This is the eighth in our series of articles describing 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. We are going to deviate from the outline we presented in the first article back in October 1993 Mainline Modeler, and put off the CoolerCrawler throttle article for a while. We have some additional signal control modules to present, which can be used for three color signals, searchlight signals, and signals using bulbs instead of LEDs. These signal modules can be wired to implement Automatic Block Signaling (ABS), Absolute Permissive Block Signaling (APB), and CTC signal control, including CTC with intermediate ABS signals and fleet mode operation.
This month, though, we have two small modules which can be used to upgrade your existing walkaround throttle. The first module blocks the operation of the reversing relay when the throttle setting is above some threshold value, to keep operators from accidentally reversing moving trains. The second module is an RC interface circuit which allows you to connect an available FM digital radio control unit to your existing walkaround throttle, saving you from replacing the whole throttle to make the move to radio control.
We are presenting 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, as well as the radio control transmitters and receivers, are available from Bill Pistello at MMRE, 625 South Princeton, Villa Park, IL 60181 (630-832-9152).
One problem with walkaround throttles is that it's easy to bump the reversing buttons when plugging and unplugging the tethered hand control unit. The resulting instantaneous reverse of the engines has put more than one long train on the ground, not to mention destroying the illusion we strive so hard to create. Additionally, when we go to radio control walkarounds, it's even easier to accidentally reverse a moving train, either by bumping the control while sorting car cards or the like, or through radio noise. We wanted a circuit that would prevent the state of the reversing relay from being changed on a moving train.
Most walkaround throttles have a control interface similar to that shown in Figure 1. The four wires of the tether are 1) positive voltage, 2) ground, 3) throttle control voltage, and 4) reversing relay lead. The throttle control voltage is held in the base unit by a capacitor, which is often set to decay at some slow rate so that an unattended train will gradually come to a stop. The throttle output is proportional to the voltage on the capacitor, although due to pulsing, momentum, and other signal conditioning it may not be strictly a linear function. The hand control unit is used to leak charge onto this capacitor from the positive voltage to increase throttle setting, and to leak charge off of this capacitor to ground to decrease throttle setting. This increase and decrease can be accomplished by push-buttons or by a potentiometer. Many hand control units also include an emergency stop button to short the capacitor to ground and bring the train to an abrupt stop.
The reversing relay is a latching relay, or a non-latching relay with a latching resistor as shown. The latching resistor gives the relay enough current to hold the relay on once it is on, but not enough to pull the relay on if it is off. The control unit is used to set or release this relay by momentarily connecting the reversing relay lead to positive voltage to set the relay, and momentarily connecting the reversing relay lead to ground to release the relay. This is the lead we want to disable when the train is moving.
The circuit we designed, which we call SafeTrainTM, is shown in Figure 2, with the component values and part numbers given in Table 1. The reversing relay lead from the hand control unit passes through the relay in the SafeTrain circuit, which is only powered by the op amp when the throttle setting is below the value set by the trimmer pot R4. In this circuit, diode D1 and capacitor C1 filter the positive voltage from the throttle base unit, which can be anything from steady DC to totally unfiltered DC depending on the throttle, and provide the power for the op amp. The diode D2 protects the op amp from the surge voltage of the relay coil. Resistor R1 allows you to limit the current through the relay; for the throttles we use and the parts listed, this resistor is 0 ohms, or just a wire jumper.
Resistor R3 sets the maximum speed to which the trimmer pot R4 can be adjusted to allow reversing. The value shown puts the optimal setting for our N scale equipment about in the middle of the pot's range, but you may want to vary this value for your situation. If R3 is decreased, the highest setting of R4 will be increased, increasing the fastest speed to which the circuit can be adjusted to allow reversing, and if R3 is increased, the highest setting of R4 will be decreased, lowering the fastest speed to which the circuit can be adjusted to allow reversing. R3 can be reduced all the way to 0 ohms if your application requires.
Resistor R2 is necessary to add a little hysteresis to the circuit. This resistor makes the throttle setting at which reversing becomes disabled a little higher than the value at which reversing becomes enabled. This keeps the relay from buzzing if you operate at or near the cutoff voltage, as when switching. The buzzing that you would get at the cutoff voltage without R2 is the result of the circuit rapidly turning the relay on and off, as if the circuit can't make up its mind. Most of the reverse blocking circuits we have seen before lack this important feature, and will buzz the relay when operating at or near the cutoff voltage, drastically shortening the life of the relay. If you adjust the value of R3 for your application, you should also adjust the value of R2. You want to keep the relationship between (R3 + R4) and R2 constant, around 4.5.
