow a multiple loco consist the get over the detect points. Any crossing made

after this time would be taken as another sequence being started . The timer that is triggered first cannot time out until the other one is triggered by the train crossing the exit detector. Then both time out in the delay time set by the 2.2 ufd capacitor and the 1.0 meg-ohm resistor. Either timer being on will activate the flashing LED and the lights. The flashing LED is not symmetrical, on to off times, so a 100 ufd capacitor has been added to even it up.

Chapter-5d-2 Page 34

Construction:

The strip board is cut to a 16 row by 27 hole size. Temporarily number the front and back (copper side) then using the copper side diagram (below) put a spot on the ones (X marked) to be cut using an ink marker. Check carefully, twice, as it is too easy to misplace a mark. Check that 35 holes have been marked. When satisfied twist a No.31 drill in each hole to cut away the copper round the hole. Burnish the copper to clean any ragged bits and check with magnifier and/or meter that the copper round each marked hole is fully cut.

Link wires:

I use a single strand insulated wire for the links like Kynar wire wrap wire. Again mark the holes and double check before soldering the wires in. There are 19 wire links. Make sure that you have them correctly placed under the IC's.

Components:

It is best to start with the low profile resistors and diodes first, observe polarity of the diodes. Check each component against adjacent ones, links, etc before soldering. Add the Integrated circuits next. I don't solder pin 3 or 6 of the 4N25s as they are not used. Some 4N25's have an internal connection to pin 6. The capacitors and connectors go on last. The outputs are on PC screw terminals and the Spot inputs on 2 pin plugs.

Checking:

When all is soldered in place get out the magnifying glass and carefully check each copper strip for solder spillover to adjacent strips. Running a craft knife blade along the gap between the strips also helps to locate spillover solder. Installation: Cut the Isolated Track sections where you have calculated. Both spots are identical in connection. Once triggered the delays will not time out until 5 seconds after the exit spot is crossed. The positive leads from the LEDs go to the Sig-1 and Sig-2 terminals. The return from each pair goes to the terminals Retn-1 and Retn-2. .

To wire for double track make the two detector rail sections on each side of the crossing common, ie they are wired together. The both sets of rails must be in the same power block and on the same power buss. Test with a meter if not certain.

However if two trains pass on the crossing section then the logic will be confused and the light etc. will not work as expected

Option 1:

This could be used to control a servo circuit, described later, to raise and lower crossing arms. A signal is produced at the "Sw" and "Gnd" terminals that should be wired to the same named terminals on the servo driver. The terminals could be a 3 pin PCB Header or just wires soldered in.

Chapter-5d-3 Page 35

Components:

Resistors: 2K2 - 2, 10K - 2, 3 pin as required. Strip (Vero) board 27 holes x 16 rows

Symbol Value No. 150R 150 Ohms 1 390R 390 Ohms 1 470R 470 Ohms 2 560R 560 Ohms 2 2K2 2,200 Ohms 2 10K 10,000 Ohms 2 56K 56,000 Ohms 1 1M0 1,000,000 Ohms 2 2N2222A NPN transistor - 2 LM555N timer IC - 2. 3 band plus tolerance brown-green-brown orange-white-brown yellow-violet-brown green-blue-brown red-red-red brown-black-orange green-blue-orange brown-black-green

Option 2: A small buzzer can be inserted into the LED circuit as shown on the schematic to make a soft repeating sound that is a little bit like the electronic crossing chime now in use. It should be soft enough to be heard for only a few feet from the crossing. Connect it in series with the cathode return

1m0 - 2, Other: 470R - 2, 560R - 2, 56K - 1. 1N4004 diode - 2, Capacitors: 0.1ufd, 0.22ufd - 2, of, 2.2ufd electrolytic 25volt - 2. 'W04' Bridge Rectifier - 2, 100ufd electrolytic 25volt - 2. 4N25 optical coupler - 2, PC header Connector - 2 and 5mm Flashing LED 3-12 volt wire going to the terminal Retn-2

Components for buzzer option:

Resistors, 150 and 390 Ohms, Capacitor, 1000 ufd 16 volt working Diode 1N4004

Resistor Color Codes

1.5 volt piezo buzzer.

4 band plus tolerance brown-green-black-black orange-white-black-black yellow-violet-black-black green-blue-black-black red-red-black-brown brown-black-black-red green-blue--black-red brown-black-black-yellow