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A rotating light is often mounted on the leading locomotive of US trains as a safety device. In this photo, it can be clearly seen on top of the crew cabin. Low-power flasher for model railways All around the world's railways, guards' vans are disappearing. In their place, battery operated flashers are being mounted on the end wagon of long trains. Here we present a project to simulate those units. And we present a different version to simulate the strobe light mounted on some American locomotives. By MALCOLM YOUNG In most states of Australia, we call them "guards' vans". In most countries of Europe they are referred to as "brake vans" while in America and South Australia they are referred to as cabooses. Whatever they are called, they are steadily disappearing from the world's railways and being replaced with battery operated flashers which are usually mounted on the coupler on the end of the last wagon. 52 SILICON CHIP As such, they are a safety device to indicate the end of the train. In America, such devices are known as "end of train" indicators. In Australia, they are commonly referred to as BOGs; short for " battery operated guard". In practice, these train safety devices are not much different from the battery operated traffic beacons placed on the barriers around road excavations. The lamp has a lens about 15cm in diameter and it flashes at around once a second or thereabouts. Locomotive flasher As one of the photos in this article shows, the leading locomotive on American trains often has a rotating light mounted on the cab, again as a safety device. Naturally then, keen railway modellers will want to simulate these modern developments and they can with the flasher circuits presented here. Both the BOG and the locomotive flasher circuits are based on the National Semiconductor LM3909 Led flasher/oscillator. This readily available device has been around for about 15 years now and is the ideal device for this application. The beauty of the LM3909 is that it will easily flash a light emitting diode even when it is powered from a 1.5V cell which is practically flat. Furthermore, the LM3909 is such an efficient device that it works at an extremely low current drain, typically around 0.5 milliamps . PARTS LIST Locomotive flasher 1 PCB, code SC09102891, 20 x 25mm 1 LM3909 integrated circuit 1 W04 400V miniature bridge rectifier 1 3mm or smaller diameter yellow or orange LED 1 .047 Farad 5V super capacitor 1 4 7 µ,F 16VW electrolytic capacitor 1 1 kn 1 / 4 W resistor BOG Flasher 1 PCB as above, or small piece of Veroboard 1 LM3909 integrated circuit 1 4 7 µ,F 1 6V electrolytic capacitor 1 3mm or smaller diameter red LED 1 1.5V AA-size carbon zinc or alkaline cell 1 AA size 1-cell holder (Tandy Cat. 270-401) 1 subminiature panel mount slide switch, Jaycar Cat. SS-0852 or DSE Cat. S-2010 . Miscellaneous Superglue, solder, hookup wire. The BOG circuit Our circuit for the "battery operated guard" is taken directly from the National Semiconductor applications literature for the LM3909 - see Fig.1. Besides the LM3909, it uses just three components: a light emitting diode, a 47µ,F capacitor and a 1.5V cell. You couldn't have a much simpler circuit than that. Well the circuit is simple enough to build up but explaining the operation of the LM3909 is anything but simple. Basically what it does it is to charge up the reservoir capacitor to about 1.2V. Then it effectively connects the charged 47 µ,F capacitor in series with the 1.5V battery and uses the 2.7V combination to briefly flash the LED. Thus, the LM3909 powered from a 1.5V cell can easily drive LEDs even though they typically require 1.6V to conduct. Green and yellow LEDs need more voltage; 2.2V or more. For a more detailed explanation of the internal operation of the What could be simpler? The battery operated guard circuit uses just one IC, a capacitor and a light emitting diode (LED) - plus a switch and 1.5V dry cell. The flashing LED of the BOG circuit is mounted just above the coupler on the end of the last wagon. This photo shows how we installed the battery operated guard components on the bed of a HO wagon. Both the cell holder and slide switch are glued in position. The switch actuator protrudes through a slot cut in the bottom of the wagon. S1 1.5Vf - - - - - - - - - - - - - - - - s1 - - - - - - - - ~ _,. 0 , 6 LED1 LM3909 2 END-OF-TRAIN FLASHER Fig.1: the circuit for the battery operated guard. The LED flash rate is about twice a second. 0 SINGLE CELL HOLDER ~ WAG ON BED Fig.2: here's how to install the battery operated guard circuit on the wagon bed. You can make the cuts in the Veroboard pattern using a large drill. FEBRUARY1989 53 TO TRACK 47000 5VW + - LOCOMOTIVE FLASHER Fig.3: the locomotive flasher derives its power from the track. The bridge rectifer takes care of AC or reverse-polarity DC voltage on the track, while the 4. 