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Rail p A Walk-A 40  Silicon Chip power MkII: Around Throttle For Model Rail­ways Want to build a walk-around throttle for your model railway? This completely new design offers all the features you could want, including pulse power, pushbutton control, track voltage metering, inertia (momentum) and full overload protec­tion. By RICK WALTERS September 1995  41 S MAIN FEATURE ­piece. tions on the hand nc fu l al ith w l • Walkaround contro eed operation. d reliable low sp an th oo sm r fo d regu• Pulse power r excellent spee fo F M -E ck ba or • Monitoring of mot n. d the latio as though it ha untts ac el od m entum) so hed off for sh • Inertia (mom ertia can be switc in n; ai tr al re a weight of dible ing. g visible and au in ud cl in n, tio ec • Full overlo. ad prot rs indicato g thrown into events train bein pr t; ou ck lo e rs op. • Forward/reve st comes to a st reverse until it fir or ultimate s current speed e pressed. te ca di in g; in er met wer buttons ar • Track voltattge hen Faster or Slo w g in se d ee sp THE RAILPOWER MKII incorporates all the features of our very popular Railpower design featured in April & May 1988. While the original Rail­ power is still a valid design, the MkII version has a lot of new features. In spite of the extra functions, the new design uses less parts and is easier to build. How can that be? Just read on. The outstanding feature of our previous Railpower design was the use of pulse power and simulated inertia, allowing the train to move off from rest at a very low speed and accelerate very gradually, which looks very realistic. Another big feature is the concept of a walk-around throt­tle. This has all the functions on a hand control and allows you to follow your train all around the layout. Nor do you need a long cable which will get tangled up as you move around. You can have a number of sockets around the layout and you can plug into whichever socket is handy. And when you unplug the hand control in order to move it to another socket, the train carries on at its exact same speed setting, without any disruption. Hand control The hand control of the Railpower MkII features a small meter and six pushbut­t ons. These are labelled Faster, Slower, Forward, Reverse, Iner­tia and Stop. The latter four buttons have LED indicators to show the selected functions. 42  Silicon Chip The Forward and Reverse buttons are interlocked. If the train is moving forward and the Reverse button is pressed, the Railpower will switch to Stop, the Stop LED will light and the train will slow to a stop. The controller will then switch to Reverse and the Reverse LED will be lit. A similar sequence occurs if the Forward button is pressed while the train is trav­elling in the reverse direction. This overcomes a drawback of the original design and all other controllers that we know of – if you mistakenly throw a train into reverse while it is moving at a reasonable speed, it will be derailed. The Stop button, when pressed, will bring the train to a realistic but reasonably rapid halt. The braking time is adjusted by the “brake” trimpot (VR2) on the main PC board. If you are shunting wagons, the inertia function can be a hindrance. Hence, it can be switched out, by pushing the Inertia button, if the train is stopped or running at a low speed. Once switched out, Inertia can be switched back in at any time. Railpower controller Apart from the hand control, the Railpower MkII consists of a plast i c case containing a PC board which has all the com­ ponents mount­ ed on it. There is no transformer as we have assumed that the typical model railway enthusiast already has a self-contained power supply which can be hooked up to the Railpower. We’ll talk more about this aspect when we discuss construction. The front panel of the Railpower is bare except for six LEDs. These echo the LEDs on the hand control and add two others, one for Power and the other to indicate Overload (short circuit). +5V 5 M1 500uA 450W 2 çç 3 10 10k METER ZERO VR1 5k 560 16 4.7k 15 4.7k 7 14 13 6x1N914 8 D3 0.1 D6 STOP S6 12 A B O0 IC1 74HC42 C O2 D O3 8 4 O1 1 K  2 3 4 LED1 RED K A K A  K A  A  LED4 ORANGE LED2 GREEN LED3 YELLOW gradually increases (assuming that the inertia setting is large) the meter reading will increase to reflect this. If the Faster or Slower button is pressed, the meter will momentarily indicate the previous selected speed setting and then move up or down to show the new setting. The new speed setting is only indicated while the buttons are actually being