Between Fillmore Heights and Point Vincent runs a reversing loop section known as the "back lead." It is frequently used by the south end Havens Yard switcher as a primary switch lead.
Since it is a reversing section, trains can't simultaneously enter at both ends without producing a short circuit. Furthermore, since most of it is hidden track two short trains can meet each other with disastrous consequences.
It was decided to add dwarf signals at each end of the lead. These will operate automatically, normally green. When a train enters one end of the lead, both signals drop to red. They won't stop short circuits but can prevent a Promontory Point meet in the middle.
To control these signals I made a small circuit board containing an occupancy detector from Model Railroad Control Systems. This detector has a MOSFET output instead of an optoisolator, and it can drive a sensitive relay. So far I really like the detector. It is a more modern design compared to the classic Chubb-O-Matic detector which still works very well. The use of surface-mount components allows it to be much smaller than the C-O-M equivalent.
I had no difficulty adjusting the detector to sense a single 10K resistor. It can be purchased in two versions. The other one has a Molex female connector next to the sense transformer so it can fit into a standard C/MRI detector backplane card, or into a smaller unit offered by MRCS.
A small power supply allows it to be connected to any 12 volt source, even a Tortoise motor thanks to a bridge rectifier at the input. If connected to a Tortoise, an electrolytic capacitor keeps the detector working during the momentary power interruption when a controlling toggle switch is flipped. Frankly my days of wiring under the layout are limited, so I intend to install this in a convenient location and vampire power from the nearest Tortoise motor.
The red perf-board is from Sparkfun. Although it is a bit expensive, it can be easily snapped to desired size while clamping it in a vise and all holes are plated-through so they solder beautifully.
Maybe this is much ado about nothing, but the forums have been quiet lately.
