Information on ordering a commercial kit or assembled and tested unit of this circuit is available from the TracTronics Price List.
This is the first in a series of articles which will describe a set of electronic building blocks we have designed to control our model railroad layouts: Rich Weyand's N&W Pocahontas Division, Bill Pistello's Union Pacific, and the Reid brothers' Cumberland Valley System. These electronic modules are designed to be interconnected with each other to create a complete control system, but can also be used independently. The circuits have been designed to professional engineering standards, and are as bulletproof and reliable as we could make them. They are also easy to use: while there is a lot of techno-talk in these articles, you can just copy the diagrams and the circuits will work whether you know how they do it or not!
The circuits we have completed and will present in these articles are:
1) SwitchWitch, a twin-coil switch machine drive circuit.
2) SwitchLock, a stall motor switch machine drive circuit.
3) SwitchMap, a diode matrix control circuit.
4) BlockLock, a CTC panel block signal control circuit.
5) DetectTrain, a current-sensing train detector circuit.
6) SeeTrain, an infrared train detector circuit.
7) MasterFlasher, a high-current programmable-rate flasher circuit.
8) CoolerCrawler, a high-performance memory walkaround throttle.
We will present these circuits both as circuit diagrams and as circuit layout patterns, allowing readers to breadboard or etch their own circuits. For those who do not wish to breadboard or etch these circuits, both kits and assembled & tested units are available from TracTronics, 1212 South Naper Boulevard, Suite 119, Naperville, IL 60540 (708-527-0000).
Many thanks to our long-time friends Steve and Scott Ackerman of ACS in Sarasota, Florida (813-377-5775) for the excellent layout work on these circuit boards, as well as some very helpful criticism of the early designs.
The rest of this first article will talk about SwitchWitch, a twin-coil switch machine drive circuit, which we designed to get the Reid brothers out of a jam on their Cumberland Valley System.
The day finally arrives, the day you've worked so hard for, the day that makes all the work worth it: the first operating session. Your friends are over, their friends are over, a couple of well-known railroaders have stopped by, it's show time.
Thirty minutes into the operating session, it's over.
A loud electromechanical buzz and a cloud of acrid smoke from under the layout have terminated the proceedings, as the twin-coil switch machine on the yard throat turnout has died. The contacts on one of the switch machine control push-buttons, not built for the high current and inductive kick of the switch machine, have welded shut, permitting uninterrupted direct current to overheat and burn the switch machine. Without the yard throat, operation is impossible.
Two weeks later, the switch machine has been replaced, everybody is back, and you've gotten smarter. You've built a capacitive discharge supply to drive your twin-coil switch machines. Now if one of the push-buttons sticks, the capacitor recharge current is all that will get to the switch machine, and that isn't enough to burn the switch machine.
Problems arise soon enough. You used a large capacitor so that you could drive your diode matrixed yard ladder, but the ladder switch points still don't close solidly when five or six switches are fired. The capacitor is large enough to make a single switch being fired sound like a rifle going off, and two of your switch machines eject their iron cores onto the floor during the first hour of the operating session. You continue to operate around the problems.
Then it all goes dead. One of the push-buttons, under the added kick of the capacitive discharge unit, has welded shut again. The switch machine did not burn, but the stuck push-button is keeping the capacitive discharge supply from recharging, and now none of the switches work. The big problem is: which push-button on the panel is it?
You put the fast clock on hold and tell everybody to take a break while you and a friend begin checking all the push-buttons on the panel with a voltmeter. Everyone else dives into the munchies after only one hour of operation.
This last situation really did happen to Bill and Wayne Reid, and it happened when several well-known model railroaders had come to the Cumberland Valley System to operate. Once the yard panel pushbutton stuck, the fast clock went on hold, and the voltmeter came out. Bill and Wayne decided something had to be done, so as never to have this great embarassment again.
At this point you might be tempted to rip out all of the twin-coil machines and replace them with stall motor drives, or take out the capacitive discharge supply and let the darn things burn when a push-button fries. Bill and Wayne were considering both of these options.
We got them out of this fix with a little electronics. The circuit proved to work so nicely, we thought others might want to use it as well. The circuit, which we call a SwitchWitchTM (Figure 1) consists of a capacitive discharge unit and two switching circuits to drive the two coils of the switch machine, contained on a single 2" by 2" circuit card.
Understanding the circuit operation is not required to be able to build and use the unit, but for those who want to know:
Q2 and Q4 are the driver transistors which dump capacitor C1 through either of the switch machine coils, L1 or L2. Diodes D1 and D2 reroute the surge voltage of the coil away from the transistors when the transistors shut off.
Resistors R10 and R11 limit the base current through the transistors. Resistors R4 and R9 make sure that the transistor shuts off cleanly; they also add noise immunity to prevent noise on the input from actuating the circuit.
Finally, resistor R5 recharges the capacitor for the next time. Diode D3 is a safety item, to protect the circuit from accidental reverse wiring of the supply wires.
