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• • ••• •• A.UXlUAJJV =Sll:1.TJ111E \li FRAREl D/J{ ·-··- REMOTE R,._ILPOWER r I . TIU.CK M \ti MAX mm. • > • • OL INFRARED REMOTE CONTROL FOR MODEL RAILROADS This infrared remote controller is based on our veiy popular Railpower walkaround throttle. All the features of the original circuit, including pulse power, inertia, braking and full overload protection are retained, and a few more have been added to make this a really deluxe model railroad controller. By LEO Sll\1PSON & JOHN CLARKE 64 SILICON CHIP Over the years, we have seen a number of solid state throttles for model railroad layouts but none matches the circuit presented here for features and versatility. While our Railpower circuit, published in the April and May 1988 issues of SILICON CHIP, featured a walkaround throttle, this new circuit goes one better with full infrared remote control. Walkaround throttles are a great idea because they allow you to follow the train closely around the layout while you control it. The walkaround throttle concept is simple - just a small box on the end of a lead which has a knob to vary the speed and perhaps a couple of switches to provide direction (forward/reverse) and braking. So it is simple and inexpensive. But for some time now we have had requests for an infrared remote controlled version of the Rail power? Why? Well, apart from the fact that so many appliances these days have remote control (and therefore it is fashionable), it does have distinct advantages when applied to a model railroad controller. There are no trailing cables to trip over and there is no need to plug and unplug the throttle as you move around a large layout. Apart from remote control, the new circuit has an impressive range offeatures, as listed in a panel accompanying this article. Before we describe the features, we should note that this series of articles will present a complete model railroad controller and then describe what you have to do to upgrade our original Railpower controller to full remote control. You can retain virtually all of the earlier circuit and just build the remote control section. Pulse power The IR Rail power uses pulse power to control the model locomotive motor. This is essentially the same method as used in switchmode power supplies whereby a DC voltage is varied by rapidly switching it on and off. This means that the peak voltage across the track is always the same, regardless of the speed setting. Varying the width of the pulses applied to the loco varies the speed. In our circuit, the track voltage is about 17 or 18 volts peak. This relatively high voltage is better able to overcome poor contact resistance between the loco wheels and track and so gives much better low speed running and starting. Features • lnfrared remote operation with 10metre range. • Pulse power for smooth and reliabte low speed operation. • Excellent speed regulation. • Adjustable inertia (momentum) . • Adjustable braking. • Full overload protection including visible and audible overload indicators (short circuit duration: one minute). • Adequate power for double and triple heading of locos. • Track/direction LED indicator. So the circuit monitors the backEMF of the motor and if this voltage drops, as it tends to when the loco starts lugging up a slope or whatever, the circuit actually increases its output voltage to help maintain the selected speed. We haven't overdone this feature though, so that a loco will still tend to slow down as it is loaded, but the speed regulation is certainly better than if this feedback was not included. Inertia or momentum Real trains have inertia, hundreds or thousands of tonnes of it. When the driver opens the throttle(s) on his • Acknowledge LED. • Preset maximum and minimum track voltage. • Meter to indicate speed setting. • 2 momentary auxiliary outputs. • 3 momentary or latched auxiliary outputs. • Forward/reverse control lockout to avoid derailments. • Zero track voltage when first powered up. • Several different IR remote controllers can be used on the same layout. loco(s) very little happens at first. It may take many kilometres for the train to get up to operating speed and similarly, when he applies the brakes, the speed does not slacken very rapidly. By contrast, model trains have no inertia at all and when full power is applied to the track, they accelerate like drag racers. Similarly, if power is abruptly removed from the track, they skid to a stop, which is hardly what you'd call "prototype operation". For