Constant Lighting

HOME What's New Modellers Notebook Construction Articles Tech Notes DCC &Sound Articles PHOTOS A Short Story for Children S.R.M. at the W.D.M. LINKS Guest Book Contact Us

Constant lighting without DCC is easily accomplished with integrated circuit voltage regulators. A simple circuit shown in easy to read pictorial format is shown here. The basic circuit, shown below, uses an LM317 or similar voltage regulator to feed a constant 1.25 volts to one or more 1.5 volt lamps.  The circuit is wired in parallel with the motor and includes a diode to make it directional and an optional capacitor to make the lights brighter with pulse power, if so desired. Optionally, one or more extra diodes or a zener diode can be added in series with the circuit to make the lights come on at a higher track voltage.  If this option is not desired, then these diodes can be left out and the single diode to the left of the regulator can be connected directly to the motor or wheels.  This flexibility would allow, for example, using one of these circuits without the extra diodes to power the headlight, and another of these circuits with optional diode(s) to power the ditch lights.  Then the headlight would turn on before the engine started to move but the ditch lights would come on after the train was underway. The basic circuit without the optional diodes will turn on the light with about 3 volts on the rails.  The turn on voltage will increase by about .6 volts for each regular diode or by the rated voltage for the zener diode.  In the circuit shown, if all five regular diodes were used, the light would turn on with about  5 x .6 + 3 = 6 volts dc on the rails. For the regular diodes, 1N4001 are cheap cheap and easily available.  For the capacitor try about 100 micro farads at 25 volts. There is a limit to the number of 1.5 volt lamps (wired in parallel) that can be connected to this circuit.  The limit depends on a number of factors including lamp currents, rail voltage, and power rating of the particular regulator selected.  Fortunately, most of these regulators, including the LM317 are self protecting, that is, excessive current or excessive power dissipation causes the regulator to shut itself off rather than burn itself out.  However, this feature does NOT protect the regulator from miswiring.  The terminals of the T version of the LM317 are shown below.      1 = Adjust (negative input)   2 = Output (to lamp)   3 = Input (positive input from diode(s))   CAUTION The metal tab is electrically connected to pin 2 on most regulators.  This tab must not be allowed to contact other electrical connections, including the engine frame. <> For those who are not electronically inclined but who would still like to build this circuit, a pictorial diagram is shown below.  This is a good first project in electronics. In the pictorial diagram, the red, yellow, blue and black wires are insulated.  They can be any colour you like (electrons are colour blind) but the colours shown here are the colours referred to later in the text.  You can make the wires as long as necessary, but do not use too large a wire gauge.  Twenty-four gauge wire is large enough.  The copper coloured wires are bare.  If you keep the bare wires short, or insulate them, you will reduced the chances of short circuits. You should solder the joints where bare wires join.  If your soldering is a little shakey, check out the soldering primer elsewhere on this site. Be sure to keep the bare wires and the metal back and tab of the LM317T integrated circuit from touching one another or any metal part of the locomotive.  You can mount the parts with double sided foam tape (the kind used to mount decoders) but be sure to use the tape on only one side of the integrated circuit or it will overheat. Note that the diode has a white band around one end.  You must connect this end of the diode to the LM317T integrated circuit.  You can use any diode from the 1N4001 family ( 1N4001, 1N4002, 1N4003 etc.) The connections to the integrated circuit are not interchangeable.  Be sure to connect the wires as in the diagram. The lights should be rated at 1.5 volts, 30 to 50 milliamps.  You can use as many as ten with this circuit. To use this circuit to operate headlights or other lights that light when the locomotive is moving forward, connect the red wire to the right hand wheels and the blue wire to the left hand wheels.  To operate reverse lights when the locomotive is moving backward, connect the red wire to the left hand wheels and the blue wire to the right hand wheels.  Left and right sides of the locomotive are the same as your left and right when you are sitting in the engineer's chair and looking forward.  For bidirectional lighting, you can use two of these circuits.