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This is a Current detector using the voltage drop across a double diode (Bridge Rectifier) to develope 1 to 1.2 volts when current is flowing through the tracks. This voltage is enough to supply the LED in the opto coupler type 4N35 or 4N25. The output of this goes to 0 volts and drives base current thru the transistor which supplies drive to the relay. This is picked with a train on the tracks or other power consumer across them.
Sensitivity with a 4N35 opto coupler is greater than 10K ohms, somewhat less with a 4N25. The 1.2 volts lost in the diodes does not effect the train if the power source is a t 16 volts but may show if it is only 12 volts DCC. The 1ufd capacitor gives a 2 second drop out time to avoid false indications because of dirty track.
This is a Current detector using the voltage drop across a double diode (Bridge Rectifier) to develope 1 to 1.2 volts when current is flowing through the tracks. This voltage is enough to supply the LED in the opto coupler type 4N35 or 4N25. The output of this goes to 0 volts and drives base current thru the transistor which supplies drive to the relay. This is picked with a train on the tracks or other power consumer across them.
Sensitivity with a 4N35 opto coupler is greater than 10K ohms, somewhat less with a 4N25. The 1.2 volts lost in the diodes does not effect the train if the power source is a t 16 volts but may show if it is only 12 volts DCC. The 1ufd capacitor gives a 2 second drop out time to avoid false indications because of dirty track.
LINK TO IMAGE - dblock.gif
Circuit Schematic