this reason, the Railpower controller incorporates inertia circuitry so that the track voltage builds up slowly when the speed control is wound full on and drops slowly when Speed regulation Another worthwhile feature of our circuit is its excellent speed regulation. This helps the loco to maintain its speed even though the gradient may change or the load may change, as in shunting. What happens is that the circuit monitors the back-EMF of the motor. This is the voltage that all motors generate to oppose the current through them and, as it happens, the back-EMF is proportional to the motor speed (EMF stands for electromotive force , another term for voltage). All the controls (except for the inertia control) are on the infrared remote control handpiece. These include the throttle (Faster & Slower), braking (Stop) & direction controls, plus five auxiliary outputs for switching relays. APRIL 1992 65 MAINS INPUT A N E FUSE, SWITCH AND TRANSFORMER SLOWER TRAIN CONTROLS FASTER 0 12VAC 0 STOP 0 CONTROL INERTIA CONTROL i-----OUTPUT INFRARED RECEIVER PCB , 0 AUXILIARY CONTROLS! 0 2 4 RAILPOWER PCB INFRARED SIGNAL 0 00 3 5 +12V MOMENTARY 3 4 5 Overload protection All model train controllers need some sort of short circuit protection because short circuits can occur quite frequently. Whether it's because a loco is derailed, or because points are faulty, or because someone deliberately shorts out the rails with a screwdriver, overloads do occur. The Railpower controller has "foldback" short circuit protection (we'll explain that later) plus a LED indicator and a buzzer to indicate that an overload has occurred. Thus, it will indicate even when momentary shorts occur, as can happen when a loco is crossing points. Power output While model loco motors rarely pull much more than one amp, some model locos can pull considerably more than this, depending on whether they have smoke generators, sound systems and lighting. So if you want to double or triple-head locos or have lots of track lighting, you'll want plenty of amps. The Railpower controller has plenty, around 4 amps or so with the specified 60VA transformer. In fact , we have tested the Rail power with as many as five locos pulling 60-plus wagons around a large layout. The Railpower SILICON CHIP OVERLOAD LED OVERLOAD BUZZER LATCHED OR MOMENTARY Fig.1: this block diagram shows all the control features of the new Railpower model train controller. The circuit has a current capacity of 4 amps, enough to power five or more typical locos to pull very long and heavy trains, and features track and overload indicator LEDs and a speed meter which indicates the throttle setting. the brake is applied. It makes the trains look a whole lot more realistic. You can adjust the amount of inertia with a knob on the front panel. TRACK INDICATOR LED OV AUXILIARY OUTPUTS WITH LED INDICATION 2 66 TRACK SIGNALS FORORO RE~SE handled it without a murmur and without even getting warm. Current output is not the only important parameter though. While most model locos are specified to operate at a maximum of 12V DC, some manufacturers specify less voltage and this should not be exceeded, to safeguard their motors. For example, Marklin Zscale (1:220) locos are specified for a maximum of 8V DC. Most N-scale locos run best with around 9V DC maximum. On the other hand, Lionel and LGB trains need around 15V to really perform. The Railpower controller can be easily adjusted for these specified maximum voltages. Transmitter and receiver Now let us look at some of the operating features of the Railpower. The features are depicted in the block diagram of Fig, 1. There are two separate units, the handheld IR remote control transmitter and the Railpower unit · itself which combines the infrared receiver PC board and the pulse power PC board. The pulse power board is identical to that used in the original Rail power described in April and May 1988. The handheld transmitter is just like the handheld remote for your VCR, CD player or TV set. It has 10 buttons which are in two groups of five. The top five buttons are for controlling the locomotive. There are buttons labelled Slower, Faster, Stop, For- ward and Reverse. The other five buttons are used to control five auxiliary outputs on the Railpower. These may be used for controlling lighting, signalling or points on the layout. Two infrared light emitting diodes (LEDs) protrude from one end of the handheld unit. When you press a button on the handheld unit, the Acknowledge LED on the front panel of the Railpower will flash