Rod Murgatroyd's Circuit Library


 Rev. 11/1/2009 

A DC Block detector, similar to a Twin-T.

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This is a modification of the Twin-T (Twin Transistor) detector developed by Bruce Chub many years ago. There are many such variations using IC's. This one sets up a voltage divider for the IC inputs which includes the diodes.
When there is a current consumer (train, resistor wheelset etc.) on the tracks then a voltage is developed across the twin diodes of 0.6 volts, positive or negative. This biases one of the IC amplifiers and it gives a low output. This is kept separated from the other and allows the circuit to determine which direction the train is going (link selected)
The output when a train is detected lights a display LED and picks the relay (voltage to match the supply) The circuit voltage return (Gnd) although common to the DC CAB buss has isolated outputs.
Sensitivity can be adjusted by 200R at the input, detect time is about half a second and drop out 2 seconds.

LINK TO IMAGE - det-g.gif
Circuit Schematic

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