I'm going through some back issues and in the August 1980 issue are the plans for the optimized Twin-T Detector. I want to build a couple to see how well they work for me.
However, in studying the circuit diagrams the Twin-T Detector only detects (led turns on) when the block is empty. The Twin-T Detector does NOT detect (led turns off) an eastbound, westbound or sitting train. One of the electronic engineers at work looked at it and saw the same thing!
Isn’t this the opposite of the way it's supposed to work?
I'm not the sharpest knife in the drawer when it comes to electroincs so maybe someone can explain how it actually works or how it's supposed to be built?
Or maybe a correction was made in a future issue?
Thanks!!
Vince
Hi, Vince,
Although I wasn't on staff 36 years ago when that article was published, I'll do my best to figure out the circuit so I can answer you. The "Load" in the basic circuit diagram in that article (Figure 1) is NOT a block occupancy indicator. It's there because for the transistors in the circuit to work, current has to be flowing regardless of whether there is a train in the block or not. When there isn't a train, current flows through the load instead.
Read the section of the article labeled "Lamps, LEDs, Relays" on page 83. It tells how to connect various things to the Twin-T to be controlled by it. As it mentions, Fig. 11 and 14 show how to use the Twin-T to power a bulb or LED that lights when the track is occupied. Figures 12 and 13 show how to connect a bulb or LED that turns on when the track is clear. (One use for such a configuration would be to power both leads of a bi-color LED that would light red for one indication and green for the other.) And figure 15 shows how to connect a relay that could be used to power the motor for a grade crossing gate or the like. These lamps, LEDs, and relays are in addition to the "Load" in the basic circuit.
I hope this clarifies the article for you. If not, hopefully somebody else on the Forum has built this circuit in the past. Happy railroading!
--Steven Otte, Model Railroader senior associate editorsotte@kalmbach.com
MBVince I'm going through some back issues and in the August 1980 issue are the plans for the optimized Twin-T Detector. I want to build a couple to see how well they work for me. However, in studying the circuit diagrams the Twin-T Detector only detects (led turns on) when the block is empty. The Twin-T Detector does NOT detect (led turns off) an eastbound, westbound or sitting train. One of the electronic engineers at work looked at it and saw the same thing! Isn’t this the opposite of the way it's supposed to work? I'm not the sharpest knife in the drawer when it comes to electroincs so maybe someone can explain how it actually works or how it's supposed to be built? Or maybe a correction was made in a future issue? Thanks!! Vince
It's not uncommon for electronic circuits to be "active low" meaning when somethign is happening, the output is at or near ground potential (usually 0 volts), which is what this particular variation of the Twin-T is. So the LED is on when there is nothing in the block and goes out when a train enters. Some signal circuits can handle this directly, or a computer interface like C/MRI. If it can't, it's relatively simple to add an additional transistor on the output to invert the signal so you get a positive voltage when the block is occupied and nothing when it is empty.
What this circuit is doing is simulating how the real railroad detection worked. The rails were energized from Edison batteries installed in battery cellers near the relay cabinets. This pulled up (literally - the relays were 'upside down' with the coil on top and the contacts below. Railroad equipment MUST be reliable and fail-safe, lives are at stake. Unlike typical relays you may be familiar with, they don't rely on a spring to move the contact, they rely on the weight of the contact. No magnetic field, the contact falls by gravity and activates a detection, or lights the red signal - so if the system fails, you get a red, not green). When a train comes along, it shorts the rails which drops the battery voltage, causing the relay to drop out. It's explained with diagrams here: https://en.wikipedia.org/wiki/Track_circuit
The Twin-T circuit was revolutionary for its time (that 19080 version is much updated from the Linn Westcott original from the 1950's. At the time it was originally developed, transistors that could handle track power had just become affordable - check June, July, and August 1958. They were $1.50 for each one - that would be over $12 today.
The downside of DC detection is you need to supply some sort of bias current to detect standing trains -the track power is off and/or the block disconnected. With command control, and later DCC, there is always power in the rails, so stopped or standing trains can be easily detected by either voltage drop across diodes, or with a current sensing coil, which is preferred sinc eit has no effect on the running power for the trains. They are also much more sensitive than the old Twin-T, so you cna use resistor wheels with relatively high resistence so there is less power lost because of all the resistor wheels in parallel. I use 10K resistors in the wheels of my cars, at 15V DCC power, that's 1.5mA per car, almost nothing.
--Randy
Modeling the Reading Railroad in the 1950's
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