Well...almost constant. Unless you have embraced DCC, locomotive headlamps are going to be OFF when the loco is stopped and there's no voltage on the track. If you want to avoid DCC but would really like your headlamps to be ON when the loco is stopped, you're gonna' need to add some batteries to your locomotive (this site's section on Car Lighting will show how this can be done). With the circuits shown here, once you move your DC throttle a bit off of zero, the lamps will quickly come to full intensity and will remain at almost constant intensity regardless of throttle setting.
GLOWING FILAMENT OR SOLID STATE?
If you like incandescent lamps (as shown above), we'll be using the 1.2mm bulbs from Miniatronics; they're the most readily-available 1.5 volt bulbs (and we need 1.5V bulbs). If you'd rather have LEDs, you'll have many to choose from; but be aware that one important property of LEDs -- the "forward voltage" (Vf) varies widely between types and can significantly affect the design of a Constant Intensity circuit. We'll exclude red and green LEDs for obvious reasons, and focus for the moment on yellow ones; there are a few yellow LEDs available with Vf=1.7V, almost the same as a 1.5V bulb and therefore compatible with the circuit for incandescent bulbs (above); one such yellow LED is Jameco #333622. Most yellow LEDs, however, have forward voltages in the 2.0-2.3 volt range, and are suitable for use with the circuits identified for LEDs. If you belong to the White or Blue LED Fan Clubs, you need to know that these critters have forward voltages in the 3.2 to 4.8 volt range, and will require modifications to the circuits shown; specifically, you'll need to add more diodes/bridges in series with those shown in order to produce the greater voltage. If you really want to use white or blue LEDs, please Email me with the particulars of your LED, and we'll cook something up.
The diode bridge shown on the schematics should do fine for Z, N and HO-scale motors; if you're in a larger scale, you'll want to use a heftier bridge (3 amps or more, such as Radio Shack 276-1173 or equivalent); the discrete diodes should work for all scales whose motors draw less than 3 amps.
For giant motors drawing more current, you're going to need bridges and diodes rated at 5 amps or greater; Radio Shack 276-1181 and 276-1661, respectively...or equivalent. Note that you do need to add a wire between the positive and negative outputs of the bridge; this is what makes it act like two pairs of diodes in series (rather that an "actual bridge").
WHICH WAY ARE YOU GOING?
Want directional control with your lighting? By adding a few more diodes we can cause the front lamp to illuminate only when the loco is going forward, and the rear lamp to glow only in reverse. This works equally well with both incandescents and LEDs. The circuit below shows how this is done:
THE PRICE YOU'LL PAY
As the saying goes, "There is no such thing as a free lunch." In order to (easily and cheaply) get a constant intensity headlamp, we've connected the lamp in series with the motor; this means that you'll lose voltage on the track. In the Basic Circuit, you're giving up about 1.4-1.5 volts; if your maximum track voltage was 14 volts, it'll now be 12.5 volts. Probably not a big deal, unless you (a) have a low-output DC throttle, (b) add more diodes for LEDs and/or directionality, (c) run your trains very fast, OR (d) some combination of a, b, and c. Diodes "drop" about 0.7 volts each (it varies a bit by diode type and current); hence, with the Deluxe Circuit with most yellow LEDs, you'll find 2.8-3 volts less on the track, and if you use white or blue LEDs, you could be surrendering 4-5 volts. Also note that directionality (see above) also exacts a price in voltage; those diodes we added to make our lights smarter just devoured another 0.7 volt from the track. Don't lose sleep over any of this, just be aware of it...and the tradeoffs you make.
I have not personally tried every variation of diode and LED.
If you're experiencing a too-dim or too-bright LED, Email me
with the particulars of your LED and we'll try to find a solution.
Some motors draw very little current (rare in the popular brands), and may cause these circuits to malfunction. IF you know you have a low current motor, or IF you observe strange circuit behavior, you may need to add a "load resistor" across (ie, in parallel with) the motor; the purpose of this is to increase the current drawn thru the diode bridge by the motor/resistor combination. A typical value for this resistor is 240 ohms, 1 watt; you could substitute two 120 ohm, 1/2 watt units in series if that helps. Note that this resistor may get warm, and should be able to breathe a bit.
WHICH IS THE "HOT" PICKUP?
Simply stated, it's the one that sits on the positive track rail when the loco is moving forward. Use any type of voltmeter (eg, DVM, VOM) to determine the positive rail, then write it down somewhere (eg, "engineer's side" or "non-engineer's side") you won't forget. It'll vary by loco model, but if you have four Kato SD-40s, they'll all be the same. Lacking a voltmeter, you could use an LED and resistor to identify polarity.
The larger the scale of your models, the easier this will be. There's not a lot of spare room in most Z and N-scale locos, but you can probably fit a bridge and a diode or two into the cab or in some previously-overlooked cranny. Construction is
very forgiving, so fit things where you can (just don't let the wires touch each other or frame metal).
If you prefer the do-it-yourself approach (or simply dislike integrated bridges), you can accomplish the same thing with four diodes. For small scales, I suggest 1N4001 types; for more current, try 1N5400 family devices. It really doesn't matter (but bridges are easier).