What voltage power supply should I use?

12V DC is the most common and versatile choice for diorama lighting. It allows multiple LEDs in series and is safe to handle. 5V USB power is convenient for portable displays. 9V batteries work for temporary or small setups.

Do I always need a resistor with LEDs?

Yes, almost always. LEDs need a current-limiting resistor to prevent burnout. The only exception is if you use a dedicated LED driver or a constant-current power supply. Never connect an LED directly to a battery or power supply without a resistor.

What current should I run my LEDs at?

Standard 3mm and 5mm LEDs typically run at 20mA for full brightness. For diorama use, 5-10mA often gives a more realistic, dimmer effect. SMD LEDs vary widely — check the datasheet. Running LEDs below their rated current extends their life.

How do I hide wiring in a diorama?

Route wires through channels carved in the base foam before adding terrain. Use thin enamelled copper wire (0.1-0.2mm) which is nearly invisible when painted. Fibre optics are excellent for tiny light points as the fibre itself is the light guide.

ModelCalcs.uk

Lighting Calculator — LED Resistor & Power Planning

Plan LED quantities, calculate resistor values and size the power supply for diorama lighting effects.

Number of LEDs

Total LEDs in this circuit. They are grouped into series strings automatically.

Supply Voltage

DC supply voltage. 12 V is the usual diorama choice; 5 V suits USB power, 9 V suits a PP3 battery. Must exceed one LED's forward voltage.

LED ColourColour sets the forward voltage (Vf): warm-white 3.2 V, cool-white/blue 3.4 V, green 3.2 V, yellow 2.1 V, red 2.0 V. Choose Custom to type the exact Vf from your LED's datasheet.

LED Current

Forward current per LED. 20 mA = full brightness for 3mm/5mm LEDs; 5-10 mA gives a softer, more scale-realistic glow and longer life. Check the datasheet for SMD/micro LEDs.

How We Calculate This

The calculator uses Ohm's law to size the current-limiting resistor for each LED string in your diorama.

Formula

R = (V_supply − n × V_forward) ÷ I_led — where n is the number of LEDs in the series string
Resistor power P = (V_supply − n × V_forward) × I_led
n LEDs in series share one resistor, up to n = floor(V_supply ÷ V_forward) (e.g. 12 V ÷ 3.2 V → 3 warm-white LEDs per string)
Each parallel string needs its own resistor and draws the full LED current

Choosing the resistor

The ideal resistance is rounded up to the next E12 standard value (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their decade multiples). Rounding up keeps the actual current at or just below the rated value, which is the safe direction for an LED. Fit a resistor rated above the calculated wattage — a ¼ W (0.25 W) part is plenty for typical diorama LEDs.

Uneven strings

If the LED count is not an exact multiple of the LEDs-per-string, the last string is shorter. A shorter string drops less LED voltage, so it needs a larger resistor — this is calculated and shown separately so every string stays at the target current.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify quantities before purchasing materials.