The circuit etch pattern and component placement diagram for the SafeTrain reverse blocking are given in Figure 3 for those who want to etch boards, rather than perfboard the circuit. The component placement diagram includes the hole locations to aid in drilling your board. Note that the etch pattern is always printed as seen from the component side of the board, per electronic industry standards. The solder side image must be reversed on the board you build, so that the text and the image are correct.
Please be very careful to install C1 to match the polarity indication in the component placement diagram; electrolytic capacitors will explode when power is applied if they are wired backwards. There are two positive connection holes for C1 in the board layout; you can use either depending on the physical size of the capacitor you use.
The circuit should be wired into your existing throttle connections as shown in Figure 4. The circuit itself can be contained either in the hand control unit or by the throttle base unit. We prefer putting the reverse blocking circuit by the throttle base unit to keep the hand control units as light and small as possible. The circuit needs no separate power, as it draws its power from the hand control unit connections themselves.
We have not gone to command control as of this writing, and don't anticipate doing so. Putting even tiny receivers in our N scale locos would mean giving up a lot of engine weight, and therefore pulling power, or having permanently connected boxcars or the like. We are also not yet satisfied with the low speed performance of the command control throttle units compared to what we have achieved with our throttles.
Besides, what we disliked about our current throttles the most wasn't the block boundaries and flipping block controls. It was the tethered hand control units. There were times during operating sessions when three trains were operating in the same area at the same time. Not only was the main tied up, so were the operators!
In looking at radio control units, however, most were designed for other applications. The transmitters have great range, but are very large, with long antennas. The receivers connect to servos which would then have to be mechanically connected to our throttle controls. Finally, radio noise can make the servos occasionally swing back and forth on their own, which was the initial motivation for the reverse blocking circuit.
We were very happy to find a radio control unit which resolves these issues. The transmitter is very small, about the size of an audio cassette box, with no external antenna. This is a perfect size for stuffing in your shirt pocket while you sort car cards. The receiver has simple electrical outputs which can be electronically interfaced to walkaround throttle controls without servos and mechanical switches. Finally, the unit is digital, so radio noise does not cause aberrant operation.
The transmitter and receiver units we use are shown in the photograph. The FM digital units include digital codes which allow more than one transmitter/receiver pair to operate on the same frequency, and which prevent RF noise from falsely triggering outputs.
The circuit we designed to interface the receiver to our throttles, which we call RadioTrainTM, is shown in Figure 5, with the component values and part numbers given in Table 2. On the right are the four signals between the hand control unit and the throttle base unit, 1) positive voltage (POS), 2) ground (GND), 3) throttle control voltage (SPD), and 4) reversing relay (DIR). On the left are the six signals between the RC interface and the radio control receiver circuit, 1) filtered positive voltage (VDC), 2) ground (GND), 3) set reversing relay to reverse (REV), 4) release reversing relay to forward (FWD), 5) increase speed (INC), and 6) decrease speed (DEC).
As with the reverse blocking circuit, D2 and C1 filter the positive voltage from the throttle base unit, which can be anything from steady DC to totally unfiltered DC depending on the throttle, and provide the power for the transistors and the receiver unit. The open-collector NPN signal FWD from the receiver is connected directly to the reversing relay signal DIR. This allows the receiver to release the reversing relay to forward by grounding the reversing relay signal. The open-collector NPN signal REV from the receiver is inverted to an open-collector PNP signal by resistors R1 and R4 and transistor Q1, and connected to the reversing relay signal DIR. This allows the receiver to set the reversing relay to reverse by supplying voltage to the relay through current limiting resistor R5.
The speed controls are handled similarly. The open-collector NPN signal INC from the receiver is inverted to an open-collector PNP signal by resistors R2 and R6 and transistor Q2, and connected to the throttle control voltage SPD. This allows the receiver to increase the throttle control voltage by supplying current to the throttle base unit capacitor through resistor R7. Resistor R7 sets the rate at which trains will accelerate when the accelerate button on the transmitter is held down continuously. A larger resistor here slows the rate of acceleration, and a smaller resistor speeds up the rate of acceleration. We normally 'pulse' the accelerate button to increase speed, and not hold it down continuously, and the value we give for R7 works very well on our layouts for this type of operation.