pressed. Micro-speak for modellers REVERSE S3 For many readers and railway modellers, this might be the first time you have come “face to face” 6 with a microprocessor. Never INERTIA FORW'D K D2 D4 A S5 S4 fear, it’s just a smarter IC than those you may have used before DECEL ACCEL but otherwise it’s just another S2 S1 1 black inscrutable chip. In essence, this Z8 micro is 8-PIN RAILPOWER MKII HAND CONTROL DIN PLUG only a bunch of counters and gates, crammed into an 18-pin Fig.1: the hand controller circuit is based on IC1, a 74HC42 BCD decoder. This chip is used to indicate four modes of operation (via LEDs1-4) with only two lines from IC package. The big advantage is the microprocessor. The meter indicates both the track voltage and the selected that we can control the logic in speed setting when the faster or slower buttons are being pressed. a manner which suits each par­ ticular application. While there On the PC board, there are four halt much more quickly and the time are only a few leads to and from the trimpot adjustments: for maximum it takes is set by the brake adjustment processor, inside the chip we can have speed, minimum speed, inertia and trimpot. the equivalent of 50 or 60 gates and brake. The maximum speed setting is perhaps five counters interconnected. Low-cost microprocessor usually set to give the maximum rated These might give an output on just voltage for the particular locomotive. one pin, should a certain sequence of The design brief for the Railpower Typically, this is 12V DC for HO scale events occur. MkII was that it had to be easier to models but it can be lower for other Just as we use standard ICs (hardbuild than the previous version, it scales such as N or Z. The minimum had to have more features and it had ware) and interconnect them to obtain speed setting is determined by the to have all pushbutton operation. the circuit functions we require, a simquality of the locomotive’s motor. Very To achieve this, we have designed a ilar design process is carried out when good models may start to move with completely new circuit which uses using a microprocessor. The difference less than 1V across the track while oth- a low-cost microprocessor, the Zilog in this case is that the design relies on a ers may need 4V or more before they Z86E08. Now don’t be scared off be- set of instructions (software) stored in start moving. By setting the minimum cause the circuit uses a microproces- the internal ROM (read only memory) speed just below the point where the of the micro. sor. Have a read of the section headed loco starts to move, more realistic and “Micro-speak for modellers” and be Thus, each time we use the microrespon­sive operation is obtained. processor in a different project, we reassured. The inertia setting controls the store a different sequence of instrucThe Z86E08 (Z8, for short) comes time the train takes to accelerate to in an 18-pin package and contains tions in its ROM. maximum speed. This adjustment Before we go further, we should 2Kb bytes of OTP (one time programranges from zero to four minutes. mable) memory. Two pins are for V+ explain the pin descrip­tions for the At the maximum setting, a loco may and ground and two pins are for the microprocessor (IC1). It has three take more than one scale mile before crystal, while the remaining 14 pins groups of pins, called ports in computit reaches its selected speed, just like are all available for control functions. er jargon. These are port 0, port 2 and a real train. port 3, abbreviated to P0, P2 and P3. Using the processor allows us to Inertia applies to deceleration as carry out complex tasks which would P0 has three I/O pins (input/output), well as acceleration so a train will take P2 has eight and P3 has three. otherwise require lots of conventional approximately the same time to come Thus, pin 15 which is labelled P20 circuitry. The best example of this is to a stop as it took to reach its selected the meter in the hand control. During is Port 2 line zero (computer people speed setting. normal running, it indicates the speed, start counting from zero, not one like On the other hand, if you push the normal mortals). We have assigned from zero to 100%, at which the train Stop button, the train will come to a is actually travelling. As the speed this pin to be the one that turns the D1 D5 September 1995  43 44  Silicon Chip 8-PIN DIN SOCKET 1 7 3 2 6 4 8 5 12 13 14 D C B A 10 680W .047 22k METER SPEED MAX VR5 5k 10k MIN VR4 5k MAX VR3 5k 16 8 O3 O2 O1 O0 4 3 2 1 K K   A  A A 470  12 12 13 5 6 15 16 22pF 22pF X1 10MHz +5V INH VEE VSS 6 