Peter's Model Railroading - Modeling Articles

Helix Elevator

I really couldn't come up with a better name than Helix Elevator, but this page describes how I designed and built circuits that detect a train's presence in the helix. The actual physical location of the train is shown through a set of vertical banks of LED's on the front of the helix.

Analysis

As part of my third layout, I had a 10-level helix the outside of which was be covered with 1/8" Masonite board. This means that for at least five to ten minutes of train travel I would not know exactly where the train was in the helix.

I have seen, and operated on, layouts where the owner has installed a closed-circuit television system for monitoring train movement in the helix. While I would have liked to have used that solution as well, I couldn't find space for the television screen. The room was just too small. Also, feedback on the train's position needs to be close to the helix for the interaction to be intuitive. A smaller, but still useful solution had to be found.

The idea was to mount two vertical banks of ten LED's to the front of the helix as shown in the diagram on the right. One bank monitored the front edge of the helix, while the other monitored the back edge of the helix. The LED's were positioned approximately at the physical level of the monitored rung in the helix. This system allowed multiple trains to be monitored, important since my helix was double-tracked.

(left LED covers front tracks, right LED covers rear tracks)

Design

The key piece of technology that made this concept a reality was the electronic circuit. I found several circuits on the Internet and built two of them. The one shown on the right was the only one I was able to get to work, and also turned out to be the simplest to build. The circuit was modeled after the one shown on this web site.

D1 is the infrared LED sending signals to T1, the photo transistor. For as along as light is received by T1, the indicator LED, D3, will remain off. As soon as the signal is broken, i.e. a train passes between D1 and T1, LED D3 turns on and stays on while the train passes through.

Note that since I positioned the infrared components across both tracks in the helix, I did not know which track was occupied. Also, since there was only one detection point at each head-end of the helix (per rung), I could only infer a train's movement by watching LED's light up and turn off. This was sufficient for what I wanted. Obviously more complex systems could be developed to monitor train travel direction and which track is being signaled.

By the way, I had considered implementing a 7-segment LED display (like those found in digital clocks) to numerically report on which level a detection occurred, but those circuits are complex to build. Also, because the display can only show one number at a time, if more than one level was triggered (i.e. two or more trains are in the helix at the same time), the highest number would win out.

Parts List

I had a 10-level helix, which meant there were 10 rungs on both the front and rear of the helix that were monitored. I therefore needed 20 independent circuits to cover the 2-times-10 detection points. The table below shows the total parts list. Note: I have listed the Radio Shack parts for your convenience, but I typically buy my components from Digi-Key. The circuits shown below were built with the parts from Digi-Key.

ID	Quantity	Part	Radio Shack	Digi-Key
R1,R4	40	680 ohm resistor 1/8W	11344827	680QBK-ND
R3	20	4.7k ohm resistor 1/8W	271-1330	4.7KQBK-ND
R2	20	10k ohm resistor 1/8W	271-1335	10KQBK-ND
D1	20	Infrared emitting LED (long lead is "+")	276-142	LN66A-ND
D2	20	1N4148 diode	11318615	1N4148TRTR-ND
D3	20	LED red, 3mm in socket (long = "+")	276-068	67-1147-ND
T1	20	Phototransistor (short lead is "+")	276-142	PN268-NC
T2	20	2N3904 transistor	276-2016	2N3904-ND
	20	Printed circuit board	276-149	6008CA-ND
	?	Wire and solder

Testing Components

I decided to build a simple circuit that could help me test the infrared components. The beauty of the circuit shown on the right is that it also detects signals from remote controls of your television or stereo! Aim your standard remote control at the circuit (at Q1) and you can verify that the circuit itself is working (LED D1 will light up when you press a button on your remote control). The design came from this web site.

I constructed mine using a 2N2222 transistor for Q2 (because that was what I had on hand), and for Q1 I used the phototransistor shown in the parts list above. Hook it up to a 9V battery and you're set!

Construction

My next step was to build printed circuit boards for the 20 circuits that I had to build. Although I have done this plenty in the past, the thought of etching that many boards wasn't too appealing. I had recently ordered the Digi-Key catalog and was thumbing through it one day when I stumbled across Surfboards by Capital Advanced.

These briliant little inventions are a big time saver. Cost-wise they are well worth it, when you compare the cost of PC board, etchant, and time spent etching. I decided that the 8-pin version (part number 6008) was just right for me, based on the prototype circuit that I had built. The component layout diagram below shows, in dark gray, where the foil sections are and, in red, where the components are to be placed. Note that D1, T1, and D3 were mounted off-board onto the helix itself.

The 8 pins on the board are used to make the external connections to the infrared components, the reporting LED, and the power supply.

(component layout)

(Surfboard 6008)

(completed circuit)

The photo below shows the proof - I really did build 20 of these. The troops are ready to march...

Conclusion

Being able to find a functional design on the Internet and then stumbling onto the Surfboards were the key things that made this project a success. See the second part of this article for information about how I installed the circuits in the helix.