Resistor And Transistor Synchronize Flashing LEDs

The ubiquitous LED continues to expand frontiers and applications with a plethora of sizes, colors, and output lumens. Flashing LEDs that incorporate internal electronics for oscillation offer embedded applications the advantage of size. However, designs that require multiple flashing LEDs are impeded by the reality that individual flashing LEDs are automatons and will blink asynchronously. In addition, these flashers can be an order of magnitude more expensive than their nonflashing brethren.

Here's a way to economically synchronize an array of standard LEDs using only two additional components (see the figure). A flashing LED is required to "set the pace" of the standard "slave" LEDs. When the flashing LED turns on, its corresponding current is sensed by the transistor via the 47-(omega) resistor. The transistor's collector current then drives the standard LEDs.

This design is scalable as long as the collector-current rating of the transistor isn't exceeded. The test circuit used 17 "slave" LEDs plus the flashing LED. Peak collector current measured 155 mA using the 3-V rail. A final version of the circuit incorporated a surface-mount transistor and resistor, which occupied less space than a standard LED. Cost and size restraints make this an ideal circuit for embedded applications such as toys or indicators.