Features:
1. DC or DCC compatible. Uses just two external detectors per track. One for the bi-directional approach section, one for the island section. Compatible with any open collector style detector, or may optionally be driven from an accessory decoder. Warning! If driving from an accessory decoder such as a DS-54 that outputs more than 5VDC, you must use a series diode to prevent the external voltage from driving into the CGC board. (Connect the cathode to the accessory decoder.) I suggest using a current detector such as the RR-Cirkits BOD for the approach section, and an I-R spot detector for the island section. (That way you will not be required to add resistive wheel sets to all your cars.)

2. Supports up to 3 tracks per single board. The two "expansion" sections on each board may be used to add two tracks on another crossing controller board if they are unused on the first board. I.e. If you have two crossings, one with just a single track, the unused 2 track expansion section of the board controlling the single track crossing can be used to expand the second crossing controller for a total of 5 tracks on the second crossing. Total expansion is limited only by your imagination.

3. Fully prototypical action. (Older prototype based on fixed block boundaries, not modern pulse coded speed detection units.)

4. Controls flashers at the federal minimum rate of 48 impulses per minute or greater. A controller is in service on the LM&PH. Here is a 15 sec. MPEG clip of the gates in action (352Kb MPEG file) The clip starts just as the engine has entered the approach block. (The fire tanker truck driver was tempted to cross, but fortunately, thought better of it.)

5. Delay before gate lowering. (includes an onboard driver circuit for a 4VDC stall motor actuator) (See Note 2 below.)

6. Bell sound IC and amplifier included on the controller board. (uses ICCT DM320 IC)

7. Multiple options available for bell sound cutoff. Cutoff may be on gates down, ring on gates rising as well as lowering, cutoff as train enters island section, cutoff in 10 sec., 20 sec., 30 sec., or continuous for flashers at crossings without gates.

8. R-C timing circuits allow for user modification of timings if desired to suit your layout and operations.

9. Flashing circuit (when jumpered for common anode LED operation) includes a realistic fade of brightness for each lamp to simulate the slow fade of prototype lamps caused by their filament soft start circuits. (soft start action not available for common cathode LED connection) See movie clip above to watch fade effect.

10. Flasher lamp driver options allow for driving common cathode LED, common anode LED, or provide open collectors for driving incandescent lamps.

11. Optional fast cutoff of bell and flashers after passing of train if no gates are being used.

12. A simple button in parallel with the island detector will raise gates during extended station stops. Gates will not lower again until the train actually enters the island section. (or they may be manually lowered by holding down the same button) For this type of operation I would suggest using a current detector with a lengthened island section. Another option would be two parallel IR detectors. The second one placed to detect the engine as it first pulls away from the station.

Schematic:
Link to schematic gif. (53Kb)

Circuit description:
Each track has it's own logic to determine if the gate should be up or down. (for each track) The actual gate control circuitry lowers the gate if any of the track circuits calls for lowered gates. Occupancy inputs are active low. Unconnected inputs are treated as empty tracks. If both blocks (approach and island) are unoccupied, then that tracks logic is locked into the clear status. This allows for proper operation if a train approaches, then reverses out of the approach section without ever crossing the grade. Further, if the island is occupied, then the gates are locked down. This will re-lower the gates even if a train has crossed the island, then backed up to "clear" the crossing, and then later re-enters the island area. Normal operation first sees a train entering one end or the other of the approach block. This block is normally sized such that a train enters about 20 seconds before it crosses at grade. (800-1200' on either side) Let us look at the logic for track 1. Block 2 (S1-9) goes low. This unlocks the latch U2, and then clocks it via the exclusive-or gate U1 pin 3. At the clock U2 pin 1 goes high because the input "D" was high, indicating that the gates should lower. Once the train enters the island area (block 1 S1-10) it resets the clock pulse, and also locks the output of U2-1 high. When the end of the train clears the island, it first unlocks the latch, then the clock pulses again. This time the "D" input is low, so the output goes low again to raise the gates. Once the train clears the area and the approach block occupancy clears, it locks the latch low.

When any one of the three track logic outputs goes high, then the information is passed to U1-8. That signal unlocks the latch controlling the flashers, which immediately start up. (at the next clock pulse from U3-3) Controlling the flashers in this manner results in perfectly symmetrical flashing. The outputs from U2-12 and U2-13 are fed to the R-C network formed by R11-C6-R19 and R12-C5-R18 to drive the emitter follower output transistors Q1 and Q2. This results in a ramp effect on the output to the LEDs and nicely simulates the soft start circuits in the prototype. (Note that this only will work for common anode wired LEDs.)

Meanwhile back at the gate driver, the R-C network formed by R25 and C15 delays the lowering of the gates for a few seconds. Feedback from R26 ensures a clean transition.

U7-8 and U7-9 assure that the bell sound has the option of continuing from the start of the flashing cycle to the end. Jumpers J3-J9 provide various options for bell cutoff, and R34 allows you to set the volume to a comfortable level. This circuit uses the DM-320 sound chip from ICCT which is a digitized recording of an actual crossing bell, not a synthesized version. Feedback of the gate position via S2-3 allows for optional bell sound cutoff exactly in synchronization with the gate position. Cutoff can be final at the point when the gates are down, or optionally can also sound again as they rise. (this depends on your prototype) Of course if you are only using the controller to properly drive your cross bucks at a rural crossing, then you may leave the gate controls unconnected. A jumper then allows the bell sound to be continuous with the flashers, and cutoff as soon as the train clears. (without any delay for the gates to rise)

I hope this little animation circuit brings you lots of enjoyment.

Note:
The BOD-H PC board can also be used to build an IR transistor based optical sensor suitable for the island block. The advantage of this is that you can immediately achieve near correct operation of your grade crossing controller without the expense of installing resistive wheel sets on all of your cars. (Of course you will do that sooner or later for your full signaling system.) <grin> If you do use an IR sensor you can not expect 100% correct operation with a backing train, and you will have a compromise on the island size. It will be a point rather than the correct 50-100' short block.

Note 2:
This board was originally designed to operate a 4V gate motor. To drive a Tortoise or similar 10-12 VDC motor requires a simple modification. A second 5V regulator is added to boost the voltage on the motor driver chip. (See schematic)

Status:
The Rev F-4 CGC unit is currently shipping. The boards are now drilled to allow the easy addition of the 10V option. Link to assembly instructions. Link to parts drawing.

Cost for a bare CGC PC board is $8.67, and the full kit including the sound chip, speaker, and connecting cables is $54.91. (quantity one with discounts starting at two units) Occupancy detection is extra, because it will vary with your situation, but figure about $12-15 per track. (two detectors per track) An expansion kit for those with more than 3 tracks at one grade crossing is also available. (A second CGC board will act as a three track expansion board by using just a minimum of parts.)