RAILROAD SIGNALS of the U.S.

 

MODEL RAILROAD SIGNAL DRIVER
by Michael Watnoski

 

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Michael does a good job designing stuff.  This signal driver he designed can drive any standard three aspect color light signal, B&O CPL, or Pennsy PL.  He has these drivers installed in a rather large private HO pike, and has experienced very few problems with them over the past seven years.


Signal Driver Overview

This board is intended for use with model signal systems using a common anode system of  3 color LEDs in each head or in the case of PRR Position Light (PL) 3 series LEDs in each aspect.  It also has the ability to drive one other LED, such as the single red LED in another head on an absolute signal or the qualifier in B&O Color Position Light (CPL) signals.  The circuit is designed to permit each more restrictive signal indication to override any more permissive indication.  The default aspect is Clear.

Mechanical Specifications

This board is constructed of fiberglass reinforced resin, 0.062" thick.  Overall size, without connectors is 1.08" x 3.20".  The board has a black soldermask and white silkscreened legend.  Edge card fingers are on 0.1" centers and intended for use with Sullins EZC10DREH or equivalent.  Outputs to the LEDs on the signal mast are on a 5 contact Waldom/Molex KK type connector with 0.1" centers.  All exposed pads and edge card connections are plated with tin/lead.

Circuit Description

This board is designed to operate from a 12 volt regulated power supply. 

All inputs are active low - that is, a short (or connection) to ground will operate the function.  Diode isolation is provided to allow multiple devices to be connected to the same electrical signal without interference.  Pull up resistors of 10K ohm are provided on the board to insure unconnected inputs will return to a known state.

All logic devices are 4000 series or 74C series CMOS.

An enable input is provided to allow approach lighting of the signal.  Grounding the \EN input on pin 6 allows the LEDs to display the selected indication.  If approach lighting is not desired, the enable jumper on the board should be installed.

The red, yellow, and green LED outputs are open collector transistors capable of driving up to 500 mA each at 40 volts with an external power source with the proper selection of series resistor.  Pin 1 on the connector, is typically used to provide power to the signal head. Normal operation is to select resistor values to match the intensity of each color LED.  Nominal values are installed to match many common LEDs.

The marker is connected directly to ground when the jumper is installed in the MKR position.  The enable jumper should also be installed when using this option.  When in the CPL position, it is driven by the output of the CD4069UB and is limited to a total of 3.5 mA divided between the LED output and the \MKR output pin.  The CPL output can be selected for absolute or permissive operation.  In the absolute mode, the marker is blanked when the main head is displaying a red stop signal.  The permissive mode allows the marker to remain lit.  In either mode, the marker is blanked when the signal is not enabled unless the home input is low.

Installation and Operation

In general usage, the mating Molex connector housing is loaded with pins attached to the signal head.  A 12 volt supply is furnished on pin 1 for common anode signal heads.  Green is wired to pin 2, yellow to pin 3 and red to pin 4.  A CPL qualifier light or fixed red signal head can be connected to pin 5.

The input edge card connector is wired with signal system ground on pin 9, and the 12volt regulated supply on pin 10.  The enable pin 6 can be jumpered to the ground pin 9, (or the enable jumper on the board can be installed), if this signal does not require approach lighting.

The \HOME pin 2 is connected to the output of the current detection circuit of the block protected.  The \Rin inputs on pins 3 though 5 can be used to force a red stop indication whenever they are low.  Note that these inputs will not force the display to be enabled like the \HOME input.  These inputs should be connected to anything that should force a red display, like a turnout set against the direction of travel or optical detectors at the ends of the block.  The \DIST line is usually connected to the \Rout line of the next signal, or the current detector protecting the following block.  This will cause the signal to display a yellow approach indication whenever the next block is occupied.

Below is a photograph of the circuit board, and below that is the schematic.
Connections to the signal are on the left, inputs are to the right.

For additional info on this signal driver, email Michael at: freestatesystems1@comcast.net


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NEW 7/8/2007
Last Modified
: 08-Jan-2014