Light Signals

Posted on Feb 6, 2014

A block diagram symbol is used to represent the PCB schematic on block connection diagrams. The following diagram shows the block diagram and the circuit that it represents. Any number of signal blocks can be connected in series. To be able to show all of the aspects the protected track section should be at least four blocks long. APPROACH - Typ

Light Signals
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e Lighting means that the GREEN signal is on only when the block after a particular signal is showing a RED signal. The Green signal is on when Terminal GI is LOW. The YELLOW or RED input functions are the same as for Normal GREEN lighting. 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 resistors for the LEDs - R4, R6 and R8 - can be changed to achieve the desired brightness from each signal LED. For example; - If the signal LEDs are too bright, external resistors can be connected at the circuit board`s outputs rather than replacing resistors R4, R6 and R8 on the circuit board. Diodes D1 and D3 at the base circuit of transistors Q1 and Q2 provide an extra voltage drop in the base circuit that allows the transistors to turn off completely. This diode is not needed at the base of Q3. 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 PNP signal circuit to be controlled directly by optoisolators. More than one block can be...

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