The circuit is placed near the switch machine and wired into the existing leads, with the leads from the push-buttons or the diode matrix connected to I1 and I2, and O1 and O2 connected to the switch machine coils. The switch machine coil common lead(s) are connected to the COM pads. Finally, V1 and GND are connected to the plus and minus leads of a 25 Volt DC power supply. Multiple pads for V1 and GND are provided in the board layout so that the power supply connections can be daisy chained from board to board under the layout. Figure 2 shows the connections to the circuit, including multiple control push-buttons if desired.
The circuit only draws 20 mA when recharging, and only needs 25 mA on I1 or I2 to actuate, so this circuit can be wired with 22 - 28 AWG telephone hookup wire. A 250 mA push-button can select 10 switch machines through a diode matrix without straining the contacts, and the diode matrix itself can be built with 1N4000 series diodes.
Since each switch machine has its own capacitor, the switch machine always gets the same energy, no matter how many other switch machines are actuated at the same time or by the same control push-button.
An enhanced version of the circuit is shown in Figure 3. This version includes a few more parts and allows the circuit to be driven by momentarily connecting the inputs I1 and I2 to ground, instead of to the positive side of the 25 Volt DC supply. This will allow the circuit to be actuated by open-collector TTL circuits, train detectors with open-collector outputs, and opto-transistors.
In this circuit resistors R10 and R11 are omitted. Instead the base current into Q2 and Q4 is provided by two PNP transistors, Q1 and Q3. Resistors R3 and R8 now limit the base current into transistors Q2 and Q4. Resistors R1 and R6 limit the base current into Q1 and Q3. Resistors R2 and R7 ensure that Q1 and Q3 shut off cleanly, and provide noise immunity.
This circuit is wired as before, but now the controls must momentarily connect I1 or I2 to ground to actuate the switch machine, as shown in Figure 4. In the enhanced version of the circuit, only a 7 mA current to ground is required from I1 or I2, allowing a 250 mA push-button to control up to 35 switch machines.
A particularly nice application of the enhanced circuit is to drive switches on modules, using two infrared opto-transistors camouflaged by foliage or ground cover on either side of the track. A modified penlight with an infrared LED can be used to actuate the switch machines without any control panel! Just connect the collectors of the two opto-transistors to the inputs of the enhanced circuit, and connect the emitters of both opto-transistors to ground. Also for you module guys: note that the very low supply current required by the circuit means four 6 Volt lantern batteries can run a whole layout.
If you don't want to drive your switch machines with other electronics, or if your switch machines are already wired with positive voltage through the controls and the common lead to ground, then you can use the simpler version of the circuit, save some money on components, and save some time rewiring the sense of the controls.
If you want to eventually run your switch machines with other electronics, or if your switch machines are already wired with the positive voltage on the common lead and the controls connected to ground, you should use the enhanced version of the circuit.
The circuit etch pattern and component placement diagram are given in Figure 5 for those who want to etch boards, rather than perfboard the circuit. All of the components and traces for both flavors of the circuit are included on the board; populate just the parts list for the flavor of board you want.
The capacitor C1 is allowed to stand on its leads above the other components at the top of the board. Please be very careful to install C1 to match the polarity indications in the component placement diagram; electrolytic capacitors will explode when power is applied if they are wired backwards.
The transistors Q2 and Q4 should be mounted with #6-32 x 1/4" machine screws and #6-32 nuts. Install the machine screws from the bottom of the board; the nuts would short the traces on the board if they were on the bottom.
For a harder snap than provided by the values given, a larger capacitor can be used, or a second capacitor can be placed in parallel with the first. For less snap, a smaller capacitor can be used. 1000 uF and 4700 uF capacitors are available from Radio Shack in an appropriate voltage range. The suggested value of 2200 uF given here was the right value for all of the switch machine installations we tested on the Cumberland Valley System.
A smaller value of R5 will speed up the recharge time of the circuit after it has been actuated, but if you use a value between 600 and 1200 Ohms, a 1 Watt resistor should be used, and between 300 and 600 ohms, a 2 Watt resistor should be used. This will keep the power dissipation of the resistor within its tolerances if a push-button sticks and R5 tries to recharge C1 indefinitely. A cheaper solution than a 1 Watt resistor is to use a second 1.2 KOhm 1/2 Watt resistor in parallel with R5. It can be soldered across the R5 traces on the back of the board.
The power supply we used on the Cumberland Valley System is shown in Figure 6. This supply will put out 25 Volts DC at 2 Amps continuously , which will supply a hundred of these switch machine circuits even if they are all actuated at once. Be sure to fuse the 110 Volt side of the transformer. If you are not experienced in wiring 110 Volt connections, get someone who knows what he's doing to help you!
We installed the basic version of the circuit on the CVS, because the controls and the diode matrix had already been wired for positive voltage to the coil leads with the common coil lead to ground, and there was no benefit to be gained by rewiring all of this. The yard ladders, the control panel, and the installation of the SwitchWitch units under the layout are illustrated in the photos. It's important to note that the units are located within a foot or two of the switch machines, to get the full kick of the circuit.
The SwitchWitch circuits have been installed on Bill and Wayne Reid's Cumberland Valley System for some time now. Everybody has been back to operate, without any yard ladder incidents. No switch machines have burnt up, no misfires of the yard ladders have occurred, and no iron cores have hit the floor.
And no more operating sessions have been interrupted!
