If you are thinking about using this circuit, you should do some simple testing. Set up the bridge (D1) as shown and connect a standard LED in place of the pins 1 & 2 and see what happends. It may not need a resistor.
IC1 is a standard opto isolator and as such has no active output. When the LED lights, the darlington acts like a closed switch, otherwise it acts like an open switch.
If you want to have an output from it, you must provide the circuit with power. Any negative voltage should be connected to pin (3) Y and the positive side connects to pin (4) X.
A simple output circuit can be made by putting an LED in series with a 1k resistor and a 12vdc power supply. Use the output pins 3 & 4 as a switch in series with the test circuit.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Website: http://bobfrey.auclair.com
If you just use a resistor on the output, you will not get guaranteed logic level outputs - TTL, CMOS, and all the other logic families have a 'dead zone' between what voltage represents a 0 and what represents a 1. The comparator output will be all the way to the logic 1 or all the way to the logic 0 voltage as the input crosses the comparator's input threshold. This also makes it easy to adjust the detection sensitivity by using a pot instead of a fixed resistor on the comparator.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
LION sees the diagram. LIONS do not understand these new fangled circuits (vacuum tubes) and the ICs are beyoind all possible belief. LION will stick to relays. Things that go CLICK. THAT him can understand.
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
No, to post an illustration here you have to host it somewhere else and follow the forum posting procedure.
Joe
i guess images in google image searches aren't permanent
greg - Philadelphia & Reading / Reading
Greg
I don't see a circuit illustration.
Edit: Thanks I can see it now
i believe the circuit below is the basic DCC block detection circuit (yes a small valued resistor should be in series with the LED). I've seen this circuit connected to a block 10s of ft from the block being monitored. I've seen this circuit with the Darlington connected to various comparitors (339, 555, 7414 schmitt).
besides the obvious, to provide hysteresis, what is the advantage of using a comparator over a simple resistor?
what kind of circuits being driven by this detector output would require anything more than a simple level shift?
Wouldn't bringing the x and y connections back to the device using the detector output allow that device to condition the output as needed?