or will light up for as long as you hold down one of the buttons. If you press the Faster button, the pointer on the meter will move up the scale. This indicates the speed setting for the loco. If the meter indicates full scale, then you are asking for full speed from the loco. Just how long it takes for the loco to reach full speed will depend on how you have set the Inertia control. If you have set the Inertia control to off (ie, fully anticlockwise), the loco will have no inertia at all and will respond immediately to any increase in the speed setting. You might want this when performing shunting manoeuvres. On the other hand, if you set the Inertia control fully clockwise, the loco will take three or four minutes to reach the set speed, depending on how much load it is pulling. If you press the Slower button, the meter reading will immediately begin to reduce to zero but again, the loco may take several minutes to reach the new speed you have set, depending on the Inertia setting. Pressing the Stop button will cause the loco to come to a complete stop and this will take between zero and about 10 seconds, depending on how you have set the braking adjustment. Forward/reverse lockout Pressing the Forward or Reverse buttons will cause the Acknowledge LED to light but you will get no other response unless the loco is stopped or running at a very low speed which is set by you. The reason for this feature is simple. If you switch any normal speed control from forward into reverse (or vice versa), it will usually derail the whole train. If your train consists of 60 wagons and several locos and the layout is a metre or more above ground level, such a derailment can be more than just a hassle - it can cause expensive damage to your rolling stock. Our IR Remote Rail power cannot cause these derailments. If you do want to change the direction of the train, the procedure is as Below: the circuitry in the Railpower unit is accommodated mainly on two PC boards: the original Railpower pulse power board at left (mounted on the lid) & the new Infrared Receiver board which mounts on the bottom of the case. The potentiometer on the front panel allows the amount of inertia to be adjusted. follows . First, hit the Stop or Slower button and allow the train to come to a complete stop. You must wait until the FR/OFF LED on the front panel is extinguished. When that happens, you can press either Forward or Reverse to change the direction of the train. You then briefly touch the Faster button and the train will build up to the previously set speed. A 2-colour LED indicates the track voltage and train direction. Green is used for Forward and Red for reverse and the LED glows more brightly as the track voltage increases. Auxiliary outputs As already noted, the new Railpower has five outputs and these are designed to operate external relays. Two of the outputs, 1 & 2, are momentary which means that they operate only while buttons 1 or 2 are pressed. The other three outputs, 3, 4 & 5, can be either latched or momentary. If they are set up as latched outputs, you press the appropriate button once to turn them on and once again to turn them off. Each output has an associated LED on the front panel which lights when the output is activated. Multiple remote controls Some readers will no doubt ask This is the board for the handheld remote control unit. The various linking options allow you to use up to three otherwise identical transmitters which all operate independently on the one layout (see text). whether they can have more than one of these infrared remote train controls on the one layout. After al( on a big layout divided up into blocks, you might want three, four or more controllers. 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JI I I I ,- ------:7 E 1k ~ 100k: INFRARED RECEIVER PCB 1 I I ~ 0 Is --.--.- ........ .... 220k +17V 100k PULSE-POWER BOARD OUT + 0.1 +12V TRIANGLE WAVEFORM VT RAIL POWER +. 2200 .J: 2.2 .J: 25VW"t!.25VW-,,!. .,. .011 47 ~ 16VW+ +12V L ________ J I o;c 15 13 I I I. I .r-1 F M2165 60VA + +12V ,----!-----,I MINIMUM~ ADJUST VR2 100k 12 11 MAXIMUM ADJUST VR1 100k 100k +1?V 8.2k r: +9.BV +12V I 27k 10k +1 _2V +12V +12V EOc VIEWED FROM BELOW 8 MOTOR BACK EMF 8 CE ~ ~ 10k GND ,~oo, re ~, Q1 '80650 SW ~ .,. +12V o.m MOTOR OVERLOAD BUZZER Q2 BD650 +11v---.