3mm Round Top Red / Blue Alternating Flashing LEDs

3mm red/blue alternating LEDs house two separate LED dies — one red and one blue — inside a single 3mm T-1 epoxy package with an integrated…

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3mm red/blue alternating LEDs house two separate LED dies — one red and one blue — inside a single 3mm T-1 epoxy package with an integrated circuit (IC) that automatically switches between them. The red die illuminates, extinguishes, then the blue die takes over, cycling continuously at a steady cadence. The instantly recognizable red-and-blue emergency strobe pattern runs automatically with no external components beyond a current-limiting resistor. The 3mm form factor is purpose-built for scale model emergency vehicles and dioramas where a 5mm LED would be oversized — HO scale (1:87) police cars, fire apparatus, and ambulances all benefit from the smaller package that fits inside scale-appropriate light bar housings and grille cavities without modification.

HO and N scale emergency vehicle models: The primary use case for the 3mm red/blue alternating LED is HO scale police car, fire truck, and ambulance models. At 1:87 scale, a real vehicle’s 1,200mm light bar scales to approximately 14mm — a tight space where every millimeter of LED diameter matters. The 3mm body fits inside custom-made or kit-bashed light bar housings drilled with a 3.2mm (1/8″) bit. One LED provides both red and blue alternation, eliminating the need for two separate LEDs and an alternating circuit. Model railroaders staging emergency scenes — a grade crossing accident, a building fire response, a police traffic stop — mount 3mm red/blue alternating LEDs in the roof housings of scale emergency vehicles positioned on layout roads. For N scale (1:160), the 3mm LED is usable in larger vehicles, though it may appear slightly oversized; consider a 1.8mm LED if the vehicle cavity permits.

Security deterrent and alarm indicators: The compact 3mm package fits inside small tinted dome housings, fake camera shells, and narrow indicator windows on DIY security panels. A red/blue 3mm LED pulsing behind a dark lens on a garage eave, gate post, or vehicle dashboard creates an effective security deterrent in a smaller, less conspicuous package than the 5mm version. The low 20mA current draw allows months of operation from a small battery pack or indefinite operation from a USB power supply with a simple resistor circuit.

Miniature wargaming and diorama police scenes: Warhammer 40K Arbites vehicles (the 40K equivalent of police) and Judge Dredd-style law enforcement terrain pieces use 3mm red/blue alternating LEDs for emergency light bar effects at 28mm gaming scale. Gaslands and other vehicle combat wargames use 3mm red/blue LEDs in Hot Wheels-scale police car conversions. Tabletop RPG terrain builders embed them in modern/sci-fi police precinct buildings and checkpoint barriers. The single two-pin LED replaces what would otherwise require two separate LEDs and an alternating timer circuit, simplifying the wiring inside small terrain pieces.

How the dual-die design works: Inside the 3mm package, two LED dies (one red, one blue) share a common lead structure with an IC that switches current between them. The external pinout is standard two-pin: one anode (+), one cathode (−). The lens appears clear/colorless when off. When powered, you see red, then blue, alternating smoothly from the same physical point. The forward voltage is approximately 3.0–3.2V (dictated by the blue die). Maximum current: 20mA. Use the blue/white column in our LED resistor calculator for accurate resistor values.

Electrical and wiring: At 12V supply, use a 470Ω resistor. At 5V (Arduino), use a 100Ω resistor. The LED wires identically to any standard single-color LED: longer lead to positive, shorter lead to negative. For DCC or AC model railroad power, add a bridge rectifier and smoothing capacitor — see the AC/DCC wiring guide. For zero-math installation, browse animated pre-wired LEDs with the resistor pre-attached. New to LEDs? Pre-wired LEDs are the easiest starting point.

Related categories: The same red/blue alternating animation is available in 5mm for larger scale models (O, G, 1:24). For single-color flashing in 3mm, browse 3mm 1Hz, 3mm 2Hz, and 3mm 6Hz. For full RGB color control under your own circuit, see RGB LEDs. For static (non-animated) 3mm LEDs, see 3mm round-top LEDs.

Use the forward voltage of the blue die (~3.0–3.2V) for your calculation, since the IC accommodates the higher-voltage die. At 12V, use a 470Ω resistor. At 5V, use 100Ω. Use our LED resistor calculator with “blue” selected for accurate results.
No. An RGB LED has three dies (red, green, blue) with separate leads you control individually. The red/blue alternating LED has two dies (no green) and an internal IC that switches between them automatically. It is a simple two-pin device with no user control over timing. Choose it for a no-code police strobe; choose RGB for full color control.
The alternation is typically in the 1–2Hz range per color — each color displays for roughly 250–500 milliseconds. The cadence is recognizable as a police-style strobe but slow enough that each color is distinctly visible.
Yes. At HO scale (1:87), a full-size 1,200mm light bar scales to about 14mm wide. The 3mm LED body fits comfortably inside custom light bar housings drilled with a 3.2mm (1/8″) bit. For even smaller models or N scale, consider the 1.8mm version.
Absolutely. A multi-point light bar build can combine one or two red/blue alternating LEDs with separate 3mm 6Hz red and 3mm 6Hz blue flashing LEDs. Each LED runs on its own independent IC, so they operate simultaneously without interference. Just give each LED its own resistor.
Yes, after converting to DC. Add a bridge rectifier and smoothing capacitor. See the AC/DCC wiring guide for full instructions.