8 7 OUT/IN 3 IC4 A 11 4051 B 10 C 9 +5V ZD1 3.9V 12VAC INPUT 74HC11 14 2 12 IC3a 13 1 BACK EMF 16 VDD 4.7k LED2 GREEN LED3 YELLOW 3 13 0 15 2 14 1  LED4 ORANGE K A K LED1 RED BRAKE VR2 5k PO1 PO2 X1 X2 P20 P21 9 P32 P31 8 180k 0.1 IC1 Z86E08 5 VCC 10 P33 GND 14 17 P22 18 P23 11 PO0 4 P27 P24 2 P25 3 P26 MODE INDICATION INERTIA VR1 5k 10k IC2 74HC42 D3 1N914 15 0.1 10k Q10 BC338 D2 1N914 D4-D7 4x1N5404 +5V E C 2200 C 10 5V 10k B E C B 0.1 +5V  BUZZER +5V 0.1 10k E Q6 BD649 C B C Q2 BD650 E 1k B Q12 BC328 470  C E LED5 GREEN 22 Q9 BC338 E Q5 BC338 C 1.8k B CURRENT MONITOR 10k 22k 10k 6 OUT  7 IC3b IN914 10k B REG1 7805 B GND IN +17V E C LED6 RED 560  4 3 5 8 470  D1 IC3c RAILPOWER MKII 2200 Q11 BC338 B 22k 10k 9 10 11 Q1 BC338 10k E B C B PLASTIC SIDE E C VIEWED FROM BELOW E B 1k Q8 C BD649 B C 0.1  10k MOTOR Q4 BD650 E E C E C Q7 BC338 B 2.2k A B 10k Q3 BC338 K I GO +17V power to the track on and off. We did not have to use this pin; we could have used any pin on P2, or for that matter, P0. We could not use P3 as the pins on this port connect to two comparators, which are used to convert the analog voltages from preset potentiome­ters VR1-VR4 to digital values, which can be used by the proces­sor. Enough on micros, let’s get back to the main story. Hand control circuit The hand control consists of six pushbuttons, four LEDs, one IC (integrated circuit) and a few resistors, diodes and capacitors mounted on a small PC board measuring 74 x 50mm. The hand control connects to the Railpower via a 9-core cable (one unused) and an 8pin DIN plug. The circuit is shown in Fig.1. Supply rails of 5V and 0V are fed via pins 5 and 1 on the DIN connector to IC1, a 74HC42 BCD (binary coded decimal) decoder. Four outputs from IC1 are used to drive the four LEDs. It was necessary to use the IC as there were insufficient outputs available on the micro­processor. By using the 74HC42, the microprocessor only needs two lines to control four LEDs. Again, due to limited processor outputs and only eight pins on the connector, the six pushbuttons are accessed by three lines. We do this by using diodes D1-D6 which are connected in a simple matrix, allowing each button to pull one or two lines to 0V. As each line, or pair of lines, is connected to ground, it signals to the microprocessor the function required. Main board The main PC board contains four ICs, a 7805 regulator, 12 transistors, five trimpots and a handful of small components, mounted on a PC board measuring 143.5 x 108mm. The circuit is shown in Fig.2. Note the eight lines of the DIN socket. These connect to the hand control circuit of Fig.1. The best way to explain the circuit Fig.2 (left): IC1, the microprocessor, controls all facets of circuit operation. As well as driving the H-bridge circuit (Q1-Q8), it reads the buttons in the hand control, the settings of the trimpots (VR1-VR4) via IC4, the backEMF and the load current. As well, it drives the mode indicator (IC2) and the meter. Q1 BC338 10k B +17V Q2 BD650 E 1k B C E Q4 BD650 C B 1k C C MOTOR E IC3, Q9 Q5 BC338 C 1.8k B E Q3 BC338 B 10k E 10k Q6 BD649 C B 10k Q8 C BD649 B E C Q7 BC338 B 2.2k E E 0.1  Fig.3: the H-bridge circuit. This controls the speed of the motor (depending on the pulse width), as well as its direction. For example, to make the motor go forward, Q8 is turned on continuously while Q2 is pulsed on and off. For reverse, Q6 is turned on continuously and Q4 is pulsed on and off. is to go through the microprocessor start-up sequence. When power is first applied, the Z86E08 microprocessor executes a series of steps. First, it sets pin 15 low; ie, to 0V. This pin applies power to the track when it is high (+5V). Pin 16 is taken high to set the train direction to forward (low for reverse). Pins 17 and 18 are both taken low, which via IC2, another 74HC42 BCD decoder, illuminates the Stop LED. The same lines go to IC1 in the hand control to illuminate its Stop LED. It then takes pins 12 and 13, which control the output of IC4, low. IC4, a 74C4051 8-input analog multiplexer, is simply a switch which can route any one of eight inputs to its output (pin 3). With pins 10 and 11 low, the wiper of the maximum speed trimpot, VR3, is connected via IC4’s output to pin 9 of IC1. The microprocessor converts the voltage on the wiper to a digital value which it stores. Pins 12 and 13 of IC1 are taken high and low in sequence and the voltages from trimpots VR1, VR2 & VR4 are subsequently read and stored. IC1 has now finished its “power on routine” and is ready to look at the hand control, to see if a button has been pressed. motor