---------, Fig.2 (left): the pulse power circuit is virtually identical to the Railpower circuit described in the April & May 1988 issues of SILICON CHIP. All the IC and transistor numbers correspond to those shown in Figs.2 & 4. IC2c and IC2d provide the foldback current protection while ICs 3 & 4 provide logic switching to the H-pack output stage (Q1-Q4). +12V +12V 100k VT VP 1\/V\ OSCILLATOR three otherwise identical transmitters all operating independently. And if you install different ceramic resonators in the circuitry, you could have virtually unlimited numbers of independent infrared remote train controllers. Pulse power principles Now let us describe the operating principles of the pulse power circuitry of Fig.2. This is virtually identical to the Railpower circuit published in the April and May 1988 issues of SILICON CHIP. The circuit of Fig.2 is pretty daunting at first so let's have a look at the core of the circuit which is shown in Fig.3. This depicts the two key op amps which provide the pulse power output or more specifically, pulse width modulation. IC1d is wired as a Schmitt trigger oscillator while IC2a is wired as a comparator. ICld oscillates by the following action. When power is first applied, Cl has no charge and the output ofIC1d is high. Consequently, Cl is charged via R1 until the voltage at pin 6 exceeds the voltage at pin 5. This causes the output at pin 7 to switch low and so Cl is now discharged via R1. So Cl is alternately charged and discharged via Rl and the resulting waveform is a triangle (sawtooth) waveform, shown as VT in Fig.4. This waveform has an amplitude of between two and three volts peak-topeak and a frequency of about 200Hz. This triangular waveform is applied to pin 13 of IC2a which compares it with the speed voltage Vs fed to pin 12. Since IC2a is wired as a comparator, its output can only be high or low, so when VT is above Vs, the output will be low and when VT is below Vs, the output will be high. The interaction of VT and Vs via IC2a is shown in Fig.4. Fig.4(a) shows .,. Fig.3: this is the basic pulse power control circuit. ICld is wired as a Schmitt trigger oscillator while IC2a is wired as a comparator. The output (Vp) is a 200Hz pulse waveform with pulse width determined by the setting of the speed control pot. VT n?WITT /'( /\ /\ VP- (a) HIGH VOLTAGE (b) LOW VOLTAGE Fig.4: how the output of IC2a varies with the setting of the speed control pot. At higher speed settings, the output pulses are longer. that when Vsis set for high speed, the output from IC2a is a series of fairly wide pulses. These give an average DC voltage across the track which is quite high, say around 9 or 10 volts. Similarly, in Fig.4(b), when Vs is set for low speed, the output from IC2a is Vp, a series of narrow pulses which have quite a low average DC voltage, say around 2 or 3 volts. H-pack output So th e pulse waveform Vp is eventually delivered to the track and loco motor via IC3, IC4 and output transistors Q1-Q6 , as shown on the circuit diagram ofFig.2. Again, comprehending how all th ese devices work together is not easy so we have reproduced the output circuit in Fig.5. This shows the four power transistors, Q1-Q4, in an "H" configuration with the motor of the loco connected between the two sides of the "H". IC3 and IC4 are depicted as a logic block with three inputs, one for speed which is Vp and two for direction (forward and reverse). Fig. 5 is really quite a lot more corn- plicated than it needs to be. Instead of using six transistors and two logic ICs, we could have made do with one small signal transistor, a power transistor and a heavy duty relay, which would have reversed the track voltage for the forward reverse mode. But while the circuit is a little complicated, it does have the advantage of being cheaper and more compact than the relay/transistor combination. It also has the advantage of having memory for the direction setting. This is important if the circuit is to be used with a walkaround control as originally described or, as now, when the circuit is married to the remote control board. The H-co.nfiguration of Fig. 5 is commonly used in industrial circuits for motor speed and direction control. To make the motor go in one direction, Q1 and Q4 are turned on while Q2 and Q3 are kept off. To reverse the motor, Q2 and Q3 are turned on and Q1 and Q4 are turned off. Putting it another way, for the forward motor direction, current passes through Q1 and Q4; for reverse , curAPRIL 1992 71 VP LOIIIC IC3, IC4 FORWARD o.m REVERSE CURRENT SENSE ... Fig.5: the H-pack output circuit. To make the motor go in one direction, Q1 & Q4 are turned on while Q2 & Q3 are kept off. For the reverse direction, Q2 & Q3 are turned on and Q1 & Q4 are turned off. spikes from