Santa Maria Valley 70 Ton #70 
Using 1.5 volt bulbs with DCC decoders!
Normally, decoders put out basically track voltage (usually 12 to 16 volts in the HO world) on the function leads. If you connect 1.5 volt bulbs directly to these outputs, their life is measured in a fraction of a second, perhaps even destroying the control transistor inside the decoder. There are several ways to control these lovely bulbs, each with advantages and disadvantages!
Decoder
Train Control Systems has built a decoder with 1.5 volt support included. The A6X HO decoder is built on an Atlas Light Board sized circuit board. It can control up to 6 functions with an internal 1.5 volt regulator. Nothing is easier! CLICK HERE
Train Control Systems also offers an external 1.5 volt regulator to interface with some of their other decoders. CLICK HERE
As of now, no other manufacturer has followed suit. Pity!
Resistor
If you know your track voltage and will NEVER operate your loco on another layout or change the track voltage on your layout, then you can simply add a resistor in series with each 1.5 volt bulb. The formula for calculating the resistor is:
Resistor (ohms) = 1000 * (track voltage - 1.5) / (bulb current in mA)
For a 15 mA bulb with 15 volts on the track, the value is:
Resistor (ohms) = 1000 * (15-1.5)/15 = 900 ohms
The advantage of this method is that it is quick and cheap. The disadvantage is that the bulb(s) may vary from not lit to blown out depending upon the design voltage and the track voltage as you move from one layout to another.
Resistor and Diodes
The quickest and cheapest and most reliable system is to add a couple of diodes to the basic conneciton above to regulate the voltage applied to the bulb(s). A forward biased (activated) diode drops about 0.7 volts, so two in series will provide about 1.4 volts over a wide range of input voltages.
In the above circuit, it is critical that the banded end of the diodes are connected as shown. The BLUE connection can be to the blue wire or to a rail (red or black). The WHITE connection can be any function lead (yellow, green, brown, etc.).
For 15 mA bulbs, the resistor will vary with the number of bulbs as shown below. These values are calculated for any track voltage between 12 and 16 volts:
| Bulbs | Ohms |
Watts |
| 1 | 390 | 1/4 |
| 2 | 180 | 1 |
| 3 | 110 | 1 |
The advantage of this scheme is that it is easy and inexpensive and provides good performance over a wide range of track voltages. The disadvantage is that the resistor can get rather warm and should not be in contact with plastic components.
Current regulator
A solid state current regulator can be used to contol as many 15 mA bulbs as you wish to light at one time (up to about 6). A LM334 Integrated Circuit can be connected with a 5.1 ohm resistor to make a current limiter that will deliver about 14 mA regardless of the supply voltage. This elegant solution runs very cool and is small, allowing for easy installations. Connect the IC as shown in the diagram below and cover with shrink tubing.
The single lead can be connected to the decoder blue wire or to a rail (red or black) wire. The other end of the bulb (or string of bulbs in series) is connected to the function lead (white, yellow, etc.).