June 5, 1997

Inexpensive LED drives flash-and-beep circuit

Dennis Eichenberg, Parma Heights, OH

Sometimes, you need an alarm that is both audible and visible in an application that's stingy with space, power, and budget. The circuit in Figure 1a is compact, efficient, and inexpensive. The F336HD flashing LED (part no. 276-036 at Radio Shack) is a useful device. It operates directly from 5V and produces a consistent pulse of light at approximately 1 Hz without a time-constant capacitor. The F336HD starts immediately when you apply power, and it's insensitive to temperature variations.

The current in the flashing LED also pulses. The pulsed current produces a consistent beep when you apply it to a piezoelectric alarm, such as the Murata (Smyrna, GA) PKB5-3AL shown. The circuit can operate directly from TTL or 5V CMOS circuits. The LED is small enough to install in the periphery of the case of the audible alarm; you can even install the LED completely inside the case if you require no visible alarm. The simple modification in Figure 1b changes the tone of the audible alarm from a beep to a chime. The 100-µF capacitor in parallel with the piezoelectric alarm produces a pleasant ringing sound. The capacitor's value is not critical; you can experiment with different values to produce various audio effects. (DI #2035)

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