myowngod wrote: 
The bulbs being used are 1.5v 15mA, grain of rice style(YELLOW circles). I was thinking of having 5 rows, 1 for each timber frame between tracks, with each row having 4 bulbs....The power supply will probably be an old transformer, maybe 12v.
There is a total of 20 bulbs in the circuit. My question is, should I put a resistor on each leg of 5 bulb rows or one larger one at the point where the wires run to the transformer? Also any ideas how many Ohms they should be? Each row would total 6v, the whole building 30v. I would rather the bulbs be dimmer then brighter, since there's so many in the building. I'll put a few bulbs on the outside of the structure near the back service doors as well
Thanks


If the bulbs have a current of 15 mA at 1.5 volts steady state, then the resistance is 100 ohms.
With a supply of 12 volts with 5 bulbs in series, the voltage drop is 7.5 volts and the total resistance is 500 ohms if you allow 15 mA of current to flow through them. The voltage that the resistor needs to drop is 4.5 volts. To to this for 15 mA, it will need to be 300 ohms.
If each rung of 5 bulbs has a resistor, each resistor will drop .0675 watts. A quarter watt resistor will do just fine. If you use a single resistor for all 4 rungs, then you will need at least a half watt resistor. What you could do on top of this is put in a 500 or so ohm pot and adjust it to dim the lights to whatever you want.
The above calculations where done easily with ohms and watts law.
Ohms law ( V = I * R )
Watts law ( P = V * I )
V is voltage in volts
I is current in amps (divide mA by 1,000 to get amps, so 15 mA = .015 amps)
R is resistance in ohms
P is power in watts.
The full wheel to get any 4 value from any combination of two others:
http://www.sengpielaudio.com/FormulaWheelElectronics.gif