7V zener diode limits the supply voltage to the LM3909. I TO TRACK F -5."1,_ _ , I I ~ ::=:.. .JI K LED1 A Fig.4: parts layout for the locomotive flasher. All parts except for the lk!1 resistor are polarised so be sure to install them the right way around. SC09102891 LM3909 see the panel accompanying this article. The locomotive flasher Fig.5: here is a full size pattern for the PCB. The locomotive version of the circuit is shown in Fig.3. While the BOG circuit above runs from a single 1.5V cell, the locomotive ver- This is the fully assembled locomotive flasher circuit, prior to installation in the locomotive. Note that the zener diode and resistor are mounted end-on to conserve space. The handed end of the zener diode denotes the cathode lead. 54 SILICON CHIP We used a 3mm LED and mounted it through a close-fitting hole in the roof of the cabin. sion of this flasher gets its power from the tracks or more precisely, from the wires to the motor in the locomotive. The circuit therefore needs to cope with negative or positive rail voltages (depending on whether the loco is going forward or in reverse). It also needs to cope with a variety of train controllers which may supply smooth DC up to 12V, unsmoothed DC of up to 20 volts peak, low voltage AC up to 12VAC (in the case of Marklin models and some other Continental brands), or pulse width modulated DC as in the case of the Railpower which was published in the April and May 1988 issues of SILICON CHIP. In all these cases, the supply voltage can be expected to vary widely. Normally, there will be some minimum voltage of 2V or more always present on the track, at which the loco will not move. Above this voltage the loco will begin to move and in normal operation 4V or more can be expected across the tracks. To cope with these conditions, the locomotive flasher has a bridge rectifier and a 4. 7V zener diode. The bridge rectifier takes care of low voltage AC or DC of either polarity while the zener diode protects the LM3909 against voltages of more than 5V. Current through the zener diode is limited to a safe value by the 1kn resistor. Super cap The cunning part of the locomotive flasher is the inclusion of a .047 Farad (47,000µF) 5V super capacitor made by NEC of Japan. Normally intended for memory backup use in computers, VCRs and FM tuners (for storing the station settings), the supercap is used here simply as a large reservoir capacitor, albeit in a very small package. So while the 4. 7V zener diode protects the circuit (and the supercap) against the higher track voltages, the supercap lets the circuit continue to operate more or less normally when the track voltage drops to low values or the loco is on a section of track which is temporarily unenergised. In the latter condition, the supercap will power the flasher circuit for about five minutes or more, which could be a considerable boost to model realism. After all, normally when power is removed from a model loco, it is completely dead. With the flasher in operation, it looks active even though it is not moving. Construction Because these two circuits use so few components there are a number of different approaches to construction. For the locomotive flasher we have produced a small printed circuit board measuring 35 x 20mm (Code SC09102891). This accommodates all the circuit components and can be comfortably fitted into typical HO scale (1:87) American diesel electric locomotives. Assembling the board is easy. The specified W04 bridge rectifier is available from Dick Smith Electronics (Cat. Z-3304) while the .047 Farad supercap is available from Jaycar (Cat. RU-6700). The 4.7V zener diode may be a 400mW or 1W rated type. Both the zener diode and 1kn resistor are stood "on end" Yes, it fits! We slid the circuit board for the locomotive flasher into a vacant space behind the leading bogie but other arrangements can be used. The power supply leads are connected across the motor. to minimise the space they occupy on the board. The LED is wired to the board using two short lengths of light duty hookup wire or a length of 2-way rainbow cable. Make sure that the LED is correctly oriented before wiring it into circuit. In general the longer wire on the LED is the anode (positive connection). Similarly, it is important to ensure that the 4 7µF electrolytic capacitor and the 0.047F supercap are connected with correct polarity. On the 47µF capacitor the negative connection is easily identified with the black strip down one side which is often associated with a minus sign. On the supercap, no minus signs are present but there are a couple of black lines down one side to in- dicate the negative connection. The negative pin is also slightly longer than the positive pin. When construction is complete, connect the circuit up to a variable DC supply. The circuit should work down to at least 2.5 volts. Alternatively, if you don't have access to a variable supply, you can use two 1.5V cells in series. A few seconds after the supply is connected the LED