This page is for a circuit for 3 Light type model railroad signal. The circuit can produce only solid - Red, Yellow and Green signal indications. The signals can be connected for NORMAL or APPROACH type lighting of the Green signal.
The circuit is designed for a 12 volt power supply and will drive light emitting diodes at approximately 10 milliamps. Other supply voltages and LED currents can be had by changing the values of the circuit's resistors.
Each circuit board has two complete signal circuits. The circuits can be used individually allowing a block from one circuit board to be used in a signal group with blocks from other circuit boards.
The signal circuits 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.
The following diagram shows block diagrams that are used to represent the circuits schematic in most of the diagrams on this page. The block diagrams reduce the complexity of the connection diagrams for the signals.
NORMAL Type Lighting means that the GREEN light is always lit unless the signal is changed to YELLOW or RED by one of the circuits other inputs.
The PNP signals circuit board is designed for NORMAL Green lighting without external wiring as Terminals 3 and 8 are connected on the board so that the GREEN input is always LOW.
The YELLOW PASS-BACK line on the diagram connects block 4 to block 1 to close the loop.
For fully operational signals, the track would be at least four blocks long.
Any number of signal blocks can be connected in series.
APPROACH Type Lighting means that the GREEN light is only on when the preceding block is showing a RED signal. The Green signal is on when Terminal 3 is LOW.
To configure the circuit board for APPROACH Green lighting a trace on the back of the board that connects terminals 3 and 8 must be cut. This makes the number 3 terminal the APPROACH Green input.
The YELLOW PASS-BACK line on the diagram connects block 4 to block 1 to close the loop.
The GREEN PASS-FORWARD line on the diagram connects block 4 to block 1 to close the loop.
For fully operational signals, the track would be at least five blocks long.
Any number of signal blocks can be connected in series.
The power supply and common connections for each block circuit are joined on the circuit board but can be separated if needed.
The values of the current limiting resistors for the LEDs can be changed to achieve the desired brightness.
If the signal LEDs are too bright, external resistors can be connected at the circuit board's output rather than replacing resistors R1, R3 and R5 on the circuit board.
If the signal LEDs are too dark, resistors R1, R3 and R5 can be replaced with resistors of lower value.
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.
The combined current from a DETECT INPUT for 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.
The PNP circuit's BOD inputs can operate at a voltage of up to 3 volts.
Approach type lighting for the GREEN signal is selected by breaking one of the traces on the circuit board and connecting terminal 3 the the BOD input of the previous block.
The signals can be controlled by a dispatcher by using toggle switches. In this case no occupancy detectors would be needed.
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.
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 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 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 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 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.
| Qty | Circuit Part Number | Part Description | Digi-Key Number | |||
| 6 | - | Q1, Q2, Q3 (Times 2) | - | IC TRANS PNP SS GP 200MA TO-92 | - | 2N3906FS-ND |
| 10 | - | D1, D2, D3, D4, D5 (Times 2) | - | DIODE SWITCHING 75V 500MW DO-35 | - | 1N4148DICT-ND |
| 6 | - | R1, R3, R5 (Times 2) | - | RES 1.0K OHM 1/4W 5% CARBON FILM | - | 1.0KQBK-ND |
| 6 | - | R2, R4, R6 (Times 2) | - | RES 22K OHM 1/4W 5% CARBON FILM | - | 22KQBK-ND |
| 2 | - | C1 (Times 2) | - | 1.0uF 50V Radial capacitor | - | P5174-ND |
| 2 | - | Terminal Blocks - 2 Position | - | TERMINAL BLOCK 5MM 2POS PCB | - | ED1623-ND |
| 4 | - | Terminal Blocks - 3 Position | - | TERMINAL BLOCK 5MM 3POS PCB | - | ED-1624-ND |
There are 2 signal blocks on each circuit board. The circuits operate independently so that one circuit could be used for and EAST signal and the other circuit for the WEST signal.
The 12 volt supply and common terminals of each circuit are connected together on the circuit board.
If needed the circuit board can be cut in half to provide two separate and fully functional signal circuits.
There are marks on the back of the board that indicate where the separation can be made.
The diagram shows two 12 volt supply lines, this was done to unclutter the diagram.
The diagram shows both blocks on the circuit board being used for the same direction. The circuit board could also be connected so that each circuit was for the opposite direction.
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.
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 crossing is clear.
A diode matrix circuit could be used to create complex signal control systems.
This circuit can also be used to control separate Red and Green LEDs to form a two aspect signal.
This circuit can also be used to control bicolour, common cathode connected LEDs.
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.
17 March, 2013