their commutators and from the pulse waveform. The Darlingtons come in a TO-220 plastic encapsulation but have a collector current rating of 16 amps peak (8 amps DC). rent passes through Q2 and Q3. In practice, for the forward direction, Q4 is turned on fully and Ql is turned rapidly on and off by the pulse waveform Vp, to give speed control. Similarly, for the reverse function, Q3 is turned on continuously and Q2 is modulated by the pulse waveform Vp to give speed control. Natty, huh? Q5 and Q6 are there solely to provide voltage level translation between the logic block, IC3 and IC4, and the output transistors. This is necessary because the logic circuitry runs from +12V while the output transistors run from +17V. Q1-Q4 are Darlington transistors which incorporate flyback diodes connected between their collectors and emitters. These diodes are necessary when driving inductive loads such as motors which will tend to generate ;, Main circuit Now let us relate the circuits of Fig.3 and Fig.5 to the pulse power circuit of Fig.2. The circuit of Fig.5 can be seen at the righthand side of the main circuit while ICld and IC2a are roughly in the centre of the circuit. In the top lefthand side of the circuit is a box marked infrared receiver board. Signals from the receiver board are connected at the six points shown in the box. Now have a look at ICla and IClb, at the lefthand side of the circuit. These two op amps are connected as :il ~ - .,HJ=5 ~0mV w OG P*1 rr ~ & & T" ff 72 • FER - Hl <:I ? I •) :1 !'i ~ & " -~ 'I fl"' SILICON CHIP ·- ~ ~'f~ window 2ms/d ~ rn r ' & & I '~ Fig.6: this oscilloscope waveform shows the voltage across the track at a low speed setting. Note that the pulses have an amplitude of about 17V. The waveform between the pulses represents the motor back-EMF. The hash on the waveform is caused by the commutator of the loco motor. voltage followers. Their function is to buffer and reproduce the voltage from the wipers of VRl and VR2. VRl .sets the maximum voltage applied to the track. This is important, particularly for Z-gauge, as mentioned earlier. VR2 sets the minimum track voltage. This is necessary because all locos have some minimum voltage below which their motors will not run. So VRl and VR2 set the overall speed range which is provided by the infrared receiver board at terminal 6. The speed setting voltage from the infrared receiver board is fed to the 47µF capacitor at the non-inverting input (pin 3) of IClc. The voltage across the 47µF capacitor is buffered by voltage follower IClc which feeds IC2a, via pull-down diode Dl. So IClc and Dl provide the voltage Vs fed to IC2a, as shown in Fig.3. Back-EMF monitoring As already noted, the pulse voltage from IC2a is fed via logic circuits IC3 and IC4 to the H-pack output stage but let's ignore those for the moment. Instead, let's flick down to the backEMF monitoring circuit provided by diodes D4 and D5 and transistor Q8. What this circuit does is monitor the voltage across the motor when the output transistors are providing no power. In other words, the speed monitoring circuit looks at the motor in between each pulse delivered by Darlington transistor Ql or Q2. How does it do it? Well, remember that for the forward motor direction, Q4 is continuously on while Q3 is off. This means that virtually the full voltage appearing across the motor appears at the collector of Q3. So the motor voltage is fed via D4 and a 2.2kQ resistor to the non-inverting input of IC2b (over on the lefthand side of the circuit). But D4 feeds the voltage down the 2.2kQ resistor all the time so it gets the pulse voltage as well as the motor back-EMF which is not what we want. So every time a pulse is delivered by Ql, the pulse waveform Vp also turns on Q8. So the pulse voltage never gets to the input of IC2b. Similarly, for the reverse direction, Q3 is always on and the full motor voltage appears at the collector of Q4 and is fed via D5 to the 2.2kQ resistor and thence to the input ofIC2b. Again, whenever pulse voltage is present across the motor, Q8 is turned on, to The remote control unit is built into a standard plastic case which is cut in half to produce a slimline unit that's easily held in the hand. Note the two infrared LEDs protruding through the end of the case. shunt it to ground. So the voltage fed to ICZb truly represents the motor back-EMF and therefore is an indication of the motor's speed. It is a train of pulses, because of the switching action of Q8. ICZb is a non-inverting amplifier with a gain of 3.2, as set by