should begin to flash. The rate of flashing does vary according to the supply voltage. If the supply is above 6 volts (ie, biasing the zener diode fully on) the LED will continue to flash for at least five minutes after the supply is disconnected. Installation In the loco in the photos, we inFEBRUARY 198~ 55 How the LM3909 Works The LM3909 has been specifically designed to flash LEDs from a supply of 1.5V or less although it will operate on supplies up to 6V. It will work in other applications too although most of these revolve around its ability to work efficiently as an oscillator from low voltage supplies. Fig.6 shows the internal circuitry of the LM3909 plus the external components needed for it to work as a LED flasher. It is reproduced from National Semiconductor data. Here's how it works. When power is first applied, the 300µF capacitor connected between pins 2 and 8 has no stored charge (ie, the voltage across it is zero), 04 is biased on and 01 is off. The 300µF capacitor then begins to charge via the two 4000 resistors and via the internal resistance selected by pins 8 or 1 . In the case depicted in Fig. 6, the internal resistance selected is 3k0. As the capacitor charges, the voltage at pin 8 falls . Hence the emitter voltage of 01 also falls while its base voltage remains largely constant. After about one second, the voltage between the base and emitter of 01 exceeds O. 6 volts and so O 1 turns on. 04 then turns off, as its base voltage is forced low. When 01 turns on, it turns on 02 and 03 which pulls pin 2 and hence the positive side of the capacitor down to OV. This means that the negative side of the capacitor will be forced down to about -1 .2V. Since the LED is connected between pins 6 and 8, it now has 1.5V + 1.2V from the 300µF capacitor applied to it. The resulting pulse of current, as the 300µF capacitor discharges, causes the LED to flash briefly. The discharge current is limited by the internal 1 20 resistor in series with pin 6. stalled the circuit board vertically within the body. It fitted in easily. The supply to the board should be taken from across the motor, using light and flexible leads. A close fitting hole needs to be drilled in the roof of the cabin, to take the LED. We used a 3mm LED which is really a little large to be of correct size for HO scale. A number of manufacturers such as HewlettPackard produce 2mm LEDs in red, orange, yellow and green. These would be much closer to correct scale. To our knowledge though, these are not generally stocked by retailers but we understand David Reid Electronics [phone (02) 267 1385) will obtain stocks if demand warrants it. The flasher circuit could probably also be fitted into N scale (1:160) locos although somewhat 56 SILICON CHIP LM3909 _..,,..,.,.:3--___.1w2n~-----------i'5::.._.+1.5v LED 03 1---- Fig.6: basic schematic of the LM3909 IC. The external capacitor is charged to about 1.2V and then connected in series with the battery to give 2. 7V to flash the LED. The whole cycle then recommences with 01 turning off and 04 turning on , to allow the 300µF capacitor to recharge . Note: this explanation is not the whole story as the circuit of Fig .6 is a schematic only; it does not show all the internal componentry of the LM3909 . Instead of the explanation above, the LM3909 could be. regarded as a bistable pair; ie, 01 and 04 with positive feedback applied around the circuit by the 300µF capacitor. Either way, the circuit does oscillate and the LED flashes at about once a second. more ingenuity would have to be employed. BOG construction The BOG flasher circuit could be built on the printed circuit board or assembled onto a small piece of Veroboard as shown in our photos. We powered the unit from a standard 1.5V AA cell mounted in a onecell holder (Tandy Cat. 270-401). This was glued to the bed of a HO wagon. A slide switch is mounted (glued) so that its actuator pokes through the underside of the wagon, so that it can be easily turned off and on but is barely visible. This circuit could also be easily mounted in an N-scale wagon although you would need to use an AAA size 1.5V cell. The circuit could also be made a great deal more compact by dispensing with the Veroboard and wiring the LED and 47 µF capacitor directly to the LM3909 IC. In fact , by using this approach and powering the circuit from a silver oxide or mercury button cell (as used in cameras, watches and some calculators), it may be possible to fit the flasher into a Z-scale (1:220) wagon. Incidentally, we have specified the 47 µF capacitor with a 16V rating. This may seem a little over the top since the supply voltage is only 1.5V in the case of the BOG circuit and no more than 4. 7V in the case of the loco version. However , unless your parts retailer has old stock it is unlikely that you will be able to buy electrolytic capacitors with a voltage rating of less than 16VW (VW means " volts working") or even 25VW. That's the way they make them these days. ~