IF you use Miniatronics 1.5 volt "grain of wheat" or "grain of rice" bulbs for illumination on your layout, then this project is tailor-made for you. Depending on the bulb you're using and the transformer you choose, you can conveniently operate up to 160 bulbs from this simple supply (assuming 15mA bulbs). By using a center-tapped transformer, we can get two separate supplies from a single transformer (thereby doubling your available current). And, since we're producing only the 1.5 volts needed by the lamps, no dropping resistors are needed. Depending on your needs, you can implement one or both sections of the supply...or, one now and the other later. Cost can range from $10-15 -- maybe less if you buy carefully; on a per-lamp basis, that's pretty darned reasonable!

power supply schematic

You'll note that there are two identical regulator circuits (the second one on a pale yellow background), one working off each side of the transformer; depending on your needs, you can implement one or both sections on whatever transformer you choose. Most any 12.6 volt, center-tapped (CT) transformer will work just fine; in the table just above, I've shown two common units (both available from your local Radio Shack) along with the number of lamps they'll drive. Remember to wire the bulbs in parallel, as shown below:

lamp wiring

hints & tips



1. Don't forget the heat sink on the voltage regulator IC(s). Depending on the load current, that little regulator can get very hot! [If yours gets too hot to touch, use a better heat sink.]
2. The on/off toggle switch is optional; I don't use one, and simply have the transformer plugged into the outlet strip that runs the whole layout. It's up to you...just don't omit the fuse.
3. If you'd like to run your lamps just a wee bit dimmer, thereby prolonging their life, you can lower the output voltage slightly by using a smaller resistor in place of the 47 ohm units shown; the chart below shows several options. Don't go lower than 10 ohms.

resistor chart

4. All resistors are ¼ watt (½ watt is also OK).
5. Feel free to substitute electrolytic capacitors; for example, three 1000uF caps for a 2200uF cap, or two 2200uf caps plus a 470uf unit for a 4700uf cap; connect them in parallel (parallel cap values add), and watch those polarities. In these circuits, any DC Working Voltage of 16 or greater is fine. You can always use more capacitance, just don't go less than what's shown in the chart.
6. BE CAREFUL wiring up the transformer primary and connecting to the 115VAC house current; this voltage is DANGEROUS! If you're not comfortable doing this, get help from a qualified friend or electrician.

Need lots more power to light hundreds of 1.5 volt bulbs?
Want to use 2.5 volt Christmas tree lamps?
Then take a look at LAMPZILLA.

Finally, if you have any questions about
this circuit, or would like a slight variation,
just Email Me, and I'll try to help.

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Updated 3-31-02