its 220k.Q and lOOk.Q feedback resistors. Its output is a pulse waveform which is filtered by a 22k.Q resistor a'nd 2.ZµF capacitor. This smoothed DC voltage, representing the motor's actual speed, is fed to the reference input of ICld, the triangle waveform generator. This h&s the effect of raising the overall voltage level of the triangle waveform VT, while its amplitude and frequency remain the same. So what happens if the back-EMF generated by the motor for a certain speed suddenly drops? The effect is to lower the overall voltage level of VT, the triangle waveform. As can be seen from Fig.3, if VT is lowered in leve_l with respect to Vs, the pulses delivered by ICZa will be longer and so the power delivered to the motor will be increased and the desired speed will be restored. Overload protection Two op amps, ICZc and ICZd, provide the short circuit protection and both of these are wired as comparators. The current passing through the motor is monitored by the 0. 1.Q 5W resistor connected to the commoned emitters of Q3 and Q4. The voltage developed across the resistor is fed via a lOk.Q resistor to the inverting input, pin 2, of ICZc. The voltage at pin 2 is then compared with a reference voltage at pin 3, which is approximately 0.6 volts. Normally, the voltage at pin 2 will be well below 0.6 volts and so the output of IC2c will remain high, as will the output of IC2d. Therefore, operation of the controller continues as normal. When an excessive current flows in the controller output, a large peak voltage will be developed across the 0. 1.Q sensing resistor and the voltage at pin 2 will rise above the threshold of comparator ICZc. This will cause the output to go low which then pulls pin 12 ofICZa low, via diode DZ. This has the effect ofreducing the width of the output pulses and so the fault current is reduced. ICZc also turns on the overload LED to indicate the fault condition. ICZc's action in reducing the fault current tends to cause a "hunt" condition whereby as the current is reduced, the voltage at pin 2 reduces and so the controller again delivers the full pulse width. This causes the current to increase again and ICZc again switches on. This "oscillation" is slowed to some extent by the O.lµF filter capacitor at pin 2 of ICZc, so that the action of ICZc is adequate to cope with shortterm overloads and short circuits which may occur when a loco is crossing points. For longer term short circuits though, ICZd comes into play. This op amp monitors the output of ICZc via LED 2 (the overload indicator). When a long duration short circuit occurs, the capacitor at pin 5 is discharged so that its voltage is below the reference voltage at pin 6. This causes ICZd's output to go low which then also pulls pin 2 ofICZa low, via diode D3. So ICZc and ICZd together act to reduce the pulse width and thereby control the output current. ICZd thus provides a 'foldback" current limiting action. ICZd also drives Q7 which sounds the buzzer whenever a short circuit or overload occurs. This very effectively draws your attention to any overloads, whether momentary or otherwise, so that any faults can be corrected. Just a small point of explanation here: the reference voltage at pin 3 of ICZc is 0.6V which may lead you to conclude that current limiting will occur for currents in excess of 6 amps peak (ie, 0.6V across the O. 1.Q sensing resistor). In practice though, the O. lµF filter capacitor at pin 2 allows higher peak currents to pass before limiting occurs. The output Darlington transistors, Q1-Q4, are fitted with small heatsinks which normally stay quite cool. If a short circuit is maintained across the track for any length of time though, the transistors will rapidly become very hot. They can withstand this condition for several minutes although the overload buzzer will be sounding stridently and the short should be removed as soon as possible. Logic circuitry Now we come to the part of the circuit which looks quite tricky but isn't; if you have stuck with the description as far as this point you will have no trouble with the logic. IC3c and IC3d are the key to it all; they are coupled together as an RS flipflop which is controlled by the APRIL 1992 73 13 10_t 12 XXX01 11 XXX10 ,. + + 220 16VWI 0.1! 02 BD140 -: 9V: ..L B + lO XXX11 5 100XX IC1 MV500 OUT 010XX A,.-5=-----(:>--L._K3-c1.,.