This circuit controls a single head Searchlight, or Target, type signal and can produce a solid - RED, YELLOW or GREEN indication. The signals can be configured for NORMAL GREEN or APPROACH GREEN lighting of the signal.

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

 - APPROACH Lighting means that the GREEN signal is lit only when the block before a particular signal is showing a RED signal. The GREEN signal is on when GI Terminal is LOW. The YELLOW or RED input functions are the same as for Normal GREEN lighting.

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• NORMAL and APPROACH GREEN type lighting of the signal.
• The detector and aspect inputs are low current and not voltage sensitive.
• Complex signal arrangements are possible adding diodes at the inputs.
• Signals are priority sensitve: Yellow overrides Green and Red overrides Yellow.

• Red, Yellow and Green - 1 aspect signals.
• The outputs can control other devices such as relays.
• Can be used with common cathode or common anode, three lead LEDs.

  This circuit uses a LM556 - Dual Timer IC in a complimentary output configuration to drive a red and green, bipolar type light emitting diode to produce the signal's three colours.

  The following schematic shows one signal circuit schematic with all of the components needed for Normal or Approach Green Lighting of the signal.

  This following diagram shows five signal blocks using complete circuits.

  Complete circuits can be used for either NORMAL or APPROACH Green lighting of the signal.

  Any number of blocks can be connected together.

  If Approach Lighting is not needed, the parts associated with this can be left off of the circuit board and Q1 replaced with a jumper.

  This following diagram shows five signal blocks using partially assembled circuit boards.

  Partially assembled circuit boards can only be used for NORMAL lighting of the GREEN signal.

  Any number of blocks can be connected together.

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

•   The circuit is designed for a 12 volt power supply voltage and will drive light emitting diodes at approximately 10 milliamps. Other supply voltages and LED currents can be used by changing the values of certain resistors in the circuit.

•   The values of the current limiting resistor for the LEDs - R5 can be changed to achieve the desired brightness from each signal LED.

    - If the signal LEDs are too bright, an external resistor can be connected at the circuit board's outputs rather than replacing resistors R5 on the circuit board.

    - If the signal LEDs are too dark, resistor R5 can be replaced with resistors of lower value.

•   The combined current from a DETECT INPUT of one block and the YELLOW INPUT from the previous block is about 2 milliamps. This low current allows the signal circuit to be controlled directly by optoisolators.

•   The circuit's BOD input will operate correctly with voltages of up to 2 volts at the inputs.

•   More than one block can be controlled by a single input device if diodes are used to separate input devices such as occupancy detectors, toggle switches and computer control systems.

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

    Toggle switches can also be used in conjunction with block occupancy detectors.

  The 556 - Searchlight Signal Driver circuit can be controlled by many types of input devices including most Block Occupancy Detectors designs as well as 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 must have a common connection at the minus of the power supply.

  Computer system interface cards often 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 and do not need to have a common power supply connection.

  A dispatcher can control the signals by using toggle switches to control the signals as shown on the block diagrams. In this case no occupancy detectors would be needed but they could be used in conjunction with the switches.

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

  The 556 - Searchlight Signal Driver circuit can also be controlled by the VT-5 Block Occupancy Detector that can be found at this site.

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 1 - 556 - Searchlight Signal circuit board without parts is 12.00 dollars US each, plus postage.

  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: Search Light Signal

  The next diagram shows diodes being used at the BOD inputs to allow more complex signal schemes. The diodes allow separate input devices to control more than one signal while isolating the BODs from each other during normal operation.

  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 STOP by the dispatcher or interlocking until the crossing is clear.

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

  This circuit can be used to determine the values for R3 and R4 that will produce the best YELLOW indication for a given LED. The circuit lets the user customize the circuit to their preference for the YELLOW indication before constructing a set of signal drivers.

  A variable resistor can be used for R4 to find the best value.

  For more information on the timer used for this circuit see second circuit on this diagram and the 555 Timer web page.

PNP - 3 Light Model Railroad Signal Circuit Page.

556 - Bipolar LED - Searchlight Signal Circuits Page.

Colour Shifting for Bipolar LEDs

  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.

13 June, 2021