Since Digitrax released their DS44 stationary decoders, I have been using them to control the Tortoise switch machines I use, using the Digitrax DT400 throttle. I had no problems using this set-up and re-used them in all of my various layouts. However, there are several limitations with this approach. First, some operators find it overwhelming to push buttons on the throttle. Secondly, only Digitrax' DT400 can control these accessory decoders, so their simple throttles are useless on my layout. Third, I recently invested in the Stanton/NWSL S-CAB system, and I am switching my layout over to battery-powered radio control. Accessory decoders cannot be controlled via the S-CAB throttle, so I finally decided that my approach needed to be changed. I wanted a simple momentary push-button interface on my layout's fascia to toggle the position of the Tortoise. I also wanted to add an LED to indicate the turnout's position. I eventually found a design on Rob Paisley's web site. It shows how to use a LM556 timer integrated circuit to control the polarity to a Tortoise (or any other) slow-motion switch machine. Various designs are presented on the page. I decided that the "Alternating Input Circuit" one was the one I wanted to build. The diagram below is a higher-level component design of the whole thing I want to build. It is only showing the things needed to control one Tortoise, but the board comes with four separate control circuits, allowing up to four Tortoises to be controlled.
I decided to order the circuit board and parts that he offers for sale on that page. This is what I got. I have 12 Tortoises to control on my layout, so I ordered three packages, where each can control up to four Tortoises.
The "Alternating Input Circuit" that he has on his web site does not use the 4.7uF capacitors that come in his package, so I just put those in my parts box. The circuit I am going to build requires some additional parts. You can get those at Digikey, but I bought mine at AllElectronics. The extra items I ordered are as follows: 14-pin IC socket (ICS-14); nylon spacers (SP-58); 100k ohm resistors (291-100K); 0.47uF capacitors (RMC-474). The 1N4148 diodes needed I already had.
I soldered the 10k ohm resistors to their respective spots on the circuit board. Next, I soldered the IC sockets to the board (I don't like soldering ICs directly). The 0.47uF capacitors are quite big, so I had to come up with a creative way to install them. Since they are not part of the basic circuit, there is no spot for them on the circuit board. I soldered one leg of the capacitors to the input holes on the board. I soldered one capacitor on the component side of the board, and the next one on the trace side of the board, each individual input trace needs one capacitor, so a total of 8 of them. Before actually installing the capacitors, I drilled a hole right in the center of the "R" and "N" texts next to each output trace. This fit the capacitors perfectly. The 100k ohm resistors are next, and they are a bit more of a challenge. I soldered those as best I could between the traces on the board to which they need to be connected (the output of the ICs, and the external legs of the capacitors). Finally, the 1N4148 diodes are soldered to the external legs of the large capacitors. Four wires stick out of the board, which is to where one leg of the momentary pushbutton switches are soldered.
This photo shows how I built the second and third iteration of the circuit. Staggering the capacitors like this made the whole board look nicer. The 100k ohm resistors are soldered under the board (trace side). This board only had three control circuits installed because the parts for the fourth one were needed on the other side of the layout.