January 15, 1998

LED flasher and triac pulser work off ac line

Dennis Eichenberg, Parma Heights, OH

A flashing LED is an excellent visual alarm. Unfortunately, the LED is a dc device and requires additional circuitry to operate from an ac source. Several circuits can perform the necessary function, but the circuit in Figure 1a is the most efficient. This circuit is also reliable, compact, and inexpensive.

The F336HD red-flashing LED (part no. 276-036 at Radio Shack) operates directly from 5V and produces a consistent pulse of light at approximately 1 Hz without a time-constant capacitor. This LED starts immediately when you apply power and is insensitive to temperature variations. The W04G full-wave bridge rectifier produces a full-wave dc waveform from the 120V-ac line. The 0.5-µF capacitor provides current limiting for operating the LED from the rectified 120V-ac line. The 100 ohm resistor protects the circuit from surges when you first apply power. The 1N4733 5.1V zener diode protects the LED from high-voltage excursions.

Some applications require a more intense alarm. A simple triac pulser can pulse a 120V-ac lamp or other resistive load of as much as 8A (Figure 1b). This circuit is also reliable, compact, efficient, and inexpensive. The circuit is similar to the one in Figure 1a, but, in this case, the F336HD LED drives an MOC3010 opto-coupled triac driver. The 180 ohm resistor provides current limiting for the 2N6343 triac gate. This configuration can pulse a load as high as 960W. You can increase the power rating by choosing a different triac. (DI #2143)

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