3 Light Signal Circuit
This page presents signal driver circuits that are simple, efficient and inexpensive to build. These circuits provide only solid - RED, YELLOW and GREEN signals.
The circuits are designed to use 12 volt power supplies and drive Light Emitting Diodes. The circuits can be controlled by block occupancy detectors that have a output voltage drop of 0.2 volts or less.
The first circuit will control a typical 3 light signal head and provides the basic function of only one light on at a time.
There are three blocks shown on the schematic. In the first block, the GREEN light will always be on when the block is empty. In the second block, the GREEN light is only on when the RED light of the first block is lit. The third signal will be YELLOW when the second block is RED.
If APPROACH lighting is not desired the "GREEN IN" terminal of each block is connected to the circuit common.
The following is is a diagram of 5 blocks of the - Simple - 3 Light Signal Circuit - connected in a continuous loop. In the diagram a train is occupying Blocks 3 and 4.
Bellow the diagram, is a photo of test circuitboard for the same 5 block signal loop. To simulate Blocks 3 and 4 being occupied, the wires for blocks 3 and 4 are connected to the minus of the power supply.
This circuitboard is designed for 'Approach' type lighting but could be changed by cutting some of the traces and connecting the GREEN IN terminals to the common.
The next circuit controls a Search Light type signal head and provides solid - RED, YELLOW or GREEN indications.
This circuit uses an LM556 dual timer in a complimentary output configuration to drive a two colour, bipolar - light emitting diode in order to produce the three signal output colours. The 556 timer is more expensive than some other devices that could be used but is better suited to this application.
The circuit as shown, includes Approach Lighting. If this is not needed then the parts associated with Q1 can be left out and the RESET pins of the timers tied directly to the supply.
A version of the circuit using a 555 timer is also shown. The operation is the same as the 556 timer version.
Colour Shifting for Bipolar LEDs
The next circuit controls a Search Light type signal head and provides the basic function of only one colour showing at a time.
This circuit uses a common cathode, two colour - light emitting diode to produce the three signal output colours.
There is no "Approach" lighting scheme for this circuit.
These circuits have been tested at the work bench but have not been used in actual practice. It would be advisable to build three circuits and test them under actual conditions to see if they are suitable.
The diodes in the base circuit of the transistors are used to provide an extra voltage drop at the base of the transistors when the 'YELLOW OUT' and 'GREEN OUT' terminals are pulled to a low state.
Any block occupancy detector that has a voltage drop of less than 0.2 volts across its output will work with these circuits. This would include open collector transistors and voltage comparators.
If the output voltage drop of the block occupancy detector's output is greater than 0.2 volts, one or two diodes can be added to the "GREEN IN" terminals of the circuits.
The values of the LED - current limiting resistors may be adjusted to achieve the desired brightness or colour balance.
The circuits shown on this page have been designed to use as little power supply current as possible.
If optoisolators are used a transistor may have to be added to their outputs to achieve a low enough voltage drop to operate the common cathode LED signal circuit. See the following diagram.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
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.
18 February, 2009