5mm Round Top Red / Blue Alternating Flashing LEDs

5mm red/blue alternating LEDs contain two separate LED dies — one red and one blue — housed inside a single 5mm epoxy package with a shared…

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5mm red/blue alternating LEDs contain two separate LED dies — one red and one blue — housed inside a single 5mm epoxy package with a shared integrated circuit (IC) that automatically switches between them. The red die illuminates, then shuts off as the blue die takes over, then red again, cycling back and forth continuously at a steady cadence. The result is the instantly recognizable red-and-blue emergency strobe pattern used by police vehicles, ambulances, and fire department command cars worldwide. No external wiring tricks, no microcontroller, no code — just a current-limiting resistor and a DC power source, and the alternating pattern runs automatically from the moment power is applied. This is the simplest possible way to add a realistic police/emergency light effect to a scale model, diorama, prop, or display.

Police car and emergency vehicle models: The primary use case for red/blue alternating LEDs is scale model emergency vehicles. Die-cast police car collectors and plastic model kit builders in 1:24, 1:43, and 1:64 scales mount these LEDs in roof-mounted light bar housings, grille lights, and rear window decks. The automatic alternation eliminates the need for a two-LED circuit with an alternating relay or microcontroller — one LED does the job of two. For more complex multi-light bar builds, combine a red/blue alternating LED with separate 6Hz red and 6Hz blue LEDs to create a light bar with both alternating and synchronized strobing sections. Fire truck modelers pair red/blue alternating LEDs with separate amber 6Hz LEDs for the mixed-color cascade pattern on modern apparatus. Ambulance builders use red/blue alternating LEDs in conjunction with white strobe LEDs for the high-visibility intersection-warning pattern. The 5mm package fits standard model light bar dimensions for 1:24 through 1:43 scale; for smaller scales, see the 3mm red/blue alternating variant.

Security deterrent and alarm indicator lights: A red/blue alternating LED mounted behind a tinted dome on a building eave, dashboard, or fence post creates a highly effective visual security deterrent. The red-blue flash pattern is universally associated with law enforcement and emergency response, and even a single small LED pulsing red-to-blue is enough to give a potential intruder pause. DIY home security builders mount these LEDs in fake security camera housings, near entry doors, on car dashboards (visible through the windshield as a car alarm indicator), and on gate posts. The 20mA power draw means a small battery pack or solar cell can run the deterrent 24/7 with minimal maintenance. Some homeowner association communities and rural properties use red/blue alternating LEDs on mailbox posts and driveway markers as visible security signals at night.

Patriotic displays and themed decorations: The red-and-blue alternation lends itself naturally to patriotic themed displays for Independence Day (July 4th), Memorial Day, Veterans Day, and Flag Day. Embed red/blue alternating LEDs in yard signs, flag display cases, parade float trim, and memorial garden markers for an eye-catching effect that does not require a complex multi-LED circuit. Combine with static white LEDs for a full red-white-blue presentation. Similarly, red/blue alternating LEDs are used in superhero themed displays (Spider-Man, Superman, Captain America), sports team displays (any team with red and blue in the color scheme), and theater/stage props where a quick police-light effect is needed during a scene change. Escape room builders wire them into “alarm triggered” sequences that activate when a puzzle state changes.

How the dual-die design works: Inside the 5mm package, two separate LED dies (one red, one blue) share a common cathode or common anode lead with the internal IC switching current between them. From the outside, the LED has the same two-lead pinout as any standard 5mm LED — one anode (+) and one cathode (−). The lens appears clear/colorless when the LED is off. When powered, you see red, then blue, alternating smoothly. Because both dies share a single lens, the color transitions appear to happen in the same physical spot, creating a convincing single-point strobe rather than the split-color appearance of two separate LEDs placed side by side. The forward voltage is typically in the 3.0–3.2V range (driven by the blue die’s requirements). Maximum forward current: 20mA.

Electrical characteristics and wiring: Because the internal IC must drive the blue die (which has the higher forward voltage), the overall forward voltage of a red/blue alternating LED is approximately 3.0–3.2V. Use the blue/white column in resistor tables and calculators. A series current-limiting resistor is required. Use our LED resistor calculator to find the correct value for your supply voltage. At 12V, use a 470Ω resistor. At 5V (Arduino), use a 100Ω resistor. For DCC or AC power sources on a model railroad, add a bridge rectifier and smoothing capacitor — see the AC/DCC wiring guide. The LED wires identically to any standard single-color LED: longer lead to anode (+), shorter lead to cathode (−). New to LEDs? Pre-wired LEDs include the resistor on the wire.

Related categories: The same red/blue alternating animation is available in 3mm size for smaller scale models. For single-color flashing, browse 1Hz slow flash, 2Hz fast flash, or 6Hz extra-fast flash in red or blue individually. For full RGB color mixing under your own control (not just red-blue alternation), see RGB LEDs. For static (non-animated) 5mm LEDs, see 5mm round-top LEDs. For pre-wired animated LEDs with resistor attached, browse animated pre-wired LEDs.

The LED contains two separate dies (one red, one blue) and a tiny integrated circuit (IC) inside a single 5mm package. The IC automatically switches current between the red and blue dies in a repeating alternation pattern. From the outside, the LED has a standard two-pin configuration — no special wiring is needed. Just connect it like any other LED with a resistor, and the red-blue alternation runs automatically.
Use the forward voltage of the blue die (approximately 3.0–3.2V) when calculating the resistor, since the IC must accommodate the higher-voltage die. At 12V supply, use a 470Ω resistor. At 5V (Arduino), use a 100Ω resistor. Use our LED resistor calculator with “blue” or “white” selected as the LED color for accurate results. The red die has a lower forward voltage but the IC manages the switching internally, so you only need to account for the blue die’s requirements.
The alternation rate is typically in the 1–2Hz range per color — each color is displayed for roughly 250–500 milliseconds before switching to the other. The rate is factory-set by the internal IC and cannot be changed. The cadence is fast enough to be clearly recognized as a police-style strobe but slow enough that each color is distinctly visible rather than blurring together into purple.
Yes — this is the single most popular use case. One red/blue alternating LED replaces a two-LED circuit with alternating relay or microcontroller. Mount it in the light bar housing and it automatically produces the red-blue strobe. For a more elaborate multi-light light bar, combine one or two red/blue alternating LEDs with separate 6Hz red and 6Hz blue flashing LEDs. The 5mm body fits 1:24 through 1:43 scale light bars; for smaller models, use the 3mm version.
No. An RGB LED has three separate dies (red, green, blue) with independent leads that you control with your own circuit or microcontroller — you choose when each color turns on, at what brightness, and in what combination. A red/blue alternating LED has only two dies (no green) and an internal IC that controls the alternation pattern automatically. The red/blue LED is a two-pin device with no user control over timing or color selection. Choose the red/blue alternating LED for a simple, no-code police strobe. Choose an RGB LED for full color control and custom patterns.
Yes, but convert DCC to DC first using a bridge rectifier and smoothing capacitor. DCC track voltage is a form of AC, and the LED’s internal IC requires clean DC power to operate correctly. See our AC/DCC wiring guide for a complete diagram. Once converted, the red/blue alternating LED works identically to how it would on a DC battery or power supply.