drive circuit which is known as an “H-bridge”. This consists of four Darlington transistors – Q2, Q4, Q6 & Q8 – and these are driven by buffer transistors Q1, Q3, Q5 & Q7. To explain this part of the circuit better, we have reproduced it in Fig.3. The H-bridge circuit does two things. First, it switches the power on and off to the motor. The rate of switching is 150Hz and the voltage fed to the motor is directly proportional to the width of the pulses. Second, the H-bridge allows the direction of the motor to be reversed, depending on which transistors are actually turned on. In this case, to make the motor go forward, Q8 is turned on continuously while Q2 is pulsed on and off. Q4 & Q7 are turned off. To make the motor go in reverse, Q2 & Q8 are turned off, Q6 is turned on continuously and Q4 is pulsed on and off. Transis­tors Q1 & Q3 ensure that the Darlington transistors Q2 & Q4 turn on hard (ie, saturate) so that their power dissipation H-bridge motor drive Before we discuss this operation, let’s look at the September 1995  45 Myths & Magic of Pulse Power Pulse power as used in the Railpower Mk I & MkII circuits is quite different to that used in some commercial train con­trollers. In the Railpower, the voltage is applied to the track in pulse form at 150Hz. At low speeds, the pulses are very short and high speeds, the pulses are much longer. This is very similar to the system used in switch-mode power supplies and is highly efficient. However, the reason we use this pulse power system is to get more reliable running. Because the peak voltage applied to the track is about 17 to 18V at all speed settings, it is much more effective at overcoming resistance due to dirty track, dirty motor brushes and commutators. The result is really good slow speed operation which means that your trains will look much more realistic. It’s magic. On the other hand, some modelling enthusiasts believe that pulse power can make motors run hot and can even burn them out. This is not true and there are a number of factors which ensure that pulse power does not damage model locomotive motors. First, virtually all motors used in model locomotives are permanent is minimal and small heat­sinks can suffice. More importantly, Q1 & Q3 per­form voltage translation of the 5V logic signals to Q2 & Q4 which have a supply voltage of +17V. Q5 & Q7 ensure that their respec­tive Darlingtons, Q6 & Q8, turn on fully. Having described how the H-bridge works, we can now see how it is controlled by the micro, IC1. As we stated previously, to select the forward direction, pin 16 of IC1 goes high, taking pins 1, 2 & 13 of AND gate IC3a high. IC3a is used simply as a non-inverting buffer, so its output at pin 12 is also high and thus Q7 & Q8 are turned on. The output of IC3a also turns on Q9 which pulls its collec­tor to 0V. This will turn Q5, and thus Q6, off. Pin 15 of IC1 is the pulse drive (150Hz) signal and this is fed via AND gate IC3c to turn on Q1 & Q2. Q9 also pulls pin 46  Silicon Chip magnet or series wound motors. In both types, the torque generated is proportional to the average current through the windings while the heating effect is proportional to the RMS value of the current. Now because we are using pulse power and the RMS voltage will be slightly higher than the average value, particularly at low speed settings, then it might be supposed that the motor’s winding would get hotter than if pure DC was applied. In practice though, two things come to the rescue. First, the motor’s inductance tends to reduce the current drain when the speed settings are low, due to the very narrow applied pulses. Second, because the narrow pulses are actually much more effec­tive in making the motor rotate and thus moving the locomotive forward, the motor then generates more back-EMF than it otherwise would with a low value of DC and thus the current is actually reduced. So in practice, the difference in motor dissipation between the unfiltered DC of most controllers and the pulsed DC of the Railpower is negligible. The big danger of motors burning out is if the motor stalls due to a bind- 5 of IC3b low, via diode D1, and this means that output pin 6 will be low, turning off Q3 & Q4. To reverse the motor, pin 16 of IC1 goes low, so pin 12 of IC3a is low, turning off Q7, Q8 & Q9. This allows Q5 & Q6 to turn on and the pulse signal from pin 15 of IC1 passes via IC3b to Q3 & Q4. Overload protection Note that the emitters of Q6 & Q8 are connected via a common 0.1Ω resistor to the 0V line. This resistor is used to monitor the current supplied to the track. If there