November 20, 1997

Alternating LED flasher uses minimal parts

Dennis Eichenberg, Parma Heights, OH

A pair of alternating flashing LEDs makes an excellent visual alarm. One of the LEDs remains illuminated, so there's little chance the alarm will go undetected, even by a casual glance. Several circuits can provide this function, but the one in Figure 1 is the simplest. The circuit is also reliable, compact, efficient, and inexpensive. The heart of the circuit is LED₁, an F336HD T-13/4 red flashing LED (part number 276-036 at Radio Shack). It operates directly from 5V and provides a consistent light pulse at approximately 1 Hz without a time-constant capacitor.

The F336HD starts immediately when you apply power, and it's insensitive to temperature variations. LED₂ is a standard T-13/4 red LED (part number 276-041 at Radio Shack). The on-state current pulse through LED₁ is approximately 10 mA; it is 0 mA in the off state. This current flows through the base of Q₁ to turn it on. Resistor R₁ biases Q₂ on until Q₁ turns on. When Q₁ turns on, Q₂ and LED₂ turn off. When LED₁ turns off, Q₁ turns off, Q₂ turns on, and LED₂ turns on. Resistor R₂ provides current limiting for LED₂; R₂'s value produces brightness in LED₂ approximately equal to that of LED₁.
(DI #2106)

| EDN Access | Feedback | Table of Contents |