__._ _ +9V 1 Bt=-~--_,L.,,K4"-(')-___. LK2 .,. LK1 PLASTIC 0 ~ ':" B ELJc VIEWED FROM BELOW ~{ ECB ~- RAILPOWER INFRARED TRANSMITTER BOARD Fig.6: this is the circuit for the remote control transmitter. It uses an MV500 transmitter chip (IC1) to drive two LEDs via switching transistor Ql. The pushbutton switches connected to the row & column address pins select the output code. forward/reverse switch S2. When S2 is set to the forward condition, it pulls pin 5 low (normally held high by a 10kO resistor). This causes the output at pin 4 to go high while the output at pin 3 goes low. The flipflop will then remain in this condition until S2 is switched over to the reverse condition. When that occurs, pin 1 will be pulled low and the flipflop will change state. Pin 3 will now be high and pin 4 will be low. (If you want to better understand this type offlipflop, have a look.at our series on Digital Electronics, in the February 1988 issue) . The flipflop determines which output transistor remains on continuously - ie, Q3 or Q4. For the forward setting of S2, pin 4 of IC3c will be high and pin 3 will be low. As a result, the outputs of inverters IC4c and IC4d will be low and Q3 will be off. Conversely, the outputs of inverters IC4e and IC4f will be high and so Q4 will be on. IC3a and IC3b gate through the pulse waveform (Vp) signals from IC2a, depending on the setting of the flipflop. Thus, ifQ4 is turned on continuously, pulse signals are fed via IC3a, inverter IC4a and transistor Q5, to turn Q1 on and off at 200Hz. Similarly, if Q3 is 74 SILICON CHIP turned on continuously, for the reverse condition, Vp signals are gated through IC3b, inverter IC4b and transistor Q6, to turn on Q2 at the 200Hz rate. Power supply The power transformer is a 60VA multitap unit available from Jaycar (Cat. No. MM-2005) or Altronics (Cat. No. M-2165). It is connected to provide a 12V AC output which feeds a bridge rectifier and two 2200µF 25VW electrolytic capacitors. This produces smoothed but unregulated DC of about 17-18V. This unregulated DC is fed to the output stage (Ql-Q4) and also to a 7812 3-terminal regulator to produce a regulated+ 12V rail which is used to power all the op amps and logic circuits. Remote control And now let's have a look at the infrared remote control side of the circuit. This is based on a 3-chip set from GEC Plessey. The remote control transmitter uses an MV500 while in the receiver we use an SL486 amplifier and MV601 receiver. The MV500 and MV601 !Cs are designed specifically for infrared or di- rect wire link transmission using PPM (pulse position modulation) signals. When connected to a suitable keypad, a maximum of 32 different codes can be transmitted. Each IC is set to the same transmit frequency, anywhere between 400kHz and 1MHz, with the frequency set by a ceramic resonator. For a given ceramic resonator frequency, it is possible to have three different transmitters and · each will have their own unique coding. The transmitter circuit ofFig.6 comprises the MV500 transmitter IC, the ceramic resonator (Xl) and infrared LED driver transistors Ql and Q2. The circuit operates from a single 9V battery and draws so little standby current (2µA) that an on/off switch is unnecessary. Ten pushbutton switches are connected between four of the row input pins (5, 7, 8 & 9) and either the column output pins (10, 11 & 12) or the +9V rail. When a button is pressed, a unique code is delivered from the output, pin 1. It drives transistor Ql via a lkO resistor. Ql then drives the base of transistor Q2 via a 1000 resistor. The 8200 resistor from Q2's base to the 9V rail ensures that Q2 turns hard off when Ql is off. Transistor Q2 drives infrared LEDs 1 and 2 via a 2.20 current limiting resistor. The peak current through the LEDs is around 1.3 amps although the pulses are very short at around 15 microseconds long and the duty cycle is quite low, at under 20%. The 220µF capacitor across the 9V battery supply helps supply the peak currents to the LEDs, while the 0. lµF capacitor provides supply decoupling for ICl. !Cl 's internal oscillator runs at close to 615kHz, as determined by the ceramic resonator (Xl) connected between pins 16 & 17. The l00pF capacitors at these pins provide the correct circuit loading for the resonator. The A and B inputs at pins 14 and 15 set the transmitter coding, as mentioned earlier. They can be independently connected to either the +9V rail or to ground. In our circuit, we show both inputs connected to the +9V rail via links LK3 and LK4. Note that the transmitter will not operate with both the A and B inputs tied to ground. Next month, we will continue with the infrared receiver circuitry and its interfacing to the pulse power circuit described above. SC