is a short circuit across the track, the voltage across this resistor will increase. This voltage is applied to transistor Q11. If the voltage across the resistor rises above 0.6V, Q11 turns on, lighting LED 6 (overload indicator) and also turning on Q12, which drives the buzzer to give an audible indication of the short. ing gear system. This risk applies to any model train controller, not just the Railpower. Pulsed DC is also reputed to cause more motor noise than with pure DC. This tends to be true, partly because the Railpower allows the loco to run at a much lower speed than would be possi­ble with unfiltered or pure DC across the track. At these much lower speeds, the motor noise is more significant; at higher speeds, the motor noise is drowned out by gear noise and wheel/rail noise. Motor noise is also dependent on the quality of the gear systems and it can be amplified by locos of brass construction. Overall though, pulsed DC as used in the Railpower gives signifi­cantly better running, greater realism and more reliable opera­tion. However, coreless motors, such as those branded Portescap or Escap, should not be used with pulsed DC as they have very little inductance and generate very low back-EMF. These motors should only be used with pure DC train controllers. However, these motors are not generally used in model locomotives and so will rarely be encountered. Not only do we get a visible and audible indication of the short but the system goes further and shuts down the voltage on the track, so that no damage can occur. This happens in the following way. As pins 4 & 9 of IC3 are connected to Q11’s collector, they will also be pulled low when Q11 turns on. This will turn off the power to the motor, whether it is running forwards or is in reverse. As there is now no voltage applied to it, there can be no current flow through the resistor and consequently Q11 will turn off. Power will be re-applied and the whole sequence will repeat until the short circuit is removed. We have previously stated in the description that pin 15 of IC1 goes high to run the motor. Actually pin 15 goes high every 6.5 milliseconds, for a time dependent on the adjustment of VR4, the minimum speed setting. If the operator presses the Faster button on the controller, the pulses from pin 15 are longer, effectively putting a higher voltage on the track. Similarly if the Slower button is pressed, pin 15’s pulse output will become shorter, reducing the average track voltage. Speed regulator & back-EMF As a model train comes to a gradient, it will tend to slow down, the speed reduction being dependent upon the motor’s power and the slope. In severe situations, the train might even stop and this is not very realistic. Our circuit compensates for the extra load on the motor by increasing the voltage to the track so that the speed setting is maintained more or less constant. How is this done? By measuring the back-EMF of the motor and using it to control the micro, is the quick answer. All electric motors generate a “backEMF” which is the voltage which opposes current flow through the motor windings. If the motor speed is high, the back-EMF is high and current will be low. If the motor is stalled, the back-EMF will be close to zero and the current will be very high. So how do we measure the motor’s back-EMF while it is running? It turns out that this pulse power system makes it fairly easy and we measure the back-EMF in the periods when the voltage applied to the track is zero; ie, between each pulse on pin 15. We monitor the motor’s back-EMF by means of the 10kΩ resis­tors connected to either side of the motor. While one side of the motor is always close to 0V (depending on whether Q6 or Q8 is off), the opposite side will always have the track voltage ap­plied to it and thus one or other of the 10kΩ resistors will feed the voltage to the collector of Q10, then through D2 and the 180kΩ resistor to pin 8 of IC1. The capacitor on this pin filters this voltage. Now the trick is to make sure that the voltage fed back to IC1 is the backEMF and not the track voltage. This is done by turning on transistor Q10 via the pulse line, pin 15, of IC1. Thus, each time a pulse appears on the track, Q10 is turned on to short the anode of D2 to the 0V line. Hence, the signal applied via D2 to pin 8 is a sample of the motor-back PARTS LIST HAND CONTROL 1 PC board, code 09109952, 74 x 50mm 1 plastic case, (Jaycar HB-6032 or equivalent) 1 8 pin DIN plug (Jaycar PP0312 or equivalent) 1 500uA FSD edge reading meter (DSE Q-2110 or equivalent) 2 yellow PC mount