This page presents a 3 Light signal control circuit for model railroads. The circuit produces only solid - Red, Yellow and Green signal indications. The circuit can be wired for NORMAL and APPROACH type lighting of the Green signal.

  The signal circuit is designed to use a 12 volt power supply and drive light emitting diodes at approximately 10 milliamps. Other operating voltages and LED currents can be used by changing the values of the circuit's resistors.

  Each circuitboard has circuitry for 5 blocks of signals. The blocks are preconnected on the circuitboard but they can also be used individually. This allows extra blocks on one circuitboard to be used in a signal group with blocks from another circuitboard.

  The signal circuit can be controlled by any device that can pass a 1 milliamp current and has a common connection with minus terminal of the signal circuit's power supply.

1. The BOD inputs are low current and not voltage sensitive.
2. Signal blocks can have more than one input.
3. Each of the five block circuits on the PCB can be used separately.
4. Complex signal arrangements are possible using diodes at the BOD input.
5. The signal LEDs are connected common cathode.

  The circuit can also be used to control Search Light type signals.

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  The next diagram shows a schematic diagram and a truth table for 1 signal block. Any number of signal blocks can be connected in series.

  On the circuitboard there are 5, preconnected signal block circuits.

  This circuit can also be adapted to drive 2 Colour, Common Cathode Connected LEDs to make a Search Light type signal.

  NORMAL Type Lighting - means that the GREEN light is always lit unless the signal is changed to YELLOW or RED by one of the blocks other inputs.

  The next diagram shows a block diagram of the 5 signal block's on the circuitboard connected in a loop for NORMAL type lighting of the GREEN indication. A schematic for 3 blocks of the circuit is also shown.

  The YELLOW IN and YELLOW OUT connections shown between blocks 2 to 5 on the diagrams are preconnected on the circuitboard.

  The YELLOW PASS-BACK line on the diagram connects block 5 to block 1 to close the loop. For fully functioning signals, the track would be at least four blocks long.

  The signals are connected in a loop to show the GREEN IN & OUT and YELLOW IN & OUT connections that would normally be between circuitboards.

  APPROACH Type Lighting - means that the GREEN light is only lit when the next block is showing a RED signal or the signal is changed to YELLOW or RED by one of the blocks other inputs.

  The next diagram shows a block diagram of the 5 signals on the circuitboard connected in a loop for APPROACH type lighting of the GREEN indication. A schematic for 3 blocks of the circuit is also shown.

  The YELLOW IN and YELLOW OUT connections shown between blocks 2 to 3 on the diagrams are preconnected on the circuitboard.

  The YELLOW PASS-BACK and the GREEN PASS-AHEAD lines on the diagram connects block 5 to block 1 to close the loop. For fully functioning signals, the track would be at least four blocks long.

•   The main advantage of the PNP based circuit versus the NPN based Simple - 3 Light Signal Circuit is that the PNP circuit's BOD inputs can operate at a voltage of up to 3 volts while the NPN - BOD inputs must be below 0.2 volts for reliable operation.

•   'Normal' or 'Approach' type lighting for the GREEN signal is determined by the placement of jumper wires on the circuitboard.

•   The combined currents from a DETECT INPUT from one block and the YELLOW INPUT from the previous block is about 1 milliamp. The low current requirement allows the PNP signal circuit to be controlled directly by optoisolators.

•   Diodes D1 and D2 in the base circuit of transistors Q1 and Q2 provide an extra voltage drop that allows the transistors to turn off completely. A diode is not needed at the base of Q3.

•   Diodes can be used to separate block input devices so that one BOD or toggle switch could control several signals. Also, more than one block can be controlled by a single input device. There is a diagram for this farther down the page.

•   The signals can be controlled directly by the dispatcher using toggle switches. In this case no occupancy detectors would be needed.

•   The values of the current limiting resistors for the LEDs can be adjusted to achieve the desired brightness. If a LED is too bright, a resistor can be added in series with it without having to change the resistor on the circuitboard.

•   This circuit provides only solid signal indications. The signals could be made to flash by using an external oscillator and placing an NPN transistor in the cathode circuit of a LED.

•   This circuit can also be adapted to drive 2 Colour, Common Cathode Connected LEDs to make a Search Light type signal.

  The PNP - 3 light signals can be controlled by many types of input devices. This includes most Block Occupancy Detectors designs, toggle switches and the outputs of computer system interface cards.

  For BODs with open collector, transistor outputs, the signal circuit and the BOD can use the same power supply and will have a common connection at the minus of the power supply.

  Computer system interface cards usually have open collector - transistor outputs. The interface card can have its own power supply but will need a common connection at the minus of the power supply of the card and the signals circuit.

  BOD's and interface cards with optoisolator outputs can be used directly but will not share a common power supply connection.

  The signals can controlled by a dispatcher by using toggle switches to control the signals as shown on the block diagrams. In this case no BODs would be needed but they could be used.

  See the Using Isolating Diodes At The Occupancy Detector Inputs section later on this page for more BOD input information.

Connecting the VT-5 DCC Occupancy Detector To The PNP Signals Circuit

  More information on this topic will appear here.

  There are 5 signal blocks on each circuitboard. Normally they are connected together for use for a long section of track but the board can be separated into as many as 5 individual sections for shorter tracks.

  The sections could be used in two and individual sections for EAST and WEST signals at the meting of two blocks.

  There are lines on the back of the circuit board to indicate where these cuts can be made.

  The next diagram shows how diodes can be used at the BOD inputs of the signals to provide more complex signal schemes. The diodes allow different input devices to control more than one signal while isolating the BODs from each other during normal operation.

  In the diagram, blocks 1, 2 and 3 are forced to RED when S1 is closed. When S2 is closed, blocks 2, 3 and 4 are forced to RED.

  A possible use for diodes at the BOD inputs is at a rail crossing or interlocking where tracks that do not have the right-of-way would have their signals held at RED by the dispatcher until the blocks are clear.

  A diode matrix circuit could be used to create complex signal control systems.

PNP - 3 Light Model Railroad Signal Circuit Page.

PNP - Search Light Model Railroad Signal Circuit Page.

Bipolar LED - Search Light Signal Circuits Page.

Signals Circuits Comparisson Page

Colour Shifting for Bipolar LEDs

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  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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11 November, 2011