Unlike the reversing relay, we cannot connect the open-collector NPN signal DEC directly to the throttle control voltage SPD as you might expect. While this works well in principle, in reality the leakage through the open-collector NPN transistor on the receiver is enough to discharge the capacitor and slow the train to a stop within a few tens of feet. Instead we use the open-collector NPN signal DEC to energize a relay which drains charge off the throttle base unit capacitor through resistor R8. Here, as for the reverse blocking circuit, the diode D1 protects the op amp from the surge voltage of the relay coil, and the resistor R3 allows you to limit the current through the relay; for the throttles we use and the parts listed, this resistor is 0 ohms, or just a wire jumper. Resistor R8 sets the rate at which trains will decelerate when the decelerate button on the transmitter is held down continuously. A larger resistor here slows the rate of deceleration, and a smaller resistor speeds up the rate of deceleration. We normally 'pulse' the decelerate button to decrease speed, and not hold it down continuously, and the value we give for R8 works very well on our layouts for this type of operation. Also, as there is no emergency stop button, holding the decelerate button down continuously when one is in trouble results in a fast enough stop to prevent mishaps.
The circuit etch pattern and component placement diagram for the RadioTrain RC interface are given in Figure 6 for those who want to etch boards, rather than perfboard the circuit. The component placement diagram includes the hole locations to aid in drilling your board. Note that the etch pattern is always printed as seen from the component side of the board, per electronic industry standards. The solder side image must be reversed on the board you build, so that the text and the image are correct.
Please be very careful to install C1 to match the polarity indication in the component placement diagram; electrolytic capacitors will explode when power is applied if they are wired backwards.
The receiver and RC interface circuit should be wired into your existing throttle connections as shown in Figure 7. The receiver and RC interface circuit can be connected to your existing throttle at the throttle location, or plugged into any hand control plug-in anywhere around the layout. Plugging it into the hand control plug-in allows you to remove it and go back to tethered operation at any time for debugging purposes, if there is a failure of the transmitter, receiver, or interface circuit, or if you just run out of 9 volt transmitter batteries.
If you use the RC interface circuit together with the reverse blocking circuit, the reverse blocking circuit is connected between the RC interface circuit and the throttle base unit, as if the receiver and RC interface circuit were a hand control unit, which, taken together with the transmitter, they are. This is shown in the photos, in which an 'unboxed' receiver, RC interface, and reverse blocking circuit are shown plugged into a hand unit plug-in on the Reid's Cumberland Valley System.
The radio control transmitter is shown in the photos. We use a different control assignment from some of the push-button radio control units we have seen, and having tried both we very much prefer the one we show here. The 'up' and 'down' buttons are to accelerate (throttle) and decelerate (brake), and the 'left' and 'right' buttons correspond to forward and reverse, which on our layouts is also left and right as the operator views the train. We put a drop of epoxy on the accelerate and decelerate buttons so that they have a different feel and are more prominent to the touch than the direction buttons, making it easier to look at your train and not at the throttle transmitter when operating.
Operation with this unit is wonderful. Tap-tap-tap on the accelerate button nudges your train to higher speeds, tap-tap-tap on the decelerate button gradually slows your train down. 'Standing' on the decelerate button stops a full train completely in a couple of feet when prototype throttle operation would result in disaster, with no need to fumble for a separate emergency button. The direction controls correspond to the operator's view of the train, and will not operate, accidentally or otherwise, when the train is in motion thanks to the reverse blocking circuit.
The photos show the bare and completed circuit boards, transmitters and receivers, and the connection to the layout through the existing hand control plug-ins. Also shown are a couple of situations in which the reverse blocking circuit can pay for itself in preventing a single operator mistake.
These two circuits are excellent enhancements for walkaround throttles. The freedom for the operator of not being wired to the layout, looking for throttle plug-ins, and being able to control the train from anywhere without being limited to the cable reach is tremendously liberating. Ducking under other operators' tethers, and the knot of operators that can develop when three trains on two mains hit the same town at the same time, are also things of the past. Gone too is the justified fear of the proud layout owner when new people operate, who might confuse throttle buttons and direction buttons and put a long coal drag on the floor.
One warning, though: do not try radio control operation if you aren't serious about doing it; being tethered will never be the same after you've tried RC! See you next time.