momentary switches (Jaycar SP-0722 or equival­ent) 1 red PC mount momentary switches (Jaycar SP-0720 or equivalent) 1 black PC mount momentary switch (Jaycar SP-0721 or equivalent) 1 white PC mount momentary switch (Jaycar SP-0723 or equivalent) 1 green PC mount momentary switch (Jaycar SP-0724 or equivalent) 1 5kΩ horizontal trimpot (VR1) Semiconductors 1 74HC42 BCD decoder (IC1) 6 1N914 signal diodes (D1-D6) 1 3mm red LED (LED1) 1 3mm green LED (LED2) 1 3mm yellow LED (LED3) 1 3mm orange LED (LED4) Capacitors 1 10µF 50VW electrolytic 1 0.1µF monolithic Resistors (0.25W, 1%) 1 10kΩ 1 560Ω 2 4.7kΩ Miscellaneous 1 cable clamp, Jaycar HP-0718 or equivalent 1 12mm x 2.5mm countersunk screw 1 2.5mm nut 2 #8 x 10mm self tapping screws 1 8mm untapped spacer 2 5mm untapped spacers MAIN BOARD 1 PC board code 09109951, 143.5 x 108mm 1 plastic case, 140 x 110 x 35mm (Jaycar HB-5970 or equivalent) 1 8 pin chassis mounting DIN socket (Jaycar PS-0360 or equival­ent) 1 10MHz crystal 1 PC board mounting buzzer (Jaycar HB-3458 or equivalent) 2 TO-220 heatsinks 5 5kΩ horizontal trimpots (VR1VR5) 3 metres 9-way cable (Jaycar WB-1578 or equivalent) 4 PC stakes Semiconductors 1 Z86E08 programmed OTP microprocessor (IC1) 1 74HC42 BCD decoder (IC2) 1 74HC11 triple AND gate (IC3) 1 74HC4051 or 4051B analog multiplexer (IC4) 1 7805 +5V regulator (REG1) 2 BD650 PNP Darlington transistors (Q2,Q4) 2 BD649 NPN Darlington transistors (Q6,Q8) 7 BC338 NPN transistors (Q1,Q3,Q5,Q7,Q9-Q11) 1 BC328 PNP transistor (Q12) 1 3.9V 500mW zener diode (ZD1) 3 1N914, 1N4148 signal diodes (D1-D3) 4 1N5404 rectifier diodes (D4D7) 2 5mm red LEDs (LED1,6) 2 5mm green LEDs (LED2,5) 1 5mm yellow LED (LED3) 1 5mm orange LED (LED4) Capacitors 2 2200µF 25VW electrolytic 1 22µF 16VW electrolytic 2 10µF 50VW electrolytic 4 0.1µF monolithic 1 .047µF MKT polyester 2 22pF NPO ceramic Resistors (0.25W, 1%) 1 180kΩ 2 1kΩ 3 22kΩ 1 680Ω 12 10kΩ 1 560Ω 1 4.7kΩ 3 470Ω 1 2.2kΩ 1 0.1Ω 5W 1 1.8kΩ Miscellaneous Solder, hook-up wire, plastic cable ties. September 1995  47 Although the circuit of the Railpower MkII is quite complicated, the PC board is relatively simple and has very little wiring. EMF, not the track voltage. The voltage at pin 8 is filtered by the 0.1µF capacitor, so that commutator hash does not give false readings. This voltage at pin 8 is compared with the desired setting and if the value starts to drop, due to the train slowing or the load increasing, the microprocessor increases the track voltage, to keep the loco running at a constant speed. Maximum speed setting Previously, we discussed VR3, the maximum speed adjustment, and described how it is used to set the maximum track voltage, to suit the locomotives being used. We also discussed the meter which has two modes, one to indicate the actual track voltage and the other, to indicate the track voltage being set by the Faster and Slower buttons. The meter is driven directly from 48  Silicon Chip pin 4 of the microprocessor, via trimpot VR5. During the setup procedure, trimpot VR3 is used to set the maximum track voltage and then VR5 is used to set the meter’s pointer to full scale, to give a 100% reading. In the same proce­dure, trimpot VR4 is used to set the minimum track voltage and trimpot VR1, in the hand control, is used to set the meter to zero. In practice, it will be necessary to do the adjustments for the various trimpots more than once, before they are correct. A 10MHz crystal is used by the microprocessor and is con­ n ected between pins 6 & 7, along with two 22pF capacitors to ensure the crystal oscillates reliably. Power supply No power transformer is included in the circuit as it is assumed that mod- elling enthusiasts will already have a suitable controller power supply or a 12V battery charger. As presented, the circuit can deliver peak currents of about 6A, which corresponds to a maximum output of about 4A continuous. A 12V charger rated for at least 4A or a 12V power transformer with a rating of 60VA is recommended. Unfiltered DC from the external 12V battery charger or AC from an external 12V power transformer is applied to a bridge rectifier consisting of diodes D4-D7. These rectify the input and their output is filtered with two 2200µF capacitors to give unregulated DC of about +17V and this is the motor supply, ap­plied to the emitters of Q2 & Q3. The +17V rail is also applied to the 7805 5V regulator which supplies all the other circuitry in the Railpower. Next month, we will complete the description of the Rail­ power MkII, giving all the construction details and setting up proce­dure. SC